Update 7/4/16: I did a comparison using the GR 1568A wave analyser that I previously repaired and described, to locate and measure the components from the fast output of my Tek 106 square wave generator. The results from this analyser and the GR are in surprisingly close agreement, well surprisingly to me anyway. The GR is very tricky to use and I was pleased to get similar magnitude values.
HP gear is usually excellent and this unit is no exception. The other piece of HP kit that I value highly is the 339A distortion analyser that with its precision oscillator and voltmeter finds a great deal of use on my bench. Having just finished up an audio project (see Audio Audio below), I got inspired to get this working again. The last time I went to use it, the signal channel was non-responsive. As so often turns out to be the case, it was an oxidised board connector and once again, Deoxit came to the rescue! Oxidised connectors and switches are the norm in old test gear. I’m not going to go on at length, just show a few examples of it in use.
Here it is displaying the internal 10kHz test / calibration signal:
This signal can be used to calibrate the instrument within +/- 1.5% at 10kHz. It can also be used to check the frequency span which here although slightly off is within limits. The frequency range of the analyser is 5Hz to 50kHz. The spurious response is specified as more than 80dB below the input reference level and can be seen here more than 80dB down after all these years. Here, the signal fundamental at 10kHz is the first spike that can be seen to exactly meet the top of the graticule, then we have 20, 30, 40 and 50kHz harmonics.
It has the capability of digitally storing one sweep that can be brought back up after a second sweep to allow direct comparison, here is an example of that feature, comparing the distortion spectrum produced by two different types of output tube, EL34s and 807s. The “dirtier” spectrum is from the 807s. The harmonics are at 1kHz intervals on a 1kHz fundamental (far left).
The sinusoidal tracking oscillator signal, swinging from 20Hz to 43kHz is available and may be used to do a frequency response sweep. The output can be varied from zero to to greater than 1 V rms into 600Ω. The sweep start is at 20Hz, so the graticule major divisions indicate 20, 43, 98, 200, 430, 980, 2k, 4.3k, 9.8k, 20k, 43k.
Here we have the same amp with at 10W (upper) and 1W (lower). The amplitude accuracy is given at +/- 0.5dB 100Hz to 20kHz and +/- 3% over the range 20Hz to 43kHz.
It shows about 2dB rise over the range, not bad for an old school amp with less than 6dB negative feedback.
I have only shown the basic features, there are many other modes of operation. One very neat feature is that the response can be cut off using the adaptive sweep control to above the noise level and this greatly increases that scan rate. This is because the filters will search for peaks in the noise and noise basically consists of a plethora of low level peaks that makes the filters work very hard. It is very useful when tweaking a circuit to watch the spectra change with the tweaking and the analyser is running in repetitive mode. Here is the test signal displayed with the baseline raised above the noise level:
Here is a capture of a frequency scan of an inverse RIAA network:
The shape is correct but the accuracy leaves something to be desired being compressed by about 4dB. The network measures better than 0.3dB except for two point that are closer to 1dB off. Since the rest of the points (I measured 11 points from 10Hz to 20kHz) are within 0.3dB, I suspect these two points are anomalous.
I have a companion blog on audio and I have just posted a new article on an amp that I designed to use up some quality parts that I had remaining from my sojourn in the world of vacuum tube audio. As a result, the design is somewhat original and possibly actually unique. I take time to craft my articles to be both informative and perhaps, to a point, entertaining. So I do like to have them read widely, hence this notice and the link is:
2/27/16: Today some new Lionel Fastrack arrived so that I could construct an oval having an increased radius of 3′, (from 2′) with 5′ straights. Pulling two Ace trains Stanier carriages, a cement wagon, a cattle wagon and a guard’s van (that’s all the goods rolling stock that I have), it managed 1590′ (0.3 Miles) at a scale speed of 79mph, just short of 10 minutes run time. Not bad at all! It is something like 80 years old. This run was achieved using the operating procedure given at the end of this post.
This is one of the many “toys” that I inherited from my Dad. Bassett Lowke first issued it in 1926, it was reissued under Corgi in 2000 (or thereabouts). Until now, I have never run it successfully, nor did he. The problem was largely due to overfilling the boiler resulting in much of the fuel available being spent forcing oily water to spout out of the exhaust. This is a result of following the official directions! More on this below. Another issue was due to fuel spitting out of the fuel vent and catching fire. Such problems are common with these small spirit fired engines and many that I have seen while browsing the internet have burnt livery. Operating outdoors, it is hard to see a flare up until the paint starts to burn! ALWAYS have a soaking wet towel to hand.
The Basset Lowke recommended fuel load is 30cc and the water 150cc. Using 150cc of water, the boiler primes heavily at the start. After several runs I have found that there is sufficient water left after the fuel has gone if I use 100cc with 25cc of spirit. This reduced quantity of water does make getting under way quicker and less messy. I use boiling distilled water, boiling it means that less fuel is spent getting the water up to temperature. On lubrication, the Bassett Lowke instructions state to remove the two plugs in the lubricator and fill through one hole until oil runs out the other. This is FAR too much (about 8cc) and will completely exhaust the burner just trying to get the valves and cylinders clear. I use 5cc of steam oil.
I have counted the (18ft) laps the loco makes pulling two coaches, and the distance is 1400ft or a little over 1/4 mile. In scale miles this is 11.4 miles, quite good for a loco having no fuel and water replenishment! You may see her in action here.
Reversing is accomplished by a valve that swaps the steam and exhaust pipes around. Here’s a close up of the motion:
If the boiler is allowed to cool without any opening to atmosphere, it tends to draw in oily goop from the lubricator that will foul the inside of the boiler, impairing heat transfer. This can be avoided by opening the whistle when the burner goes out. I washed the boiler out many times using boiling water to clear this as much as I could, a lot of oil came out. If you turn the loco upside down (with the safety valve removed) put it in forward gear then rotate the wheels repeatedly using the palm of your hand in the reverse direction, it will expel the contents of the boiler via the safety valve hole.
After washing the boiler, I did the reverse process (with the safety valve installed) still using boiling water, to wash out the steam passages. I did this in both forward and reverse.
The valves and pistons have no seals instead relying on the viscosity of the oil. At the low operating pressure of 15 psi, this is completely effective, I see no steam leaks from either the valves or the piston rods. It is possible to get the pistons out of the cylinders, the rod end caps are a light press fit and may be carefully pried out if necessary. I did this during my efforts to wash out the steam circuit.
The wicks were in a nasty state. The carbonised mess at the top of the main wick meant that it was not wicking very well. I realised that this issue had contributed to the problem with fuel being expelled from the vent and catching fire. Even though the burn rate was far too slow, the loco would still get hot. Since the fuel was not being burnt quickly enough due to the fouled wick, the pressure in the fuel tank was rising resulting in expelled fuel. Like many things, it is all too obvious once seen. Here are the old main and vapourising wick with the type of ceramic wick material (left over from 35028 Clan Line) that I used to pack new wicks. If you replace your wicks, make sure that the vapouriser wick only protrudes something like 3/16″. If the vapouriser wick is too large, the ensuing large flame will simply rob air from the burner jet above it.
Here is the burner in the loco:
Here is the oily state of the loco after a run, at least we know that lubricant is getting through the valves and cylinders!
Ditto the lubricator:
So, to summarise, this is my recipe for operating this loco:
1/ Ensure that the lubricator is empty (see 10 below). This is important otherwise you will get oil into the boiler in the next step.
2/ Ensure that the boiler is empty, you can do this using the blow out procedure described earlier in this post. This step is to ensure that the boiler is not over filled due to adding the correct amount of water on top of residual water.
3/ Fill the lubricator with 5cc* of reputable steam oil. DO NOT use the prescribed method of filling until oil flows out of the second hole, it will be over full and the consequent hydraulic locking will make it extremely hard to start. Do this with the loco horizontal to avoid oil going into the boiler. Replace the plugs firmly. Since I changed to this method, the loco will self-start without any pushing. In comparison to how things were before I developed my own running procedure, this actually is miraculous!
4/ Add 100cc* HOT distilled water to the boiler and fit the safety valve. (BTW, the valve lifts at approximately 15 psi.)
5/ Add 25cc* of methylated spirit or wood alcohol to the burner tank, fit the fill plug firmly.
6/ Carefully install the burner into the loco. The jet bar goes up between the second and third axles. Ensure that it is locked in place.
7/ Using a taper through the rearmost slot in the frame, light the vapouriser wick. Wait about a minute and similarly, light the burner using a taper through the middle slot in the frames. You will need to light up in the shade otherwise it is impossible to tell when the burner has caught.
8/ KEEPING YOUR HEAD WELL AWAY FROM THE TOP OF THE LOCO, Place it on the track and wait. When the loco tries to move, if necessary, you may help it to expel the condensate by pushing it from behind! However try waiting another minute, the loco may self-start.
9/ Open the whistle when the burner goes out. Leave it to cool.
10/ Once the loco has cooled, remove one of the plugs from the lubricator. Put it in forward and holding it front down, turn the wheels in the reverse direction using the palm of your hand to expel any residue from the lubricator.
11/ Replace the lubricator plug and remove the safety valve. Now, holding the loco upside down, turn the wheels in the opposite direction to the reverser setting and expel the remaining water from the boiler. This will ensure that the boiler is not over-filled the next time you fill it.
* These quantities were arrived at through many test runs. I purchased two ACE Trains Stanier coaches that are quite heavy and it was necessary to further refine the quantities, especially the oil. I made ABSOLUTELY SURE that there was water left in the boiler once the burner was exhausted. I also made sure that there was oil left in the lubricator. If you follow my guidance, it is UP TO YOU, to ENSURE that you have water left once the burner is exhausted. Do this by starting at 150cc and them come down in 10cc increments. I would no go below 100cc, the loco raises steam quickly and runs beautifully when starting from this level.
SAFETY! ALWAYS have a soaking wet towel to hand. This will save your loco and maybe more in the event of a flare up.
Suitable size O rings may be used to replace the fibre washers if they are torn.
Enjoy it running!
5/22/16:Click here to See it pulling a train.
I have indulged a desire long postponed, and that is to have a high quality working model steam locomotive. Though I do have the skills, I have realised that I am not willing to commit the time (several thousand hours) or the money for machine tools to build from scratch. Having got this clear, I decided to purchase a kit and assemble it. Though I would prefer 5″ gauge, such machines are extremely costly so having been watching the gauge 1 scene (1:32 scale, width between tracks 45mm) for several years, I decided to purchase a rebuilt Merchant Navy (RMN) kit for the princely sum of $5600 from Aster Hobbies who have been producing gauge 1 live steam models for the last 40 years or so. The models are produced in limited runs and I got lucky in locating the kit for this model (in England). Well, here she is, 35028, Clan Line.
I was born in 1957 and steam ended on the Southern Region in England in July of 1967 with 35023 Holland Africa Line hauling the last down steam train from Waterloo to Weymouth. She had “THE END, THE LAST ONE” written in chalk on the smokebox door. We lived in Farnborough, about 1 mile away from the main line from Waterloo. I can remember Bulleid Pacifics tearing through, 100mph speeds were known, a couple of times officially too! I once met a man who used to fire on Merchant Navies and he told me that though it was not sanctioned, 100mph speeds were not unusual. He quit the railway when disiesel came, too boring for an active, “up and at it” type. (Almost all British steam locomotives, being relatively small, were hand fired.) While I appreciate the sheer audacity of Oliver Bulleid in his design for the original “Spam Can” Pacifics in both Merchant Navy and West Country/Battle of Britain guises, (the later being referred to as light Pacifics having greater route availability), I think that the rebuilt locomotives were and still are, the finest looking loco’s ever, bar none. These locomotives have three cylinders each having a set of Walschaerts valve gear (unlike the Gresley Pacifics that had outside gear only, the motion for the the middle cylinder being derived from the outside gears using a system of levers.) The unrebuilt loco had outside admission cylinders and the outside cylinders were carried over into the rebuilt loco’s. The use of slide valves for the model naturally leads to outside admission and so the appearance of the valve gear having the radius rod connected to the combination lever below the valve spindle is correct. Forgive me but if you find all this baffling, I do give a basic explanation of the valve gear further into this narrative.
(The use of three or four cylinders was common British locomotive practice to overcome the cylinder diameter limitations imposed by our restricted loading gauge. Other than power, a big advantage of multiple cylinders, especially three, is that smaller balance weights for the reciprocating masses are required resulting in less forceful hammer blow on the rails* and thus greater route availability. The downside is horrible access for maintenance and oiling. The driver would have to get underneath the loco between the frames to oil the inside motion. I have never read of an accident occurring….)
*As a balance weight – located inside the rim of each driving wheel – comes toward the rail and then retreats the vertical component of the motion momentarily ceases, imparting an impulse to the rail just like a hammer blow. Damage to the tracks from this effect was known with large 2 cylinder American passenger locos when pressed to the very high speeds of which they were capable. The superb N&W J class 4-8-4 when tested on the Pennsy reached 110mph, and track damage resulted. (Note: No 611 is preserved and currently in steam. I was blithely unaware of her overhaul at Spencer shops NC and living in Greensboro NC on the route back to Roanoke, could have seen her in steam. Drat. I have been to Roanoke twice over the years just to gaze at her.)
A good reference source for all things Bulleid Pacific is “Bulleid Pacifics At Work” by Col. H.C.B. Rogers OBE.
Here is the box!
Here is the end of the box showing contents and serial number (141 200). A plate with the serial number is affixed to the underside of the right hand (fireman’s side) running board:
This kit requires much more work than simply screwing the parts together (using a multiplicity of tiny metric screws of various sizes). Having read the experiences of others I chose to start with the tender to get used to the instructions and diagrams:
Water tank and hand pump:
Top view with fuel tank removed showing fuel sump:
Complex brake rigging (faux):
The tender is functional and contains both the alcohol fuel and water. There is a hand pump under the cover on the rear deck to fill the boiler. The loco has an axle pump to keep the boiler level up as she runs. The front portion of the tender contains the fuel tank which feeds a triple wick burner, the wicks being around 12mm diameter, each having some 30 wick strands. The burner is force draughted in proper locomotive fashion by the hot gases being drawn through the two boiler flues (that contain superheater elements) by a miniaturised Lemaître style multiple jet exhaust ejector, albeit having 4 pinholes instead of the five 2.5 inch diameter jets that the full size loco is equipped with. This is not a toy, it is truly a miniature locomotive.
Moving on to the loco, the starting point is with the cylinders. To lap the machine marks off the valve and corresponding cylinder faces. A sheet of 1000 grit wet and dry is provided:
The 80 lb surface plate lives on top of my woofer under the CD player! It stays there too, I now avoid lifting such things alone though the CD player is also 80 lb! The amp next to it weighs around 250 lb, I am glad I made it so that the power supply and amp can be separated, it can also be rolled over forwards on the lexan side plates to permit access if needed.
OK, getting back on track as it were, here are the components for the middle cylinder with a completed outside cylinder. A slide valve can be see at top left with it’s steam chest at the centre bottom:
Completed outside cylinders and middle cylinder components with small bottle of steam oil:
Complete middle cylinder with crosshead and slidebars:
Herewith the crank axle (in this case, the middle axle) and eccentric for the inside cylinder valve:
I forgot to photograph the assembly of the frames that are held together and apart by members called stretchers. The instructions admonish the builder to take great care to ensure that the frames are aligned to each other properly, I used the surface plate shown in the cylinder/valve lapping process above to do this.
Here are the frames and wheels:
Steam valve gear is designed to allow the relative phase of the piston and valve motion to be shifted by 180° to permit reversing. The mechanism that does this also allows the percentage of the piston stroke at which steam admission is cut off to be varied so that as the loco gains speed, it can be “notched up” resulting in the steam being used more expansively. Various forms of this mechanism have been around since 1841, the earliest being called Stephenson’s valve gear but of course, it was not invented by him (a precedent for Edison I suppose). Belgium Engineer Egide Walschaerts came along with his mechanism in 1844, and it became the most common locomotive valve gear up until the end of steam on railways. The major component of both gears is the expansion link that oscillates back and forth with the rotation of the wheels. The link carries a die block that may be slid under driver control from one end of the link to the other to obtain variable cut off and reversing. In the case of Walschaerts, the expansion link provides one component of the valve motion via the die block and radius rod, the other component being derived directly from the piston rod, the two motions being combined at the valve spindle by amazingly, a combination lever! Whodathunkit? Stephenson’s gear also combines two motion components but differently, both being derived directly by eccentrics on the axle. (Don’t you just hate it when you get eccentrics on the axle?) Interestingly, the two motions sum to a somewhat square characteristic so that the valve opens quickly, slows while open and then closes quickly which is quite clever, especially when you consider just how long ago all this was thought out. It makes all the variable valve timing hoopla for cars seem somewhat less extraordinary.
Here is one expansion link with radius rod and die blot in the slot. The two plates sandwich the link/radius rod/die block and have trunnions on which the expansion link swings. The elbow connects to the rod that provides motion from a flycrank mounted on the end of the driving wheel axle. The slot in the radius rod carries a bearing on the end of the reverser lifting arm that allows the rod to move back and forth while being held at the required position in the expansion link.
Here it is assembled:
Here is a picture of the left side motion assembled. The flycrank on the rear driving wheel is where the speedometer drive is located, of course faux at this scale!
Here are the three cylinders installed in the frames complete with the middle valve gear, the pipes fitted so far are for exhaust. All three cylinders drive the middle axle. The shaft, sometimes referred to as a weigh shaft, will carry the three reverser lifting arms so that as the reverser is operated, all three die blocks and radius rods are moved together to the required setting. I made a point of drilling a small indent into the shaft where the locking screw for the middle reverser lifting arm engages since it will be hard to access later if it moves. The outside ones can be adjusted relative to the middle one during valve setting. I locked all the motion screws using 222 Loctite, that can be undone if needed. In particular, it is essential to ensure that the cross heads are securely screwed home on the pistons shafts, ditto the valve couplings.
Talking about valve setting, the point is to first align all the reverser lifting arms at the centre, then set the valve slides on the valve rods such that the port openings are equal at both ends of the cylinders and to do this in both forward and reverse gear. (Having the centre axle solid mounted as it is, must help this procedure.) The instructions suggest that if perfect settings cannot be obtained in both forward and reverse gear, to make the forward setting “perfect”. I did have to do that though the reverse settings are still very close to equal. Quite good.
Here, I have fitted the plumbing ready for an air test of the motion. The superheater and four jet exhaust nozzle are clearly visible:
This picture shows the drive system on the rolling road ready for an air test (that was successful). The stainless steel tubes are the super heaters that will later live inside each boiler flue. A fitting is attached where the steam outlet from the boiler would be to allow an air line to be connected. The two U shaped tubes at the front are connected to a displacement oiler. The oiler works (hopefully) by allowing steam to pass down one tube into the oiler where it condenses forming water that pushes the oil up into the other tube. If the top of the smoke box isn’t oily after running, there is reason to take a close look to ensure no blockages!
Superheater and manifold:
While we are on the motion, here is the underside much further along also showing the combustion chamber. The bogie with it’s side control spring and rear pony truck are fitted too. The axle boiler feed pump is between the centre and rear drivers, driven from an eccentric (the buggers get everywhere don’t they) on the front axle.
Now to the boiler, here we have the two superheater elements partially inserted into the boiler flues. The inclusion of a superheater is good news. Heating the steam out of contact with the water makes a lot more work available, furthermore, it saves wasting a lot of the fuel in getting the cylinders hot enough to prevent stalling on condensing saturated steam. Having some experience of small locos that do not have superheaters and having done three steam tests on the rolling road, I can attest that this scheme is much more than cosmetic!
Here is the combustion chamber showing two water tubes that sit in the rather fierce flame from the three large wicks. The flame is sucked around the back of the boiler and into the flues. The chamber is fully lined with ceramic insulation cloth. When the loco is getting up steam using it’s own blower, (steam nozzles located in the exhaust ejector) the flame almost howls! It raises steam VERY quickly. The steam blower is effective from roughly 2 bar (29 psi), half full operating pressure. Below that, it is necessary to use a heat resistant suction fan inserted in the chimney. I used a hand vacuum cleaner run from a variac with a couple of feet of copper tube from the chimney. I also drilled large holes into the tube to permit cooling air to be drawn in, also I used a box fan to blow the fumes outside. It works very well but I do not recommend it, the vacuum still got very hot and smelly, the fumes are most likely toxic too.
Here is the completed boiler, note the water level gauge glass. The long lever is the steam regulator and the small lever behind the regulator is for the steam blower:
Here, I am trying the boiler in it’s shroud, onto the frames. (It may be helpful to cut up a detergent bottle and make a thin plastic sleeve the guide the boiler into the shroud.) It took many tries to get everything aligned properly! If you look carefully, you can just see the water feed to the boiler between the rearmost driving wheel and the boiler. Getting this bend right was tricky, there is barely enough room to sneak between the corner of the combustion chamber and the wheel. The pipe as made had the correct curve shape but it was in the wrong place! I annealed the copper to get the bend out and reformed it where it needed to be. I have attempted to illustrate this in the problems area at the end of this narrative.
Here is a view of the multiple jet exhaust nozzle and the end of the boiler flues. The two longer jets are for the steam blower that is used to force the draught when the loco is stationary, the pipe emerging from the boiler feeds the blower jets and is controlled by a valve on the back-head. The top spigot is the steam delivery union from the regulator.
I forgot to take pictures of the smokebox construction however, I did find it necessary to open the smokebox after some runs. You can see the outside of the exhaust ejector venturi. The small pipes are for the lubricator.
Words to the wise on steam testing. Watch the water glass constantly to ensure that the axle pump is working. If water does not come out when you open the bypass valve, the pump is not working. For the first test at least, leave the return water connection off so you can see the return water flow. WATCH THE LEVEL IN THE TENDER! It gets through water surprisingly fast.
I did something VERY stupid. The fuel tank dispenses the fuel into an open well under the tender. There is a tube with a slant cut end that dips about half way down into the well and a needle valve opening in the bottom of the tank that lets the fuel run into the well. It maintains a constant level because when the fuel covers the end of the cut tube, no air is admitted to the tank, cutting the flow off IF YOU REMEMBER TO FIT THE FUEL CAP! I forgot on the second run and wound up with a nasty fire. I can’t smell wood alcohol* and it is hard to see alcohol burning until it has set something else alight! In mitigation, I did have a soaking wet towel ready and that saved the day, snuffing the fire out instantly with no mess. As with a frying pan fire, there is NOTHING to beat a wet towel. I am so glad that I knew this and maybe these words will save somebody else’s pride and joy.
*The British use methylated spirit that has a strong smell and is coloured purple. Very sensible.
Many components required significant filing to make them fit without forcing, certainly more than simply removing paint here and there.
One expansion link hanger was bent such that it was unusable, here is a picture showing the other side hanger for comparison. I held my breath and carefully bent it straight. Fortunately, the (lost wax) cast parts are brass, both tough and malleable!
Regarding the incorrectly formed water feed pipe, here is a picture showing the bend. You may spot the slight crinkles where the bend was and the pen mark I made to relocate the bend to sneak between the frame and the corner of the combustion chamber. The front corners of the combustion chamber are very close to where the frames rise over the rear horns and it’s width almost occupies the space between the frames making the fit very right. I studied things for quite a while before seeing how to relocate the bend because it is hard to see what’s so with the boiler in place, yet that is what must be accommodated!
All three reverser lifting cranks have forks that fit around the slot in each radius rod, these were all distorted such that I had to carefully tweak them open.
A much more serious problem had to do with the driving wheels. The centre wheels have no suspension by design. The front and rear driving wheels do have horn blocks and springs. I found that the centre wheels were barely touching the rails with the front and rear horn blocks topped out. I checked this by removing the springs and put engineer’s blue at the top of each horn. Sure enough, the centre wheels still barely made contact and there were two blue witness marks left on the top of each horn block. I carefully filed 20 mil (I trained as a toolmaker so precision filing is a skill that I have) off the top of each horn block. By the way, “mil” is NOT an abbreviation for millimeter, it is a correct term for 1 thousandth of an inch!
I found it necessary to work on the expansion link assemblies quite a bit, beyond the lapping of the inside surfaces per the instructions. I found it necessary to shim one pair of expansion link plates further apart, I use paper which provided a small by necessary increase of 2.5 mil. Here is the link with paper shims:
I also found it necessary to reduce the length of the die block pin on one radius rod, the pin was slightly proud of the die block causing the reversing action to balk, here it is after filing the pin down slightly:
I found that one connecting rod was fouling the underside of a slide bar. The lower slide bars are unfinished lost wax castings and looking carefully, I could see that one end was quite a bit thicker than the other causing the screw that secures it to project too far. Simply reversing the bar solved the problem.
Try to test the action of the axle pump when you do the air test, it is imperative that this works properly. I had a bit of a panic when I realised that the water had disappeared below the bottom of the gauge glass during the first steam test! The action of the inlet check ball must be correct and the clearance for it can be adjusted using the fibre washers. For the most part, I don’t use fibre washers, preferring metal to metal plus a smear of sealant so I had things too tight. David Stick speaks to this issue in his expert narrative:
I’m OK with these issues, I enjoyed the fitting challenge but the price of the kit at $5600 led me to expect better.
The vendor seems to think that no significant filing is necessary. I’m not clumsy or stupid, it was necessary. Interestingly, one area of fit that seems to have troubled many, the running boards, was not a problem for me. However, one of the lost wax cast steps up to the running board did need significant filing to fit. Oh, there is a hole on the frame side of both steps that does not align with the corresponding hole in the frames at all, they are only visible on very close scrutiny and contribute little if anything to the structure so I simply omitted the screws that are intended to be fitted. More than one experienced kit builder has suggested that Aster did not have such quality issues until they came out with the rebuilt Merchant Navy kit. The diagrams that come with the instructions are superb. The instructions need careful review. In many places, the order of assembly simply doesn’t work. I did not keep notes and I am not going to try to recall the details. Suffice it to say that trial assembly at every step is essential and it is well worth the effort! A handsome model indeed.
Update, 12/28/15. Since the boiler is of the forced draught type, it needs a device to create a draught until the steam pressure is sufficient to force the draught by blowing steam up the chimney venturi. These devices do not seem to be readily available. Aster Models UK sell them but they are quite expensive so I decided to see what I could cobble together. I have a 4 inch computer fan and I was concerned that the heat would be too much for it. So I fixed it to a plastic funnel, stuck it in the chimney and tried it. (Serendipitously, the funnel interfaces perfectly with the fan.) It worked just fine, even though aerodynamically speaking it is all wrong. I “should” be using a centrifugal blower. As you may expect, there was some melting of the pointy end of the funnel. It seemed to me that if I could fit a coupling onto the funnel that would sit snugly around the chimney rim rather than inside it, all would be OK. So a trip to Home Despot saw me with a PVC coupler and some copper fittings.
Here is a cut down funnel, a PVC coupler and a 1″ copper coupler. The copper coupler fits nicely around the rim of the chimney. I soldered a short length of 1″ copper tube into it to form a transition from the chimney rim to the funnel. The disc was cut from copper clad circuit board (using hole saws with suitable clamping) to form a centering device for the transition piece. The scollops in the PVC coupler were also formed using a hole saw, again with suitable clamping. I had no trouble with this operation, even though the hole saw centering drill was in fresh air, it went perfectly.
The copper transition piece was pushed into the funnel up to the abutment formed by the coupler. The PVC coupler fits the smokebox such that the flats rest on the smoke deflectors, this would ensure that the somewhat top-heavy contraption would sit solidly and not tend to fall off!
Here’s the resulting interface to the smokebox / chimney:
And here it is in place. The top of the funnel is just visible. I cut out a plywood flange to fit behind the rim of the funnel which allowed me to clamp the fan to the funnel. All the joints were made using silicone sealant hence the messy appearance. I took advantage of the annular chamber formed by the pvc coupler around the copper transition piece to fill it with water and made a tiny pressure relief hole into the funnel. This way, the highest temperature that can communicate to the funnel is the boiling point of water. Having tested the complete device, this precaution seems to be unnecessary. It works very nicely, providing just enough draught to make lighting up easy and steam is raised in about 3 to 3 1/2 minutes. The loco blower can take over at 50% pressure (2 bar). I will finish it by fitting a Ni-mH battery pack.
I dried and vacuum sealed the HV transformers in beeswax at the end of August. Since then we have have 4 months of high humidity before the onset of winter. I have just run the two scopes for over 48 hours continuously with no HV issues at all. Previously they would run maybe 4 hours before the HV collapsed. To recap, this issue is well know in Tek circles and came about with the change of sealant from beeswax to epoxy and I surmised that it was due to the tendency of epoxy to absorb moisture 0ver time. Having baked one transformer out previously, it then worked but as expected, failed again after a few months in my damp basement. So, I decided to bake the transformers out under vacuum in molten beeswax, then gently restore the atmosphere and let them cool. It looks as though this solution does work.
This post is part of my effort to complete and clean up loose ends as I disperse my collection.
I have two of these superb instruments, the final bow of the valve era for scopes and what a bow! Sadly though, that bow was assisted by a mostly transistor Y system. In mitigation of this heinous offence, the magical X system was mainly valved and could have been implemented using valves alone. I am not going to provide a full-blown discussion of the 547 scope at this juncture, rather to focus (pun intended) on a problem specific to the 546/547 models, failure of the HV system due to the substitution of epoxy for the traditional beeswax HV transformer seal.
The problem is typically that the scope will work when turned on and then, after some time, the HV collapses. If using it, this is first evidenced by the traces lengthening as the CRT sensitivity increases with the falling HV, though the traces actually brighten, at least initially. I first experienced it working through the lengthy calibration procedure. I discussed the issue with Stan Griffiths several years ago, he told me that it was thought to be dielectric absorption due to the epoxy. Hmm I thought, I wonder? So I baked a transformer out and it worked properly for a while, however it soon failed again; I am not the only one to have done/discovered this. My thoughts are: That the issue is not an intrinsic epoxy dielectric issue, rather power lost to heating of any moisture present in the epoxy; epoxy does absorb moisture (by diffusion) so what if I could dry it out and then seal the result? Knowing that beeswax is basically moisture impermeable, I decided to bake a transformer out in molten beeswax, evacuate it and slowly allow the pressure to return to ambient thereby sealing the newly dried transformer. I described my thoughts to Bruce Baur and Stan Griffiths by email back in May of 2014 but it has taken a while to get a “rountoit”.
Accordingly I bought a vacuum pot and pump for the quite reasonable price of $150. The pot is aluminium with a thick lexan top. The instructions provided with it insist that one MUST NOT continue to pump once the gauge has dropped to 29 in Hg otherwise there is a danger that the pot will implode! With respect, this is bunk. In round numbers, one atmosphere is 15 lbf/Sq in corresponding to 30 in Hg. Pumping down to 29 in Hg results in an external pressure of approximately 14.5 lbf/Sq in (or “squinch” as one of my excellent teachers used to say). Pumping down the final inch results in the effective external pressure rising to 15lbf/Sq in, that is to say roughly an increase of 3%. If this increase will cause the pot to collapse then it should not be on sale for this purpose. In fact for external pressure, vessels are customarily designed with a safety factor of 3*. That means it should be impossible to collapse the pot when pumping down from atmospheric pressure.
(*For external pressure, the design safety factor on yield has to be greater than that used for internal pressure because the failure mode due to compression results in failure by buckling which unlike direct tensile failure, is very difficult to predict accurately. Geometric factors such as dents are critical.)
I used USP beeswax that I bought from Amazon, for a fair price if there is any such thing anymore.
Here is the HV transformer and rectifier assembly partly removed:
Transformer removed showing heater power loops:
First, I put two transformers in a can with beeswax powder in the oven for 24 hours at 170 °F. (The melt temp is around 140 °F and the lowest setting for my oven is 170 °F) There was a nice smell of honey! To ensure that the wax remained molten as I evacuated the pot and returned the atmosphere, I needed to safely heat the vacuum pot. I heated a 5lb weight in a small oven at 220 °F that I then placed into the pot just before placing the transformers with their wax. The vacuum pot was placed into a large saucepan with water that I gently heated until I could just see wisps of steam escaping up the sides. (The weight also served to keep the vacuum pot firmly down in the saucepan.) I then transferred the transformers into the pot and evacuated it. I did go to far though because the wax started to boil below about 25 in Hg of vacuum. At that point, I turned the heat under the pan arrangement off and allowed the atmosphere to return by leakage, having turned the pump off. This took about 30 min and at the end, the wax remained molten as I intended. I then removed the top and hung the transformers over the tin to drip (having soldered on copper hook wires before starting the process).
If I do this again (I have a third transformer) the only change I would make to the procedure is to hold the transformer under vacuum carefully at 20 to 25 in Hg for some number of hours (with the wax molten of course) before letting it leak back to atmospheric pressure.
Ready to evacuate:
By the way, my mother always looked in the garage if anything was missing from the kitchen!
I ran the first unit for 10 hours straight and it did well. In particular, the oscillator screen grid voltage was a few tenths lower than when I turned it on (not powered it up, what a pretentious BS way of saying it). The HV had stayed dead stable; however stability of the SG voltage is the best indicator, it will rise if the oscillator load increases, topping out at 152V or so. Note the real meter (on the 3000V scale) on the HV test point!
The oscillator tube is run quite hard so you should check the transconductance. The few I have are around 25% down. The other thing is the B+ feed to the oscillator is via a 270Ω carbon resistor. If run hot, these develop internal cracks and that can be a reason for failure. The resistor in my unit measured 434Ω cold, most likely higher when hot so I replaced it with a wirewound resistor.
The question now is whether this fix will stand the test of time. I keep this equipment in an unconditioned basement and there are 10 weeks or so of high humidity left so if it works in the winter, that will be a solid sign that all is well.
8/30/15 Update: I have three of these transformer units, all three were failing in the way described and all three now hold up over a 12 hour period. I will test again at the onset of winter once the units have been exposed to what is left of the summer humidity.
5/14/2015: Update, The Benson (bottom of this post) is now working. Amongst many aspects of life that occasionally please me (I am something of a curmudgeon), are mechanical timepieces. I don’t have the money to collect top quality pieces however, really good mechanical clocks and watches are available at reasonable prices. Here is what I have amassed over the years: First, my parent’s long case clock (not a grandfather clock), that I bought in Guildford, England for my parents Ruby Anniversary with contributions from my two sisters, in 1987. My parents have now passed and the clock found it’s way to North Carolina. It has been overhauled once following being sadly knocked over by my father in his last year. (I have it screwed to the wall.) It hurts to reflect on my father falling, grabbing for something to save himself and taking the clock down with him, it is also life….. I can’t say enough for my parents, truly great people. I think that they both knew that I feel that way. It has a 66 BPM weight driven movement with Westminster, Whittington and St. Michaels chimes. I keep it on silent for the chimes are LOUD!
A wall clock that I bought new in Cary NC around 1993. It has worked without attention continuously so the quality of the modern movement (German) must be decent. It has a 80 BPM spring driven movement and the Westminster chimes are not too intrusive.
Here is a pre-WW2 Kienzle 160 BPM Westminster chime movement, given to me by a good friend in Durham NC. The clock had been dropped and the case smashed. My father made this oak bracket for it back around 1994 during a visit to me in NC. I have had it repaired, it turned out that the mainspring arbor was cracked. Since then it has run well with pleasant chimes. It lives on top of a Tek 191 constant amplitude signal generator on the top shelf of my lab bench.
I have long been attracted to carriage clocks and here is a nice Turner & Blight (British) brass one with bevelled glass that I bought on Ebay for $100. It has a 300 BPM balance wheel movement having 11 jewels. It is in perfect condition. I did have to strip it and lubricate the mainspring. Since then, it has kept very good time, certainly better than 1 minute per quarter, rather good I think. Although the balance wheel does have balance screws (presumably not just cosmetic given the accuracy), I doubt that it would keep time so accurately when subjected to the jolts of a carriage!
Several years ago, I bought this beautiful blue faced Franchi Menotti watch for $200 on Ebay. I don’t know the history of it other than it was cosmetically perfect complete with a virgin Franchi Menotti band. However, it didn’t run for long as one of the legers dropped off the face. I had that repaired and then later, the winding stem came out! It is so pretty that I broke down and had it repaired. I was told that it is quite old and needed amongst other things, a new mainspring. This still surprises me because the case and strap were perfect, completely free from signs of wear. It does have a ETA (Swiss manufacturers association) type 2824-2 25 jewel 480 BPM automatic movement. The 2824-2 has been extremely well copied by the Chinese and so that may be the origin of this one, however, if what I was told regarding its age is true, it will be a genuine ETA movement. Who knows? Other than what the repairer told me, I can find no information on it at all. It has cost me $400 for repairs, putting the actual cost in the same range as the Tissot:
The Tissot Le Locle, also has a ETA 2824-2 movement. In contrast with the Franchi Menotti, it is somewhat chunky and together with its black face is more handsome than elegant. I keep them on a winder and in that state, they are accurate to better than 5 seconds a day. When worn, the timekeeping does change but not a lot. They both have display backs: the Tissot also has some rather fine engraved decoration on the back.
Here is my paternal grandfather’s pocket watch. In America, this is considered a “ladies” watch due to its small size, my grandfather was no lady however! It has an English gold engine turned case that has had the movement replaced with a gorgeous15 jewel 300 BPM piece by Waltham that according to a site on Waltham watches, would have been made in 1908. The evidence for replacement of the movement is that it does not fit the case perfectly. (I imagine that if I could read the hallmarks on the case, they would indicate date the case to be well before 1908.) It does run however there is a story behind that. Around 1970, it stopped and the local jeweler (R. J. Hibberd, Ltd of Aldershot) claimed that the main staff was broken and that it was “not worth repairing”. Thinking about that, it seems unlikely, I cannot imagine an event that would break the main staff and not also the balance staff. I recently asked a repairer about it and it turned out to simply need cleaning! It now runs, Hmmm. The Waltham movement is top quality and has removable jewel settings.
The classic engine turning is very worn.
Next is my mother’s Vertex Revue, bought at the aforesaid Hibberds and given by my father to my mother at Christmas of 1965. I remember that it cost £20, an online inflation calculator suggests that would be £375 today (2015) which sounds about right. The 330 BPM movement still runs.
I think my father may have purchased this one sometime in his retirement. It is both ornate and pretty, also the centre of the dial is lightly rose-hued (I was not able to capture this in the picture though it does show on my cell phone but not my Mac). It runs but is weak and badly off beat. The quality of the case is clearly superior to that of the movement, unlike my grandfather’s watch where the replacement by Waltham is a very fine American movement. I have dismantled it and on the back of the face it says James Ducomann, successeur de Fritz Voegeli, with the number 20. So, it is clearly of French origin, at least as a brand. It has a bar movement with cylinder escapement, and the following note comes straight from Wikipedia: “The horizontal or cylinder escapement, invented by Thomas Tompion in 1695 and perfected by George Graham in 1726, was one of the escapements which replaced the verge escapement in pocketwatches after 1700. A major attraction was that it was much thinner than the verge, allowing watches to be made fashionably slim. Clockmakers found it suffered from excessive wear, so it was not much used during the 18th century, except in a few high-end watches with the cylinders made from ruby. The French solved this problem by making the cylinder and escape wheel of hardened steel, and the escapement was used in large numbers in inexpensive French and Swiss pocketwatches and small clocks from the mid-19th to the 20th century.” So we can see the French connection and as expected the movement is “inexpensive”. However, the case is superb, beautifully made from silver (not sure what carat grade yet) and I surmise that these watches were intended to be primarily decorative. The cylinder escapement is not popular with watch makers and technicians. We will see if I can get it in beat. Cleaning and lubricating it comes first. Fortunately, I have a partial movement that I can use as a source to replace some of the screws that are missing. It has 9 jewels, 2 each on the second and third wheels, 2 on the escapement wheel and 3 on the balance wheel (see repair notes below). The cylinder is hard metal, not jeweled as far as I can see, it is extremely hard to see! Something I noted as I dismantled it that I do not like is that the spring barrel is not supported on both sides, it has just one bearing leaving it cantilevered. Surprisingly, it does not seem to be sloppy however, this feature cannot make for even engagement of the barrel with the first wheel.
Well, I have cleaned and lubricated it. Then, came trying to “improve” the beat of the escapement. The balance wheel has quite a bit of end float, maybe 10mil (0.01″). On the face end of the pivot, there is an end jewel, that is probably intended to take end reaction due to the escapement wheel ramps. I noticed that it’s setting was not seating quite flat and removing it and investigating, noted some tiny burrs on the seating area. I took a sharp blade and carefully pared the burrs away and the setting is now in visibly more intimate contact with the seating area. The next thing was that I noticed that the two outer turns of the hairspring were coming into contact as the balance swung. Much extremely fiddly work with bright lights, a loupe and a needle and amazingly, I actually managed to prevent the turns from touching rather than ending up with a mangled hairspring. The balance amplitude is now greater than 180° when nearly run down which is a big improvement. Now, it runs about a minute fast face up, and a minute slow face down and pretty much on beat (300 BPM) when held vertical. This is a huge improvement and though the timekeeping is poor by our modern standards, for what is after all a vintage dress watch, is quite good. I am happy with the results of my amateur fiddling!
Finally here is a Benson railway watch in sad condition. My father had a pristine example of this watch when I was a kid, I think it was stolen. (From him that is!) The dial is chipped and the balance staff is broken. It has a Swiss 15 jewel movement which is very likely original. J. W. Benson once was a fine London manufacturer however in WW1, the company was badly bombed and thereafter became a retailer of Swiss watches and also those of the English maker, S. Smith & Sons.
I purchased a replacement working movement from Ebay. Unfortunately, though it is of the same type it must have been from a different model and has arabic numerals (with black hands, one of which was missing). Still, it is nice to take two partial old watches and create one working time piece, otherwise in the natural course of things, at least part of the resulting watch would continue to be junk. Initially I intended to take the working balance and install it into the original movement. This plan was scuppered when I realised that one of the pallet jewels was chipped, this most likely occurred when the balance staff was broken, there is a small but deep dent near the rim of the outer back that had printed through onto the inner back also. It must have been dropped quite hard. Here it is, cleaned up and working with the original gold hands, very classy.
I lubricated the movement using a synthetic watch oil, hopefully it won’t harden with time. I then timed the 300 BPM movement and it indicates about 30sec slow per day, both when vertical and horizontal, suggesting that the escapement is in good order. (I use a free Android App for timing called “Clock Tuner”.) A view showing the hallmarks inside the outer back. This watch has two hinged backs in the manner of a key wound watch. The inside back is also halImarked, it is quite a bit of extra silver. I can’t imagine why they kept the second cover for this keyless type. However, all other examples that I have seen have two hinged backs.
This is a post that I started many months ago. Now that I am winding up my restoration activities it is time to post it, complete or not:
I probably paid too much for this, but it did come with the manual and these machines appear to be rare and most definitely have a place in history, for it seems likely that Hughes beat Tektronix to the market with a storage oscilloscope. The seller did not know anything about it, sadly it had been pillaged of its 8 12AU7s and 2 12AX7s. (I am giving the seller the benefit of the doubt here.) Fortunately, I had enough strong 12AU7s and 12AX7s on hand to replace them. This example has serial number 635.
It is quite nicely made but not even close to Tektronix standards. The bandwidth is a very limited 500kHz* and the timebase triggering is fussy at best. The timebase does not have a trigger shaper (usually a Schmitt trigger followed by a differentiation CR), this probably accounts for the general fussiness of the timebase. The Tektronix 564 storage scope came out soon after and offered 10MHz, a proper timebase, dual timebases if desired, not to mention a wide range of X and Y plug-ins. It was also usefully smaller. Speaking of the Hughes manual, it is awful, there are several circuit and description contradictions making it difficult to follow the (extremely limited) calibration directions. At the age of 56, even with diopter 2 reading glasses, I need a magnifying glass to read the schematics.
* The WB/4 Y plug in limits this further to 250KHz.
The ESSENTIAL thing with these CRTs is to NEVER place them face down. I don’t yet know on 1/30/2014, if this one is OK.
Here, I provide a description of the principal of operation of the Memotron tube, that I have stolen from the manual and re-written somewhat to hopefully make things either more clear or less muddy.
The Memotron tube has two electron guns, a writing gun and a flood gun.
There is a dielectric storage mesh placed before the viewing screen. The flood gun sprays the dielectric storage mesh with a (supposedly) uniform barrage of low-velocity electrons leaving the dielectric surface at the flood gun cathode potential. The high-velocity electron beam from the writing gun charges regions of the storage surface positive by secondary emission*, thus creating areas which are partially transparent to the flood electrons. The flood electrons which pass through are accelerated to high velocity producing a continuously visible image of the positive electrical charge pattern stored on the dielectric surface: as long as the flood gun is emitting, the positive and negative potentials of the charged pattern on the storage surface are maintained thus sustaining the image.
A second mesh called the collector mesh interposed between the writing gun and the storage surface is used to discharge the charge pattern on the storage surface the image by briefly lowering the voltage of the collector mesh.
This technology is sometimes referred to as bistable because a given area on the storage mesh will either be at flood gun cathode or collector mesh potential.
*Bombarding electrons can impart enough energy to surface electrons to cause them to break free, electrons breaking free in this way is known as secondary emission.
* 2/2/2014, well I know now that the CRT does store but I suspect it is not in its prime! The contrast is poor and the background brightness has to be quite high before the trace can be seen, more on this in the design notes below. I knocked up a “pinger” consisting of a 3H choke in parallel with a 1µF cap in series with a 10k resistor. I placed about 20Vdc across the circuit to store energy then quickly disconnected it resulting in the following damped oscillation (timebase is 20mS/div):
Contrast Enhancement Oscillator.
There is a blocking oscillator that applies +15V, 10µS positive pulses at a rate of 1000PPS to the storage mesh, supposedly to enhance contrast by decreasing the background brightness. Though the pulse amplitude is adjustable, adjusting it appears to make no difference at all. This is most likely due to the condition of the tube, however in truth, I have not made the effort to fully understand the action of the pulses on the CRT storage system.
This simply reduces the CRT collector potential to nearly zero at the same time as briefly stopping the contrast oscillator.
This is a simple DC coupled long-tailed pair with gain such that 9v pk-pk on the input terminals will produce full scale deflection of the CRT beam. The amplifier is served by a plug-in that in this case is a DC coupled differential pre-amplifier type WB/4. A dual channel chop/alt plug in WB/D1/11 was also available.
The sensitivity of the WB/4 is 10mV/div to 50V/div with a bandwidth of 250kHz. The legend on the front says “wideband”. Even during the era of this scope, 250KHz was not wideband! The chassis is mounted on rubber anti-vibration mounts however, the sensitivity of the unit does not seem to warrant such a detail. Perhaps this is more important when capturing the slow events that this scope is clearly intended for. I would think that it was used to observe mechanical systems, especially since an aircraft company (Hughes) put for the effort to create it. There must have been a critical need for it. It would be interesting to learn just what is was used for, maybe something, in the arrogance of hindsight, that we now regard as obvious.
This DC coupled amplifier is similar to the Y amplifier except the degree of coupling between the cathodes can be varied to control the gain. The gain is set to cause the sweep to occupy exactly 10 divisions across the screen, best done using a marker generator. The amplifier may accept an external sweep and in this mode the gain may be continously adjusted by 10:1, a further 10:1 being available on the Ext Sweep portion of the Time/Div switch. The sensitivity is 0.5 to 50V/div with a bandwidth of 250KHz.
The timebase is switched on and off (trace, flyback) using a bistable multi-vibrator which is pretty typical practise. However, there is no Schmitt trigger between the vertical signal and the control multi. There is a cathode coupled trigger amp thats allows triggering on positive or negative going signals. The DC output level of the trigger amp is variable so that the trigger signal may be shifted relative to the DC trigger point of the control multi, in this way the DC level on the signal at which the timebase starts may be set of varied. Because there is no fast squaring (Schmitt Action) of the triggering signal followed by differentiation to produce a sharp trigger pulse, this system is unsuited to fast events.
The timebase essentially consists of a bistable control multi and a positive going sweep generator. The multi is anode triggered with the triggering signal being applied via a diode (so that it is negative going) to the input tube that is off, awaiting a trigger. This forces the anode voltage down, this change is communicated to the grid of the on tube causing it to cease conducting and the multi to flip states and the off (input) tube turns on, the anode voltage falling further. The multi remains in this state for the duration of the sweep. The resulting negative pulse is used to turn the timing capacitor discharge tube off and the capacitor starts to charge. Linearity of the charge process is ensured by bootstrapping the charge to a control tube across the charge resistor, maintaining the voltage across the charge resistor constant and thus the charge current also. The sweep voltage rises approximately 90V and is communicated to the grid of the off tube of the multi, causing it to turn back on resulting in a positive going pulse at the grid of the sweep discharge tube, turning it on, and discharging the timing capacitor. With this action, the input tube of the multi is now returned to the off state and the circuit is ready to receive the next trigger. Delay between the discharge process and the next sweep to ensure that the discharge is complete is accomplished by an RC time constant on the grid of the discharge tube causing it to turn on a little after the multi flips back to the sweep state. This is a simplified hold-off, in better designs the hold-off time constant is switched to different values with the sweep range switch. There are 6 ranges, 10µS/div to 1S/div together with a 1X, 2X and 5X times multiplier such that the range of the timebase is 10µS/div to 10S/div. There is also a 10X continuous multiplier control.
This is a clever solution to the problem of coupling the unblanking signal (positive pulse from the timebase control multi) to the beam control (usually grid) element of the CRT. What Hughes did was to turn a self excited oscillator on and off using the unblank pulse. The oscillator runs at around 10.5MHz. The oscillation is fed via a low impedance transmission line to a receiving coil and capacitor that is tuned to the same frequency as the oscillator. The resulting voltage is doubler rectified and applied to the grid of the CRT. Neat.
This consists of a HV oscillator driving an air cored transformer at around 300KHz. A sample is taken from the rectified output and via an amplifier is used to control the screen voltage of the parallel oscillator tubes thereby regulating the output in the usual way. The PDA voltage is 4.3KV while the grid and cathode supply is around -2.85KV.
“Low” Voltage Supplies.
Per Tektronix (and everybody else after Tek showed the way), there is a precision -200V rail, this is used as the reference for stacked +200V and +450V rails. There is also an unregulated +325V rail.
The first job was sorting out replacements for the missing tubes and using Deoxit on all the tube sockets. Then came replacing burnt out power supply protection resistors, a couple of the caps required re-forming and most probably replacement soon. Once this was done, the regulated supplies came in a little low but all in tolerance, the drop-out point was well below line voltage. It was not long before I realised that something was not quite right with the HV (CRT) supply which is a 300KHz high frequency oscillator type with the oscillator amplitude being modulated by amplified DC error feedback from the output to the oscillator screen grid regulator. It uses 2 1X2B rectifiers and on checking them, the negative supply rectifier was very weak so I replaced it too. I was then able to get the negative (-2.85kV) and PDA (+4.3kV) supplies in tolerance though the regulation is not solid and the voltages do vary a little as the intensity control is operated. I can find no obvious reason for this, all components are good and the screen regulator amplifier tube is good too. Here are a couple of views of the HV PSU:
I particularly like the air-cored, pie-wound HV transformer and the intensity and focus control universal joints. The rectifier heaters are supplied by the loops around the base of the transformer.
Interestingly, while the PSU reservoir caps were OK (well, after reforming 2 of them anyway), I did find it necessary to replace the regulator output shunt caps. (Caps that are not designed to handle rectification charging currents are less robust, insulation-wise.) One of them was tucked at the lower back corner of the PCB with the power transformer on the foil side so I had to remove the power transformer from its mounting. If you click on this picture, you will just be able to make out the offending cap behind the wiring in the lower right-hand corner:
And here is the transformer side with the plug-in cage, the CRT and shield and the HV PSU removed:
If you click on the picture, you will be able to see that the power transformer is off its mountings and actually could be swung right out of the corner to reveal the foil pads for the offending capacitor. While you are there, look for the nice universal joints near the front panel that drive insulating rods that connect to second universal joints on the intensity and focus pots in the HV PSU.