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.
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.
Having decided to dispose of my collection, this is the first item I put on ebay and it sold in the first day, clearly I priced it too low at $135 however, it is the first thing I have ever sold on ebay, live and learn!
My ebay name is triodeguy.
I have many items that have not yet made it to the blog and as I sell them I will post the description and pictures so as at complete this documentation project, albeit without the technical and restoration narrative.
Here is the description I used on ebay:
GE regulated power supply used to power a transmitter modulator. Who knows, you may have listened to your favorite station when powered by this unit!
I fitted a terminal block to the cut wire ends. The wire nut connects the line to the B+ transformer to allow the B+ to be remotely switched. A B+ time delay could be installed here.
The raw B+ supply is configured as choke input, given that the massive 10H choke is rated for 340mA and that there are 3 6AS7/6080 series tubes, I would expect this power supply to be conservatively rated for 300mA.
The rotary switch allows the meter to indicate total current, current through each section of each series tube or output voltage.
I have powered it up and it delivers 300V DC that can be varied from 255V to 415V.
I load tested from no load to 300mA (at 300V) and the voltage did not shift more than a volt however, it IS old and I will not guarantee your satisfacton.
This is pro gear and has just one electrolytic capacitor, the rest are paper in oil.
Use vintage the pictures to form your own impression of the condition of this unit, NOTE, while testing it, I did find one dead 6AS7 section so I replaced that tube with a good 6080.
Again, being vintage, I DO NOT guarantee proper operation so this unit is sold as is.
It weighs 58 lb and measures 13″ x 10.5″ x 18.5″
And here are the pictures I used:
3/19/2015: I sold 7 scopes, 2 meters a power supply and a signal generator this weekend, 4 of the scopes are full size (Tek 500 series and HP 150A). I am experiencing a sense of relief that this wonderful equipment is wanted and going to good homes. I am also relieved to know that the burden of stuff in my home is steadily reducing. I have moved into a new phase of my life and want to be able to move freely without having to deal with (literally) tons of stuff.
11/5/2014: I have decided that it is time to start letting go of stuff. If you are interested in acquiring any of the items I have written about, please comment on this post and I will get back to you.
8/9/14, UPDATE: I took and passed the general class today. Now the learning (leaning) curve begins……
One area of vacuum tube technology that I have not (yet) explored is (amateur) radio. I have a technician license and have been putting off doing anything more because I found the antenna (and ham jargon) issue daunting. One thing is that I want the feedpoint at the house, not in the middle of the yard! This makes a classic dipole less attractive given my house and yard. A friend (ham mentors are called Elmers) suggested a loop and sent me an excellent article describing the setup and performance of a loop antenna. Because it is a loop, the feedpoint can be anywhere so I have mine where I want it! The loop length is made equal to the wavelength of the lowest frequency of interest, in this case 3.7MHz giving 272ft. The larger the area enclosed by the loop the better, so the ideal shape is a circle. Mine is a pentagon because that gives me the best compromise given the wire length and the trees I have. The feed is RG8X 50Ω coax. I now need to learn how to use it!
I have a Heathkit SB-102 transceiver that I have repaired and checked out into a dummy load. It has a solid state Variable Frequency Oscillator (VFO) that is supposed to be significantly more stable than a tubed VFO. Using a MFJ Deluxe Versa Tuner II with the Heathkit, I obtained Standing Wave Ratio (SWR) figures of 80M, 1 to 1.1; 40M 1.2 to 1.3, 20M, 1 to 1.1 and 10M 2.5 at best. I need to upgrade my license so that I can use voice (SSB) on bands other than 10M! Some people use a 2:1 balun with a loop and I do wonder if this would yield better performance at 10M? I am a beginner….
By the way, the J in MFJ may stand for junk. The vanes of the variable condensers in my unit were rubbing due to massive slop in the spindle bushings that allowed the vane assemblies to flop around. I bought the unit on ebay however, it shows no signs of use much less abuse, so I think this issue is due to rubbish quality parts. I was able to slip plastic strips into the spindle bushings and remedy the problem. Even so, I would not use it with solid state transmitter finals.
The wire is 14ga stranded and insulated house wire. I placed loops of rope around the trees / branches that looked to be good locations for suspension. The loops are fitted with pulleys. Each suspension point on the wire is fed through an insulator tied to another rope (1/4in Daycron) that passes through the pulley down to ground level. In this way, the suspension points can be dropped down to ground level when the wire snaps (it will for sure). The length combined with the elasticity of the suspension ropes provides some accommodation for tree sway however, the mid-point suspension has a dead weight load consisting of two house bricks. (The dead weight tension method is used for the suspension of railway power catenaries.) At the lowest point, the chimney it is 24′ high and at the highest, around 40′. I hope this arrangement will accommodate most wind activity! The feed point is tied to the chimney of my house (which seems a better scheme than tying it to my neighbours house). It consists of a small plastic Radio Shack project box fitted with an eye bolt for the anchor. The ends of the antenna come in, then out and back in again through small holes providing sufficient grip to prevent them pulling out. The feed cable comes in through the bottom of the box and a tie wrap inside prevents it pulling out. All the entry points are sealed with silicon goop.
Oh, I had great fun playing with a potato gun to launch 3 of the suspension points……….
I have decided to take on writing up at least some of the GR kit in this collection. This unit was for sale at an attractive price given that it included a very neat GR 1201-B unit regulated power supply. It did not work properly, ceasing to pulse whenever I turned the pulse width control much either side of centre. I did not do well in diagnosing the problem, diving in too deep before simply studying the thing. The result was a cycle of picking it up and putting it aside. A very perplexing issue was that the pulse width was off by a factor of ten and I got into all sorts of trouble with that one, sure that it was acting as a frequency divider even though that made no sense. Finally I saw it, a previous muddler had (re)connected the pulse width pot incorrectly, damn! And so it now works quite well. The Pulse Repetition Frequency was low and again, manual in hand went through all sorts of component value conniptions (based on the manual) only to realise upon studying the circuit calmly, that the tube condition would affect the PRF. The manual makes no mention of this yet looking at the circuit it must be, and is. I simply went through a number of tubes until the PRF came in correct at the calibrated position of the PRF pot. Such dependence on the tube is not necessary and is surprising to me considering the reputation GR has.
I don’t have a lot more to say about it. It works well after many the self-made detours. The manual claims that it can deliver rise times of less than 18nS and fall times less than 10nS. I was able to see 18nS and 10nS on my Tek 475 (250MHz bandwidth) but not less. It does find quite a bit of use on my bench.
Repetition Rate, 2.5Hz to 500kHz with calibrated points in 1-3 sequence from 10Hz to 300kHz plus 500kHz. Continuous coverage with uncalibrated control.
Duration, 100nS to 1S in 7 decade ranges.
Pulse Output Levels: + and – 40mA pulses, each 40Vpk into internal 1k load. DC coupled with DC component negative wrt ground.
The 1201-B power supply provides a regulated floating 300VDC up to 70mA and 6.3VAC at 4A. In the pulse generator, the floating voltage is referenced to ground at -150V and +150V.
The pulse repetition generator is a Schmitt trigger with a RC circuit charge/discharge appended. The C of the time constant is charged from the plate circuit of the A section via the R of the time constant that is between the plate and the grid, thus the charge and discharge of the grid drives the A section. If the A section is off (plate high), the capacitor will charge through the R until the A section turns on*, turning the “B” section off. This action happens suddenly due to the regenerative switching action of a Schmitt trigger. The B section has an inductor in its plate circuit and the sudden release of energy due to the B section turning off produces a sharp pulse that is used to trigger the output pulse timing circuit. When the A section turns on, its plate voltage will fall and the capacitor will discharge until the A section turns off again. In this way, the oscillation swings takes place within the hysteresis of the trigger. The reason for PRF dependency on the tube as well as the RC values is that the hysteresis of the circuit will vary from tube to tube. The PRF ranges are controlled by switching in different values of C and continuous control is provided by making R variable from the front panel.
* The manual incorrectly states this as off. Here is the repetition generator circuit:
V101 serves as a current source when the circuit is in generate mode (switch position O), allowing the Schmitt trigger to freely swing around its hysteresis point. In position A, it acts as a driver for the Schmitt trigger to allow the circuit to be driven from an external source.
The positive trigger pulse from the repetition generator is applied to the pulse timing circuit and then the whole bloody thing goes belly up, or at least it did until I spotted the wiring blunder.
Q101 is normally on and with it, V103A and diode V103B, holding the voltage at the junction of R118 & R122 at a level determined by the setting of R125, the Pulse Duration control. Since Q101 is on, V105 is also on producing a current in the positive pulse output load resistor, R130.
A positive pulse trigger pulse from V102 (the pulse repetition generator Schmitt circuit) turns Q101 off and with it, V105 and V103A. Q102 turns on, and with it V106, producing a current in the negative pulse output load, R133. C2 begins to charge (ramp up) via R118 and the grid voltage of V104 rises until this tube conducts and the Schmitt circuit V104A and B changes state, sending a positive triggering pulse to Q102, turning Q102 off and Q101 on, re-establishing the initial circuit conditions. The circuit is now ready for the next cycle initiation pulse from the Pulse Repetition Generator. The pulse duration is therefore controlled by the time constant R118, C2 and the initial potential on C2 established by the Pulse Duration control.
The pulse output circuit consists of pentode V105 and V106 that are switched as described above by the transistor bi-stable, Q101 and Q 102. In the initial state, Q101 is off and with it, V105 causing the output pulse to rise to ground, going low when the circuit changes state. V106 operates in the opposite direction. The screen voltages set the zero bias currents at 40 to 45mA and since the output loads are 1k, the pulse amplitudes are 40 to 45 volts in maximum amplitude.