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Retirement Home for Vintage Oscilloscopes.

NAVIGATION: You can access the posts from the page "Equipment List & Posts" above, the items I have written about are in blue and clicking on them will take you to the relevant post.
Note; CLICKING on the pictures and diagrams will enlarge them!

Oscilloscopes fascinated me as a kid, the ability to see what is going on in a circuit is magical and like many other kids interested in electronics in those days, I built my own simple oscilloscope, based on the Heathkit design that was published in the fourth edition (1968) of the Radio Society of Great Britain handbook. My first professional oscilloscope was a Cossor 1035 MKIII which started my odd obsession! I now have one in the collection, you may locate it by going to the "Equipment List & Posts Guide".

I am expressing this aspect of my interest in vacuum tube (thermionic valve) technology by creating something of a retirement home for oscilloscopes and some other test gear. I say that there was a golden age from the time that Hewlett Packard was formed lasting until perhaps the early 80s. I am particularly interested in Tektronix scopes; Tektronix was created by Howard Vollum and Jack Murdock in 1947. Howard and Jack were interested in creating not only truly excellent products but also that their employees and their families had great lives, a holistic approach. And that was before anybody started talking about holistics; I think that for people of the calibre of Vollum, Murdock, Hewlett and Packard, it was a matter of common sense and common decency. Quoting from Marshall Lee "Why did Murdock and Vollum institute a broad range of profit sharing and employee ownership in the company? Because neither man had as a goal the accumulation of personal material wealth", and "it just seemed the right thing to do." Compare this attitude to the bunch of rapacious thugs that constitute the body of corporate CEOs and CFOs today. I recently met somebody who knew Howard as a customer and said that he was his nicest customer. Of Jack Murdock, one of the original four partners, Milt Bave, said of him "He was probably the nicest person that I've ever met". And Tektronix was hugely successful so what was that about nice guys coming last? The corporate environment of today, savaged by the nonsense of Thatcher-Reagan “market forces” – and as a result inhuman and predatory - is a far cry from that golden age.

Speaking about Tektronix scopes, Jim Williams (author of Analogue Circuit Design) says of them that they simply radiate intellectual honesty. I respectfully agree, in fact, my experience is that truly well engineered devices (of all kinds) are imbued with the spirit of their creators. Other than Tektronix, I also have scopes by Cossor, Telequipment, Solartron, Heathkit, Siemens, Triumph and a couple of AN/USM units along with two solid state HP AN/USM 281 units, a solid state 180A, a 122A and an early HP 150A. Currently, my collection stands at some 36 oscilloscopes along with signal generators, meters e.t.c.

I will be documenting my collection here together with what I have learned that may be of use to you.

Historical and technical sources include:
Oscilloscopes, Selecting and Restoring a Classic (Tektronix) by Stan Griffiths, W7NI,
Winning With People: The First 40 Years Of Tektronix by Marshall M. Lee,
My Years At Tektronix by Miles Tippery,
Tube Testers and Classic Electronic Test Gear by Alan Douglas and
vintageTEK.org, website associated with the Oregon Tektronix museum.

Heathkit ID-22 Beam Switcher

This device was intended to permit viewing two signals on a single beam oscilloscope. The input signals would have to be synchronised and though I haven’t tested this, probably of the same frequency. To cut straight to the chase, here are two signals displayed on a Knight KG-630 at 1kHz:Heath ID 22 Knight HG 630 1KHz

At 10kHz:Heath ID22 Knight KG630 10KHz

The chop rate can be selected from 200, 500, 1100 3700 Hz (as measured on my unit and rounded to the nearest 100). Here is the schematic:heathkit_id22_electronic_switch_sch

The circuit is quite straightforward. There are two input cathode followers connected to switch tubes.  An astable MV generates the chop squarewave that is used to alternately bias the switch tubes off via driver cathode followers. The separation between the traces may be controlled by altering the relative “ON” DC potential of the switcher cathodes. A further cathode follower buffers the output to the oscilloscope. Cathode followers are also provided to allow a sync signal to be taken from either input, the scope must allow an external sync signal. This is so that the scope doesn’t “see” the chop waveform and try to sync to both that and the signal, resulting in a useless muddle on the screen. It is necessary to play with the chop rate along with the scope sweep frequency to get a decent display. Both the pictures above were taken with the chop at 500Hz.

As designed, the sync CFs are connected to the input grids. I found that this resulted in the input being loaded down especially as the frequency increases so I relocated the sync CF grids to the input CF cathodes. MUCH better.

Amazingly, the coupling caps in this specimen appear to be of fairly low leakage so I left them. I did add a B+ resistor to get the B+ within the electrolytic voltage specification. This is due to increasing line voltage over the years. However, the capacitor must have been marginal voltage wise, even back in the day. Other than lubing all tube pins, switches and pots, it worked. For audio application where the frequencies presented to each input will be the same, I imagine this device would have been a boon in the days when professional scopes were too expensive for most amateurs. On the other hand, I expect most people played with it, as I have!

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Gauge 1 Steam, BR Standard 4MT Tank Loco 80097.

By and large the British Railways standard locomotives, designed under RA Riddles, were successful machines, capable for the work they were intended to perform, reasonably efficient, reliable and easy to maintain. They were also handsome, and I think that the BR 4MT Tank loco is one of the most handsome tank locos of them all. The 4 is the power (the classification for steam is strictly not power but tractive effort and runs from lowest to highest being 9 as in 9F) classification while the MT stands for mixed traffic, i.e. freight as well as passenger trains. I remember as a little boy being taken to meet the crew of one of them at Waterloo station and was strongly impressed. This loco is contemporary with the Rebuilt Merchant Navy Pacific (35028) that I have. I duly located and bought one to partner with said Pacific. Two versions of this model have been released, first by the Gauge 1 Model Company and later by Bowande (Wuhu). The G1 MoCo is now defunct and their designs have to some extent passed on to Accucraft. Both model runs sold out quickly, it is a popular loco! I finally located an elderly one that turned out to need to urgent attention (more below). Given the extent of necessary repairs it needed, I paid too much for it, c’est la vie. The previous owner must have realised what a mess it was in but didn’t come clean with the selling agent I think.

The G1 MoCo model is spirit fired, and is equipped with a draft blower that was completely blocked in my example. It has two cylinders with faux outside Walschearts valve gear (nicely done) and inside valves with slip eccentrics for reversing. It also has an axle and a hand water pump, as is usual with spirit fired gauge 1 loco. (The Bowande version is fired by gas and has no blower or water pump.)

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A handsome machine, the A4 too! Maybe the scales below will weigh the relative merits of the designs.

To the troubles: The blower was blocked and this led me to wash out the boiler. Quite a bit of sediment both oily and solid, came out. I washed it out with distilled vinegar three times but was unable to clear the blower line. So, the manifold had to come off. It is held to the backhead by 4 M3x0.5 screws, the upper two screw into blind holes while the lower two are open into the boiler. Well, the top screws came out fine. But, the lower screw heads simply fell off when I applied a screwdriver. I didn’t panic. (Not bad for me.) I pried the manifold off using a wrench on each of the protruding square shaped lugs on each side of the manifold so no danger of scratching the faying surfaces. (The lugs house the blower and regulator valves.) Having done that, I started poking about in the two lower screw holes and thank goodness, the screws had corroded completely away so all I had to do was run a tapping drill and tap through each. With galvanic corrosion between steel and brass or copper, the steel looses. (In fact, it is possible to remove a broken tap in brass or copper by dissolving it using an alum solution and heat. I initially thought I might have to do that.)

I cleaned everything up including the blower line and then re-attached the manifold just using silicone grease (Dow Corning) as the joint compound. I took care to make sure that the threaded holes were well packed with grease. I use this on all the pressure fitting threads too. There is no need to use fibre washers under them. For what’s it’s worth (probably not much) I will replace the screws with stainless steel items when they arrive.

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Here’s the cab and side tank covers. I was glad to discover that this assembly is easy to remove, just two screws in the back (just above the buffer beam), two screws into the cab floor from underneath and two screws in tabs at the front of each tank.

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I wound up removing the side tanks also. The tank containing the hand pump had been gooped up with RTV and the transfer tube between the tanks was blocked! There was no need for the RTV, the tank doesn’t leak! The joint between the pump base and the tank was the likely culprit. Again, I sealed the surfaces using silicone grease.

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This view shows one end of the water balance tube for the two tanks.

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The tube has fiddly sprung clips on each end and I have nothing suitable to grip them with so I put a 16 SWG copper wire retainer through the ends and simply snipped it away when the clips were in place.

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Running the loco:

Well, I now have fierce blower action.

I repacked the burner tubes with 22 strands of ceramic string, leaving about 3/8″ exposed.

Having filled the lubricator and the boiler, I fitted the fan, opened the spirit tap and lit her up. Pressure rose nicely. I opened the drain cocks and the lubricator valve slightly (too much and the loco will run without opening the regulator). Once around 20lb was showing, I opened the blower and removed the fan. Pressure climbed quickly to an indicated 50PSI. Opening the regulator a little and pushing, plenty of oily water was ejected from the drain cocks. Once that was done, I closed the cocks, opened the regulator again and away she went! She runs with little temperament. The only thing is that the axle pump only just keeps up and that was with no load. Still, with occasional top ups of the tanks she run beautifully for quite some time and kept running until the gauge had fallen to well under half pressure (this includes about 50′ of 1% gradient each lap). The loco clicks as she runs and I have noticed this clicking when watching this design run on YouTube. The axle pump is driven via a bell crank, there are five pin joints between the eccentric and the pump plunger resulting in considerable lost motion, I suspect that with the obvious wear this loco has, the extra lost motion is responsible for the clicking. I also expect lost motion may be responsible for the weak pumping action too. That’s a project for another day because the pump is buried under (or over) the inside valve gear.

Oh, no sign of leaks anywhere! The silicone grease works well.

 

 

 

 

Gauge 1 Steam, SNCF 232U1 (Gauge 1)

11/26/17: My friend Barry Coward visited from England and made this excellent narrated  (by your truly) video of this loco running.

9/9/17: I’ve just had a very fine run with 232U1. I have equipped her with radio control, on the regulator only. This makes running on my graded track much less fraught! The RC transmitter and receiver come from Peter Spoerer in England. The transmitter is a neat device that is purpose built for train control and fits neatly in the palm of one hand. I don’t like the usual big units that look like video game controllers.232U1 RC on Regulator

 

The RC installation is completely reversible, the servo bracket is clamped under two existing screws on the footplate. I tucked the battery pack in the gap between the fuel tank and the tender and the receiver simply lies on top of the tender inside the coal rave. I will make a shield to go over the servo for when I attempt coal firing. RC is not necessary on the blower (fortunately because there isn’t much room if I want to coal fire with RC fitted) because with the very soft blast of a compound, (I have found) it is necessary to run with the blower slightly open so there isn’t a concern about the flame popping back when she comes to a halt. The water usage is notably lower than that of the Merchant Navy however, fuel economy is comparable. I run both locos in full gear because it is easier and the reversers are too hot and fiddly to adjust once steam has been raised*. The fuel economy of 232U1 was terrible. I discovered that a previous owner had raised the burner screen about 3mm and cut higher wicks. I returned it to the original state and the economy improved a lot. What was happening was that the fuel was evaporating too fast for the available air supply so I had a fuel rich and thus relatively cool flame. I also put some silicone grease around the smokebox door, this resulted in the loco maintaining pressure with a much smaller blower opening, though the corrected burner will be helping this too. She does suffer the compound starting problem, not on the level or from cold, but on the gradient. The Merchant Navy has no problem restarting this train on the grade but 232U1 will not. This is because only the two smaller cylinders are available at starting. However, once she is moving and compounding (using all four cylinders) the pull is STRONG. I reckon she could pull perhaps double the weight up the grade.

* Also the run time with each, something over 30 minutes is quite enough for me. It’s time for tea after that!

Here’s the run:

8/20/17: The European 4-6-4 (or 232 using French nomenclature) is known as a Baltic,  and sometimes incorrectly as a Hudson. However, the 4-6-4 wheel arrangement first appeared in 1911 as du Bousquet’s last design, a 4 cylinder compound, and came to be known as a Baltic because the Nord Express served the Baltic coast. The 4-6-4 was introduced in America by New York Central in 1927, to haul crack passenger trains along the Hudson River valley and became known as Hudsons and fine they were too. Having digressed a little, back to the French story. SNCF 232U1 was the last of a batch of 8 Baltic locomotives commissioned just before WW2. Marc de Caso was the designer and the opportunity was taken once again to compare compound expansion with simple. 4 of the batch were built as 3 cylinder simples, 3 as 4 cylinder compounds having Dabeg rotary cam poppet valve gear. Whilst these compounds showed about a 12% savings on coal over the simples, the valve gear was not satisfactory. The last of the batch was not built until after the war and de Caso’s proposal to build the final one as a 4 cylinder compound having exceptionally long travel Walschaert’s gear with piston valves was accepted. (In full gear, the travel of the HP valves was 9.2″ and that of the LP valves, 11.7″, resulting in large valve openings at short cutoff settings.) The locomotive was successful, showing a further increase in coal economy of around 12% over the poppet valve compounds and able to sustain 3300HP at the drawbar. She was efficient and apparently easy to drive,  also having a mechanical stoker. Our British locos were too small for such to be of any real benefit, supposedly. Diesel and electric was coming and so 232U1 remained unique, nevertheless she racked up over a million km in 12 years, with no problems. She is preserved at the transport museum in Mulhouse and is perhaps the best indication of what steam technology could accomplish. I say best because while the French had other successful compounds, notably the Chapelon 242A1 that was also unique and somewhat more powerful, this one was perhaps the most rounded design, ready for a future that didn’t come. de Caso knew that 232U1 was the swan song for French steam locomotion and supposedly the streak along each side represented a swan and she became know as la Divine.

For some reason, British engineers never really made compound expansion work in the sense of coal economy. There were a couple of notable failed attempts, failed because the engineers involved tried to take too many steps at once. The French tend to be academically brilliant and able to translate theoretical predictions into measurable results. After all, the application of thermodynamics to heat engine cycle analysis originated with Msr Carnot and the French steam engineers took it seriously.

Moving on to the Aster Gauge 1 model. To me, the successful design of a proper compound expansion model in gauge 1 is an extraordinary accomplishment. Given the attitude of British engineers to compound expansion, it is ironic that the designer who made such models possible was a British citizen (J.T. van Riemsdijk). He was Mechanical Engineering curator of the science museum when I was little. I remember the apparently endless glass cases containing working loco models, each operable on compressed air by pressing a button (paradise for a little boy), many of which were 1 gauge!  Aster produced 4 compound models based on his work of which this specimen is 232U1 serial number 1! It is further notable in having a wet firebox capable of burning coal as well as spirit. The model comes with a grate and ash pan as well as a spirit burner. The spirit tank in the tender may be replaced with a coal bunker. Most fire tube models at this scale have a dry firebox and burn spirit only, as is the case with the superb Aster rebuilt Merchant Navy, that I described in an earlier post. A 1 Gauge luminary told me that the Rebuilt Merchant Navy is a museum quality model. I beg to demur, yes it is superb and an excellent performer, but museum quality it is not. The 232U1 on the other hand most certainly is of museum quality.

I wonder what’s in this case?20170815_145416

Aha! A warning, this is NOT a toy!20170815_143611

Inner lid:20170815_143706

Under the lid, assembly and operation manual and book of exploded assembly drawings:20170815_143622

Now to the loco and tender, still in the original (and slightly oily) tissue. The tool laying on top is the flue brush.20170815_144036

Tender first!20170815_14445320170815_144604

The loco has outside Walschaerts valve gear. Rocking levers transmit the motion to the valve for each inside cylinder. The original did have inside combination levers though.20170815_170137

Four cylinders, inside high pressure, outside low pressure. These are cast bronze, I think.20170815_165819

Leading crank axle for inside cylinders. 20170815_165840

The eccentric on the middle axle drives the boiler feed pump. What I can’t show is that the suspension has equalization beams, another technique that was eschewed by British Engineers. I can’t help think that if the Bullied Pacifics has been so equipped, they would have an entirely different reputation! I have described the equalisation of the front truck further down.20170815_165852

At the front is the Roscoe displacement lubricator. The leading truck not only has compensated suspension but the proper swing link system to guide the loco smoothly into turns. As the truck moves off centre, it causes the front of the loco to lift a little, the lifting force causes a turning moment that will cause the front of the loco to move in the direction of the curve, smoothly following the truck (or bogie).20170815_145721

Here’s the swing links either side of the pivot bearing. Each axle box is sprung. The force from each pair of boxes is coupled to the truck by a beam that is pivoted at the centre, thereby providing load equalization.20170815_170001

Here’s the rear truck with the spirit burner affixed, also showing drag/control links.20170815_170431

There is a link each side that connects the truck to the loco such that as the truck moves from side to side, it also turns slightly to follow any turn radius the loco is traversing. The links also couple the pull to the truck, the link at the back is the draw bar so the pull of the loco is coupled through the side links via the truck to the draw bar. There are side bolsters that support the weight of the back end of the loco on the truck.20170815_170444

Detail view of left side drag/control link:20170816_140749

And now to a very special feature of a small model, the wet leg firebox complete with crown sheet and stays. The hot end of the five flues are visible. The two large flues contain superheater elements. As the label in the lid states, this is NOT a toy!20170815_165913

Here is the grate and ashpan in place: The four screws engage in keyholes in the side bolsters of the truck, making the truck easy to remove for changing the burner or cleaning the grate and firebox.20170815_170534

Here I’m holding one of the sprung loaded ash hopper doors open:20170815_170910

Speaking of flues, here’s a view through the firedoor into the smokebox:20170815_171516

Here’s the backhead. Clockwise from top left, whistle, regulator, gauge glass, pressure gauge, screw reverser and blower. Tucked down in the left corner is the control rod handle for the LP cylinder drain cocks. At the bottom right is the feed water bypass valve. The reason for the feed water bypass is that the maximum capacity of the feed pump is designed to keep ahead of the steaming rate. There is a pump bypass valve that can be adjusted to balance the feed, the excess being returned to the tender20170815_170957

Another view of the backhead and cab:20170815_172809

At the front end, here’s the appropriately filthy inside of the smokebox!20170815_171714

Double chimney:20170815_171738

Top of boiler showing nicely detailed access flaps, presumably, some would have been for sand. The front one has been used to cover the boiler fill plug. Two safety valves (4.5bar) at the right.20170815_171813

And back to the tender, it is hefty!20170816_200745

Detail view of the steps showing the catenary warning!20170816_20080620170816_201025

Hatches showing water tank and hand pump, again superb detail:20170816_200827

And finally the tender front showing the water bypass return, spirit connection and water feed connection. 20170816_200907

I will close with the words of John van Riemsdijk written in 1997 and taken verbatim from the Gauge 1 Model Railway Association book “Modelling in Gauge 1 Book 2”:

“The latest, and perhaps last, compound was the French streamlined Baltic, SNCF no 232U1. Baltic, not Hudson: the first 4-6-4 tender engines were also built for the French Nord main line, and in 1911, and were called “Baltics” because that was where the trains they were meant for ended up. 16 years later, the Americans took up the 4-6-4 wheel arrangement and called it a Hudson, but U1 is definitely a Baltic. The real engine proved one of the greatest locomotives of all time, and is on view with one of the original Baltics in the museum in Mulhouse. The model is unsurpassed and perhaps unsurpassable. It has only two sets of valve gear, but this is extremely robust and closely copies the same arrangement on the real engine. As compared with the Chapelon Pacific, the cylinder sizes are the same, but the greater weight of the coal-fired boiler improves the adhesion and allows higher working pressure to be used effectively. There are drain cocks on the L.P. cylinders, but I find that it clears itself without their use. In fact you can tie it onto a very heavy train, then raise steam, and when at full pressure the engine will quietly move off with its load, the puff only becoming audible when the cylinders warm up.”

Please Note: If an official of the G1MRA feels that I have appropriated these words err, inappropriately, notify me and I will (reluctantly) remove them.

 

 

Heathkit SB-620 Scanalyzer

SB-620 FrontDespite my interest in vintage electronics now taking a back seat to other things, I have been interested in obtaining one of these for a while and recently acquired one, a dead one. These devices are usually known as Panoramic Adaptors and were used by the Signal Corps. There was another well known commercial one suitable for radio hams but I cannot recall who made it and a quick Google search hasn’t helped.

The purpose of the device is to allow a radio operator to see what’s going on close by the frequency he is working or considering working, out to +/- 25kHz or so. Functionally, it is a narrow range spectrum analyzer. The fixed ranges are 10kHz and 50kHz, a variable range allows the unit to scan out to +/- 250kHz or slightly more, depending on the IF frequency of the receiver it is looking at. There were different coils available for the local oscillator and a builder/ user would have to set the LO up for the IF frequency of interest. In my case, I have a Heathkit SB-102 that has an IF of 3395kHz and fortunately the unit I have was set up for 3395kHz though apparently with the wrong oscillator coil, more on that below.

It has turned out to be one of the more interesting pieces of kit that I have encountered. The operating principal is the use of a frequency changer (mixer) and a sweeping local oscillator. The oscillator is swept by a voltage that is derived from the X axis sweep voltage and applied to a varactor diode. As the oscillator sweeps, when a signal is present that causes a difference frequency of 350kHz that signal is passed by the IF strip in the analyzer, is then rectified and presented to the Y axis. The signal being synchronous with the X axis results in a stationary peak or a “pip”. There is a crystal filter before the IF strip that sharpens the resolution to 1kHz. The sweep is generated using a neon relaxation circuit.

The first job was to bring the thing back to life. There was a poorly patched in replacement CRT negative supply capacitor that did not bode well and sure enough there was no HV-. I soon established that the corresponding transformer winding was open. This is actually helpful in an odd kind of way because such failures are usually (in my experience) due to insulation failure and that means the transformer is rendered completely useless. I took the transformer apart far enough to discover the remains of a burnt lead out from one end of the winding. I surmise the winding burnt when the aforementioned capacitor failed. So the transformer could still supply the B+ and heaters, all I had to do was to sort out a work-around for the HV-. I wound up (pun intended) making a voltage quadrupler that would be powered by the B+ winding. Even with current draw multiplication due to the quadrupler, since the HV- chain only draws around 200μA the additional load on the B+ winding is negligible. However there was still an issue. Because the B+ is also rectified by a full wave doubler, the B+ winding is elevated some +250V and if I connected directly, that elevation would subtract from the negative HV. So I capacitor coupled the system to the B+ winding. Further it is necessary to connect the correct side of the chain – marked “A” below – to the top end of the B+ winding otherwise it will not work. Here’s a picture of each side of the quadrupler with the diagram:Heathkit SB-620 HV Repair V Quadrupler

I found it necessary to increase the value of the caps for each stage to maintain the successive voltage gain, you can see from the measured voltages noted on the diagram that the chain steps fairly uniformly to the desired voltage. It worked out perfectly! It also fitted perfectly:SB-620 Bottom

I went through the calibration protocol and encountered a problem that took a while for me to comprehend (my knowledge of RF is sketchy at best). On setting the local oscillator (LO), I found that it seemed to tune well however, the instructions go on to test whether the LO is set at the correct frequency, i.e. 350kHz under the incoming IF frequency or 350kHz over. The method is to raise the incoming frequency by precisely 700kHz and an image peak should appear to the centre of the screen. Well, it didn’t. Said another way, the LO should be 350 above the incoming IF such that if the incoming signal is raised 700, it will first pass through the LO frequency and finally be 350 above so that an image signal will pass the analyzer IF strip. I later revisited Stephen Lafferty’s notes on constructing different LO frequency arrangements (the original parts are as rare as rocking horse poo) and he noted that for an incoming IF of 3395, the LO should be at 3745 whereas mine was at 3045. I queried this claiming ignorance of matters RF and Stephen confirmed that his figure is correct. Stephen’s article is here. We went back and forth a little, both being puzzled. The problem is that the LO coil (L3) is not marked with a part number. We concluded, I think, that the coil in my unit is not correct for an incoming IF of 3395kHz. Stephen had used a trimmer for the LO tank capacitor, and I followed his lead. The coil in my unit was already adjusted close to minimum inductance and so the fixed 56pF cap had to be reduced in value to raise the LO frequency. I replaced the cap with a 7 to 47pF trimmer and was easily able to obtain the correct LO frequency and the 700kHz image per the instructions. HOWEVER, the law of unintended consequences intervened: Before making this change I had taken a picture showing combined 1 and 4 kHz modulation with the peaks occuring in the correct locations thus:SD-620 1 & 4kHz Modulation Scan

(The small pips are harmonics. Note, the pip centre control is excessively touchy on that range hence the poor positioning in the picture above. The flash is due the the scan being very slow compared with the shutter speed.) After the modification, I was unable to reproduce this result with the modulation peaks occurring at approximately 1/2 scale. After much thought (well what passes for thought in my slow old brain), I realised that the reduction in fixed capacitance in the LO circuit meant that the swing of the varactor now had increased influence, approximately double, and that the bandwidths were now double the intended settings. Rather than screwing around with the varactor swing voltage settings, I chose to  increase the capacitance and settle for the LO being 350kHz below the IF frequency at 3045kHz. It just works better and is more satisfying to play with this way. Here’s what it looks like squashed up by the doubled bandwidth:SB620 1 and 4kHz modulation, wrong scalingI then decided to address the excessive pip centering sensitivity; The pip centre pot is 5k. I established the resistive setting with the pip centered then replaced the pot with a 2k part with appropriate resistors each side. This reduced the sensitivity usefully while still permitting the pip to be moved to the side of the screen when using the 50kHz sweep range. Again, it makes the device more fun to play with.

The next problem was that the sweep circuit then failed on the 10kHz range. This requires the slowest sweep rate and the charging resistance is more than 100Meg, the currents being in the few micro-amp range. I checked the values of the various resistances and they were surprisingly close. I also replaced the capacitor (the green cap in the picture above) but it still would not sweep on the 10kHz range. So, once again, a puzzler. I decided to see if perhaps grid current draw from the X amp was stalling the relaxation circuit. Changing tubes didn’t work, I tried several. However, I found that both the plate resistors were almost double the specified values despite being marked correctly. They are too small in size for the job and in my experience, carbon composition resistors develop cracks when run too hot over time. I replaced them with correct values and sure enough relaxation action resumed (if that isn’t a contradiction in terms). This might seem surprising however, it’s a Kirchoff thing, the currents entering a node must sum to zero. In this case the node is the triode and if the plate voltage is too low to cause sufficient conduction from the cathode, the current to balance the node must come from the grid. (I’ve encountered this before and at that time it stumped me for a while. I was developing a hybrid cascode for a phono stage with a fet in the bottom and a triode in the top. The triode was pulling the grid voltage reference down and I finally realised that I needed to increase the plate voltage.)

The rest of the calibration is pretty mundane though I encountered another issue when the instructions direct one to amplitude modulate the incoming IF at 50kHz to set the scan width for the 50k setting. Neither of my signal generators (GR 1001A and HP 8610A would accept AM at that high of a frequency. It’s not necessary, I found that if I modulated at 10kHz, the harmonics took care of the rest of the band. It’s not a laboratory quality instrument, so the deal is to mark the scan width control for the setting that gives correct placement of the modulation frequencies on the screen and (presumably) check it from time to time. Here’s what 5kHz modulation scanned on the 50kHz range looks like, showing that the harmonics can be used for sweep width setting certainly out to 20kHz.SB-620 10kHz and harmonics

Here is what 1kHz FM looks like on the 50kHz range showing the IF passband: I didn’t understand what I was seeing and again, Stephen came to my aid: “To explain it in detail, I would need to know a little more, but generally it seems typical to me. The key question is whether you mean that the modulating frequency is 1kHz or the FM deviation is 1kHz. I will assume it’s the modulating frequency. From that, I would estimate that the peak FM deviation is 7kHz and the span of the display is 40-50kHz. (I estimated the peak deviation by counting the 1kHz peaks from the center to the edge of the deviation.) Then the modulation index is m = Fpk/Fm = 7kHz/1kHz = 7.”

SB-620 1kHz FM

I managed to get it to see the IF from my SB-102. The sensitivity is just adequate and Stephen built a 17dB gain stage into his. The noise floor does show well on mine with good peak amplitudes on quite weak signals (my antenna basically isn’t). Still, here’s something from the 40M band, the primary signal is from France:SB-620 Scanning on 40M

 

 

HP 3580A Audio Spectrum Analyser.

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:

HP 3580A Internal 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).

807 : EL34 PP Spectrum 8W

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.

EL34 PP Log Frequency Sweep, 20Hz to 43kHz, 10W Upper, 1W Lower

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:

HP 3580A Adaptive Sweep Demo

Here is a capture of a frequency scan of an inverse RIAA network:

DSCF1444

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.

Here is a link to a concise description of how this analyser works and how to operate it from the University of Saskatchewan.

 

 

Audio, Audio!

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:

https://richardsears1.wordpress.com/2016/06/30/class-a-807el34-pp-with-transformer-gain-stage/

0 Gauge Steam: Bassett Lowke Stanier Mogul

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.

Bassett Lowke Stanier Mogul 1

 

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:

B-L Mogul, flycrank trailing

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.

I found myself wondering about the state of the wicks in the burner. This is a simple vapourising spirit burner with a row of 16 jets (well, holes).
B-L Mogul burner

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.

B-L Mogul old wicks

Here is the burner in the loco:

B-L Mogul Burner Alight

Here is the oily state of the loco after a run, at least we know that lubricant is getting through the valves and cylinders!

B-L Mogul oily after running

Ditto the lubricator:

B-L Mogul 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!