Project Time! #4

Last time I showed a Hewlett-Packard 3465A multimeter that I bought pretty cheap. It was described as powering up but misbehaving and I got it for one reason – to do science to it.

For some time now I wanted to build my own “multimeter”, though that’s a bit of a stretch as far as the name goes – it would probably only do DC voltages but maybe also resistance if I could come up with a good enough current source. Autoranging because actually it’s easier these days to have an MCU or some logic drive a couple of relays than to design/find a complicated gang switch. And if I could get my hands on suitable precision shunts then DC current measurements would also be possible. I did not exactly expect to build a quality instrument, no, rather it was supposed to be a learning exercise.

Now, one could just get an ICL7106 and build a 3.5 digit meter out of that and a few extra parts (like the 7-segment displays) but that’s not how I roll. I wanted to make my own dual slope A/D converter, with (MOS)FET switches, voltage reference and all the control logic including auto-zero, averaging and digital calibration – the whole works. Eventually though I figured that maybe trying to fix a broken meter rather than building one would be a better idea considering my goals and lack of experience.

The HP multimeter was promising but ultimately a failure – in the sense that all I did was take it apart, remove the dead NiCd cells, clean it, flood the switches with contact cleaner, put it back together and it worked. So well in fact that I was impressed and decided not to tinker with it. It’s a really good device – I actually ended up using it quite a lot.

So, back to square one, I got me another cheap multimeter – a Fluke 8600A. I’m not very fond of Flukes even if some people consider them to be the superior brand. Obviously this is 40+ year old tech and times (and quality) have changed but comparing the guts and performance of both HP and Fluke I have to say the HP wins, hands down. Here’s the seller’s photo:

and a link to EEVblog forums post that will show you how the Fluke should look inside.

Well, mine has a few issues here and there:

Power filtering electrolytic capacitors are gone. So is the 5V rail regulator along with its heatsink. Also, and that’s the major pain in the butt, so is the transformer – all missing. Worse yet, whoever removed the voltage regulator was not very proficient with the soldering iron:

Turns out the very pin that connects the regulator to GND is also the common ground tie point between digital and analog sections of the meter. So with the via all but destroyed I had pretty high impedance between DGND and AGND and it took me quite a while before I found on the schematic these two should be in fact connected.

Well, I can supply the power (+5V, +15V and -15V) from external PSU for now. But what’s this, the metal shield around AC to DC converter is missing as well. Also, I see some burn marks:

Not exactly sure how it happened but the precision wirewound resistor in the ohms converter burned up, and somebody replaced it with what seems to be a metal film 1% resistor. But I have to say they did a pretty good job because originally it was a 9.95K 5W 0.1% – good luck finding something like that. The replacement is 9.96K which is good enough except maybe temperature coefficient but not much can be done about that. Same for the PCBs but at least the carbonized material around the leads has been removed. Here’s what the add-on cards look like up close:

The voltage divider on the photos is already missing the 50 ohms calibration pot, I removed it because it was open. Possibly went kaput in the same event as the ohms converter? Didn’t have a spare on hand so for now I just put in 2 resistors just to be able to continue testing. You probably noticed the missing markings on one of the reed relay coils, somebody was messing with that one too.

Meter had problems with autoranging so the first thing I suspected was the small ROM that’s used to drive relays – it’s known to die. That turned out to be mostly good, one bit was glitched in the 200mV AC range. Some people claim this part is actually a bipolar PROM, not mask-ROM, which would make the zero-bit recoverable with a suitable programmer (unlike modern EPROMs, bipolars have all zeros by default and are programmed to ones by burning connections inside the chip). That was however moot point as I don’t have such programmer, and shortly after one of the outputs died – could be I killed it by shorting with probe tip at some point.

That’s the ROM testing setup, since it has only 32 addresses I can just test all of the values in 10 minutes by switching the inputs with wires. The output that died is that single LED on top, the photo and notes I made prove it was working up to a point.

The fuses were missing (both the line power and the current measurement protection), and the whole thing would pretty much always show overload no matter what mode or range was selected. This was eventually traced to busted FETs switching input and zero to the A/D. In general I had to:

  • rebuild the power supply
  • find and replace faulty logic chips
  • fix issues with A/D converter
  • clean input jacks area and redo wiring
  • fix broken AC to DC converter
  • replace burned PCBs (and dying reed relays in the process)
  • recalibrate everything (which required replacing a selected resistor)

I’m going to get into the details of all this eventually, for now though I’d like to present my idea of replacing the faulty ROM. I’ve seen people use a more modern EPROMs like 2764 (which is also obsolete now but at least obtainable and mostly supported), but that is 28-pin wide body part. The only way you can fit that in is on wires, that’s ugly.

With some minimal pin swapping I’ve managed to use GAL16V8 instead because the contents of the ROM is simple enough that it can be turned into logic equations that fit on the part. And the GAL is small enough that it actually will fit into the original socket:

The results were promising but two issues came up. First one was some weird latching in invalid state when switching away from DC current mode that caused the GAL to basically short-circuit the 5V line. Good thing I had this running from external PSU with current limits. This particular problem was easily solved by adding a 10uF ceramic SMD capacitor right on the chip, as close to power supply pins as possible. I have to say Fluke apparently did not belive in any logic chip power filtering, there’s basically nothing of the sort on the whole PCB and the 2-layer layout means pretty long traces. So long in fact that the usual 100nF wasn’t cutting it, 1uF almost fixed it completly, so I picked 10uF to be on the safe side. That and the whole issue with MLCC caps derating under DC bias.

Second problem was also DC current mode related – the way Fluke implemented it was by cutting power to the ROM and all the relays while it was selected. This is because current measurement is still manually ranged on these early models, by selecting the correct shunt resistor, and the A/D is permamently set to 200mV range to keep the burden voltage low. Now this power cutoff worked fine with ROM becuase it has open-collector outputs that in turn drive PNP transistors (that in turn drive the relay coils). For that to function properly the transistors have pull-ups to 5V that shut them off when ROM is not driving the base low. GAL however is a modern part with push-pull CMOS outputs protected by diodes to VCC and GND. So those resistors would backfeed 5V to the GAL, via those diodes, providing it with enough power to actually function and drive some of the transistors when it shouldn’t. So I had to add a mod, a small NPN transistor that would isolate the GAL from GND when the power was cut. This has a minor downside of shifting the GAL ground up by some 0.2V in the on state, but that’s not an issue in this circuit. See if you can spot the tiny SMD part (note no external base or bias resistors, these are built into the transistor package):

This solution works very well and is almost a drop-in replacement, except for that small mod. It’s not a big deal though as it will also work properly with the original ROM in the socket – which was one of my goals (in case I’ve managed to revive the ROM somehow).

BTW, the reason I got that Fluke and not something else in the first place was this add-on card:

This is opto-isolated digital output board that sits over the main PCB, I have plans to try and build an interface between it and a Raspberry Pi, which would allow me to turn this experiment into a networked logger. A dumb one since this board only provides output and can’t affect the meter operating mode or range, but that is still pretty useful to me.

The Wretched Automatons

Phoebes are done so I will open orders for GDEMUs this Saturday (2021-02-13) at 12:00 CET. Then, in a week or two – depending on how fast I can ship those GDEMUs – I will open orders for DocBrowns. I have a small batch of those almost ready.

News are slow lately but I have some non-ODE related side projects that might interest some of you, though I’m trying to figure out if it’s even worth mentioning the ones that I’ve little to show for. But there is one that I consider “done”, even though I might just revisit it soon. But it served it’s main purpose – which was to teach me a lesson 🙂 More details after the weekend.

UPDATE: GDEMU orders are now open closed. Give me a day or two to process the orders and send out confirmation emails.

Somnolent, contd.

You will be able to order Rheas during this Saturday (2021-01-23). Let’s make it USA-friendly time since I haven’t done it in quite a while, so the orders will open at 20:00 CET.

Orders from UK are allowed – for now at least. This new VAT system is not cool though. It’s one thing if EU does it on a common platform for all its countries and it’s free and has the same rules in every country. If we have to pay monthly fees for legal access to UK market, and have to keep up with all the changes in the UK law and procedures from now on. then it’s not really worth it for small operation like ours is…

UPDATE: Rhea orders are now closed. Confirmation emails will be sent out in a day or two.

UPDATE 2: I will open Phoebe orders this Saturday (2020-01-30) at usual time (12:00 CET).

UPDATE 3: Phoebe orders are now closed. Confirmation emails will be sent out in a day or two.


I will take orders for GDEMU on Saturday 2021-01-02 at noon (12:00 CET) as usual. I expect this to be a short run and after that it’ll be either Rhea or Phoebe next.

I’m also considering advance orders for Wizards. Long story short – I will probably treat this ODE differently and prefer modders to buy a few pieces at once rather than do individual sales. This is because it takes some soldering skills to properly install Wizard and it’d be even better to have a proper crimping tool to do all the wires nicely, with connectors, rather than solder them directly to the PCBs. Plus the size of the Wizard makes it difficult to ship it as cheaply as I have done so far. Basically there’s lots of small issues completly unrelated to the ODE itself that are making this one more difficult. Packing for shipping alone is so time consuming…

Due to the whole brexit thing I will not accept orders from UK at this time. This is temporary restriction – I wish to avoid the initial hiccups and have my packages delayed, bounced or taxed at the border. Once all the new customs procedures are well in place I will be happy to resume shipping. So, probably in a month or two?

At the same time I remind everyone else that I will cancel orders that I can’t ship due to any unforseen COVID-19 restrictions on travel (which tend to make air shipping impossible). Hopefuly though there won’t be any more of these and year 2021 will be less crazy.

UPDATE: GDEMU orders are now open closed. Orders from UK are conditionally accepted and might be delayed until further notice. Please allow a day or two to build the list and send out confirmation emails.

Project Time! #3

What can one do with a piece of vintage tech like HP 6920B? I found a few uses for it already:

  • testing “magic eye” tubes that require some 250V for anode/screen voltage
  • testing capacitors for DC leakage up to 1kV
  • reforming electrolytic capacitors
  • testing gas-discharge voltage-regulator tubes
  • testing Geiger–Müller radiation detectors
  • testing electroluminescent backlight panels
  • testing transformers (ratios, self-heating and winding insulation)
  • and zapping stuff with 1kV AC/DC

I’m probably going to invent some other uses for it as well but originally I wanted it for some repair fun and one particular purpose – as a 1kV DC source to recalibrate my AimTTi 1908P multimeter:

First let me introduce the 1908P a bit. I got some other AimTTi gear as well so I expected their multimeter to be decent. Different people have different needs so here’s my personal pros and cons lists:


  • 5 and 1/2 digit modern digital multimeter with reasonable specs
  • it’s cheap, even taking into account Chinese offerings like Rigol and Siglent
  • has Ethernet port for TCP/IP connectivity (as well as serial and USB, and GPIB/IEEE-488 if you need it)
  • no fan, no noise, no holes in the case for the dust to get in
  • 2s boot-up, of which 1s is the display test
  • decent screen with good backlight
  • manufactured in Europe
  • built-in battery pack for off-line operation


  • it’s only 120k counts
  • not very fast, 4 readings per second at full resolution (less in dual-measurement modes)
  • no rolling averaging to reduce input noise
  • diode test goes only up to 1.2V due to non-selectable range even if the output voltage is about 3.5V
  • only 10M impedance on input on DCV ranges (less in AC mode obviously)
  • built-in battery pack for off-line operation

I’m really starting to dislike USB these days. I use it a lot and I’ve seen all kinds of issues with it – cable length problems, flaky hubs and connections in general, driver problems. In fact custom, broken, never updated drivers are the worst and can often interfere with other devices that worked perfectly before. So I always look for Ethernet ports on new equipment, I don’t trust hardware manufactures to come up with decent drivers or any at all if you aren’t on Windows or it’s not the version they support. Serial port is second best option but it’s slow and typically offers no HW error correction, so a single bit glitch in the data stream can easily bump a digit up or down in the reading and you’d never know it. GPIB/IEEE-488 is… old. And costly. And very proprietary. If you already use it a lot you won’t mind but newcomers are better off looking for something else.

The 1908P has Ethernet port and it’s a huge plus for me. And the price is still acceptable, unlike say Rigol offerings that give you cheaper USB-only models or the full-fat ones for 500 USD more. The 120k counts is more of “5 digits” than “5 and a half” in my book but I’m not going to argue with established shady marketing, it’s not just AimTTi doing this. I very much like the no-fan design and the LCD display is OK, not great but also not annoying me in any way. The battery pack is a bit of a double-edged sword – nice when you need it but keep in mind it will die at some point. Mine in fact did (was probably defective) and that most likely caused the brick I had when updating FW – AimTTi did good though and repaired this for me for free.

No averaging, 10 meg input and limited diode mode are not great but also not a dealbreaker. I might at some point need more than 4 reading/s at full 5-digit resolution but I don’t yet so that’s also not a problem. All in all I am happy with the price and performance – or rather, I was, until I discovered a glitch.

Long story short: When in 1000V DC range and below 100V (so only with manual ranging) there is a weird non-linear jump at exactly 99.00V that skips all the .01 to .05 values and goes to 99.06V (so from 99.00 to 99.06). I asked AimTTi about it and they suggested recalibration of the 1000V range. Turns out the calibration procedure requires a 100V input at one point, which is pretty much the only thing I’ve changed in the calibration data and it only made the “99V problem” worse. So clearly the jump/skip is some sort of FW issue that tries to improve the linearity of this range by having both 100V and 1000V references stored – but messes up the cross-over point. At least that’s how I see it. AimTTi has since ignored 3 of my emails – both about this issue and another one with their MX180TP PSU.

I’ve read many good things about AimTTi support but I have to say my personal experience with them is spotty at best. They will fix bricked equipment and issue FW updates for the serious bugs (not to brag but the Interface firmware version 1.07 for MX180TP exists because of me). The less obvious stuff though? Won’t even reply to the email.

And so, to conclude this – would I buy this multimeter again, knowing what I know now? Yes, it’s not like Rigol/Siglent or the Keysight/Keithley (which are often made by Rigol/Siglent in the lower price range anyway) have spotless record when it comes to FW bugs. And the price is still very good. But can I recommend AimTTI to others? No, I can’t. If you really need a good bench meter then you should probably look into the (considerably) more expensive K-brands. If on the other hand it’s for personal use then Chinese meters have more features, color LCD graphical screens and you probably won’t mind having just the USB available anyway. Unless you live in UK and can drive to AimTTi in person if need be.

Dewy Fields

Phoebe orders will be open this Saturday at noon CEST. New Zealand, Argentina and Vietnam are back on the shipping list.

UPDATE: And done. As usual give me a few days to process all the orders and then I’ll will send out confirmation emails.

UPDATE 2: I should be done with Phoebes this week. It’s taking longer than expected because PayPal is glitching and showing all paid requests as “Sent” even though I haven’t actually entered the tracking number yet. Long story short it’s a lot more work for me to sync PayPal with my own spreadsheets. In other news, I will be opening GDEMU orders this Saturday at noon (do keep the winter time change in mind!).

GDEMU orders are now open closed. Give me a day or two to process the orders and send out confirmation emails.

Project Time! #2

HP 6920B adventures part 2 – why recapping is not repairing. Lets start with “spot the differences” game:

Why did I recap this device even though I said I won’t? Two reasons: I noticed something odd on capacitor C6, and I had some issues with DC output wandering a bit too much and I suspected another degraded cap. Curiously C6 tested OK on all parameters (capacity, ESR and leakage) but it looks suspicious to me:

That almost seems like it started to vent at some point, possibly because it sits right next to two 7W power resistors that get over 100C hot.

Did the recap help? Nope, actually things got worse afterwards. But before I venture into that – a small digression. I’ve seen some photos of recapped PCBs, including one 6920B, that had capacitors looking like that replaced with modern electrolytics:

Please don’t do that. These are not aluminum electrolytics, these are high stability hermetically sealed tantalum capacitors with temperature range up to 125C. Military and aerospace use those, you really can’t find anything much better – look up KEMET T140 series. Doesn’t matter if old or not, these don’t really age. Do not change them unless you suspect an issue, and even then you should replace them with good quality tantalums as well – that’s what I did. In fact none of the ones I’ve replaced measured out of spec and these are ’76 caps.

So what was the issue? Well, I noticed the DC output tends to wander up/down and it’s not just down to ripple. After the recap, once the device got properly warm (after 2-3 hours) it got so bad the voltage started to go up well outside the spec and on 1V range I was unable to even turn the output on, without significant load being present, and not hit overvoltage condition right away.

Look at the current sensing shunts in the inverter when output is switched on:

See the ringing? There’s clearly some sort of resonance around 500Hz, and then also continuous oscillations at about 3.3kHz. But when cold these tend to die down on their own – this is output-on steady state:

Watch what happens when temperature starts to rise:

Yup, it gets worse, but still just manages to die down. And when you reach high enough temperature inside the device:

On 1V range these oscillations never die down and pretty much take over as the primary operating mode – over 1A of load is needed for the device to have any chance of regulating the output.

After some more head scratching and poking around I concluded the issue must be with the feedback loop and voltage regulation in SERIES REGULATOR module (see schematic in previous post). HP eventually replaced Q10 transistor (still germanium on my 6920B) with a different one and recommended adding a ferrite bead to emitter lead. Unfortunately they only gave HP replacement part number and I wasn’t very keen on replacing Q10 anyway.

Eventually I came up with another mod, a bypass capacitor of exactly 4.7uF (for best result, 1uF or 10uF give worse performance) for VR2 keeps collector voltage on Q11 clean enough to prevent Q10 from oscillating on it’s own. And boy, does that make a difference for both start-up and steady state inverter currents:

No more DC output wandering, hot or not. No need for output load to keep things stable. 1V range starts and stays clean every time. I’m pretty proud of that one. So here’s both mods I’ve made to my 6920B, first one prevents DC stage in AC REFERENCE from oscillating, the second cures the ringing in INVERTER when switching phases:

Ignore the weird spots, must be the side effect of spraying 3M cleaner nearby. You can’t even see that, it’s only visible using the camera flash. You’ll notice I reused the tantalum capacitor I removed from the PCB, after all there’s nothing wrong with it. Why is it needed at all here? Well, HP did add another cap nearby too, in change #19, but that didn’t work for me. Was this the issue they were trying to fix, or was it something else? Is this inverter ringing something common in 6920B devices – perhaps so, I’ve seen (current) owners mention this DC instability more than once. Was it there from the start or does it happen with age, which is also likely considering all the germanium transistor used and these like to get noisy. Who knows, it’s fixed now, hopefully for good.

Also, the output indicator bulb got replaced and I’ve also changed the power cord socket, it was broken on the top and then also cracked on the side. That actually wasn’t easy at all, the original socket is riveted to the case.

And one last photo:

This is 1V DC output as measured by HP 3465A – which I might describe a bit more in another post someday. The 3 at the end is well within spec (+/- 10) and I could dial it to zero but this way I get slightly lower offsets in 10V and 100V ranges, so it’s a good spot.

Project Time! #1

I’m going to start this with my latest toy – HP 6920B Meter Calibrator. I got it because I need a 1kV DC voltage source. What for? Well I actually need to calibrate a meter but that’s something I will explain at a later time. Suffice to say this device is not enough on it’s own.

A good few years ago I tried building my own 2kV DC PSU for an oscilloscope CRT. That didn’t go very well though I’ve learned a lot. Biggest issue was backfeed to the control circuit due to parasitic gate capacitance of the switching MOSFET – I was driving a lot of current at 12V to get 1kV which was then doubled. These days I’d probably try less step-up at the transformer level and more stages of the multiplier, possibly also NPN instead of N-MOSFET or maybe a driver chip in between PWM controller and the power transistor. Point here is, making these voltages is not easy (or safe) so I decided to just look for something to buy.

There were a few candidates but then I found the 6920B on ebay for about 200EUR shipped. It can do much more than just output 1kV DC – and it’s a bit of a restoration project too so I was happy to pay that price. And it works, although it was pretty filthy inside – but being manufactured in 1976 and having vent holes on all sides will do that. Here’s a photo from the inside before I started cleaning it:

A not-so-short list of what was wrong with it:

  • pretty dirty both inside and outside
  • all the protective loom tubes are sticky
  • the main 10-turn potentiometer gets stuck
  • pilot light (neon bulb) lost it front panel cover
  • output indicator light bulb burned out
  • output switch contacts oxidated
  • selector switches contacts dirty or oxidated

I really didn’t want to do that but in the end I just showered the guts with hot water. Took several days (and that’s outside in the sun) to dry, mostly because the water that got into the wire loom tubes couldn’t evaporate easily. I did desolder and remove the potentiometer before applying water. It’s usually a poor idea to let the transformers get soaked in water but a) there was no other choice unless I desoldered each an every wire to both PCBs and b) the cores seem to be coated with some protective lacquer. The windings will dry out eventually – you just need to be patient.

The wire loom tubes being all sticky (and dirty) – that’s new to me. Could be cigarette smoke residue, though usually the PCBs would be affected too. It could also be this particular polymer is slowly breaking down over time and releasing this goo on it’s own. Water didn’t do much to it, I had to just go over each piece of tubing I could reach with a cloth and rub it clean. Again I really didn’t want to desolder everything after I noticed (removing the 10-turn pot) the wire strands are prone to breaking right next to solder points and there isn’t much slack to just strip more of the wire.

Potentiometer is pretty much sealed but there is a screw that can be removed and the hole is just big enough to inject some fluids into it. So in went 100% IPA and some 3M contact cleaner (it’s a wire pot), that got it moving and improved the wiper contact. But after a few days of drying and playing with it by hand I noticed some signs of the wiper getting stuck again so I injected a few drops of pot cleaner/lubricant. That’s some good stuff and it wors nicely now, no more issues.

Nothing can be done about the pilot light right now, but at least it works. I will have to come up with some replacement cover – the whole light assembly is one part and with the front broken off (that’s not typical) the neon bulb is exposed and not held by anything in place.

Output indicator is also a part that is supposed to be replaced as a whole, and trying to get it out of the front panel and apart left me with several plastic pieces that were meant to be one. The good news is, after some creative use of glue and drill bits, I now have the orange cover firmly back in the front panel and the internal bits await a new bulb that should just slide into the front cover once I solder the wires. The locking tabs are gone now but the wires are rigid enough to prevent the bulb from sliding out. Finding a small enough 2.7V bulb is a problem but I got a reasonably good replacement candidate already and a few more options on their way to me – I just need to pick the best one.

Rotary selectors were treated with 3M cleaner, work perfectly now. Then I’ve made the mistake of spraying some of that into the output switch and got a nasty brown mess of old, dirty grease oozing out – it had to be desoldered, opened up and properly cleaned. It can be done and gives a nice access to the contact points – those are silver and were already completly black. I washed the whole thing in 100% IPA and used silicone grease when putting it back together. Seems to work just fine.

So here’s how it looks now:

Does it work? Sure does, I got it re-calibrated following the manual and the DC output is pretty much perfect. So was AC for a time, but after about 6 hours of use something went bad – and now AC is too high. So much in fact that it cannot be compensated by recalibration. Many hours of looking at the schematic and head-scratching followed. This part is bit more technical and you might want to download the manuals for full schematics and some brief explanations of how this device works (it was designed around 1965, back then manuals came with schematics and even theory of operation). The block diagram:

After poking around a bit I concluded my issue is with AC reference, which actually is explained a bit in the manual, here’s a simplified version of it:

It regulates the AC voltage coming from transformer down to 1Vrms using a resistor divider, and one part of that divider is a photo-resistor coupled to a light bulb. That bulb is driven by an amplifier that senses the 1V value via a feedback. There’s actually two parts to that amp – first an AC stage that converts the AC do DC and then the second one compares the resulting DC with a known voltage from DC reference circuit, then drives the photo-resistor. Well that’s all nice on paper but the actual implementation is a tad bit more complicated and here’s the actual schematic of that part. I put come colors on it to make it easier to explain though I’m not sure if it helps or not. Well, lets get to it.

First I checked the ripple on the light blue line – it’s not insignificant but the whole device is a bit of a mess when it comes to ground reference – called -S on the schematic. And BTW my eyes are not that good anymore, at first I thought it says -5 and this confused me a lot. You’ll notice the AC REFERENCE CKT. is referenced to ground but powered by 3 different sources:

  • +12.6V from DC REFERENCE for the low power stages
  • +6.2V as the reference point through R69 calibration pot
  • some unspecified voltage from CR24/CR25 diodes (about 50V actually)

It’s that CR24/CR25 line that has ripple, but that PSU branch is not tied to ground. Instead it’s connected in various ways to other parts of the device. Ripple is most likely caused by the CR3 thyristor that discharges C17 – indeed it’s C17 that has highest ripple on it, and it’s sawtooth-shaped. So while all this is kinda suspicious, I think it’s actually meant to be like that.

Well then, there are 2 stages here, just 6 transistors, how difficult can that be to analyze? Voltage from sensing point is routed to base of Q19 which provides main amplification. It’s 100k collector resistor is too much impedance so there is Q20 which is just a follower. C19 is just to prevent high frequency noise from being passed through and it’s not shorted. Might be open but as far as I can tell isn’t, and that would not cause the problems I have. Then there is Q21 which drives the CR30/CR31 rectifier via C20.
Q21 gets pretty hot due to high collector voltage (about 30V) but the way it shares it’s emitter current with Q19 should offset any temperature-related changes in operating point. There is something interesting here, the diodes are not in parallel but there is the R66 resistor in there too. Which means the rectifier is not balanced, it has easier time pulling the output lower (via CR31) than pushing it higher (via CR30 and R66). Output from this stage is at R67.

The second stage is a differential amplifier made out of Q22 and Q23, driving the bulb-regulating Q24. Lightbulb itself is powered by 15V regulated by VR7 – and it’s also this regulated voltage that provides current to CR34. Voltage drop of CR34, about 800mV, is feeding the collector of Q22. Any temperature rise in this diode will lower it’s forward voltage, which in turn is going to limit the collector voltage of Q22 and in turn help compensate it’s temperature-related Vbe drop. Then there is the negative feedback path of C21 and R74 – this serves 2 purposes, to reduce amplification of any AC still present on the input and provide a bit of a low-pass filter to slow down any rapid changes in the output. That being said some AC does make it through to the collector of Q24 but the lightbulb has a decent “inertia” of its own not to care too much.
One thing I’ve noticed about this design is any wires connected to base of Q22, even through C21 and R74, will act as antenna and send this amp into multi-megahertz oscillations. Perhaps it doesn’t do that on it’s own but I will mod this part by adding about 100nF capacitor between Q22 base and ground to prevent that. Or maybe its collector instead of ground if that works well – would be easier.

When you look at the Q22/Q23 pair you realize the base of Q23 is held at 0V via R72. Then it stands to reason that the pair would be in balance if base of Q22 is also at 0V. And sure enough, it pretty much is. This also explains why there is an imbalance in the first stage rectifier – notice the DC reference is +6.2V via a 10k resistor. And first stage DC comes through 2k resistor. Then, to get 0V at the tie point the first stage must produce mostly negative voltage, at -6.2/10*2=-1,24V. Well, mostly, there is a helper feedback path here via CR32/CR33 that provides some negative voltage from the AC ouput of the reference.

And this is how this block keeps the balance. Except it doesn’t, but it still regulates, just imperfectly. So why? Well the “0V” at base of Q22 is actually -17mV or so (depends on temperature). If you calculate just how much (or should I say, how little) voltage regulation the R69 calibration pot provides, it’s pretty much at the limit already. Q22 is already biased to negative voltage on the base and yet the output of the whole circuit is too high. Shorting one of the CR32/CR33 diodes, to provide more negative drive from the output, helps but not a lot. Adding external negative DC bias does cancel out the error. But then it looks like this:

So what is wrong here? I’m not very good with analog stuff but after ruling out DC leakage via C21 I’m inclined to belive one of the Q22/Q23 transistors is faulty. Well, not precisely that, both still work but at least one of them has a widely different amplification than the other and the pair is now badly mismatched. This causes rather significant input offset which is outside the specs of the calibration. So I need to match two NPN transistors and replace these ones. Unless someone has a better idea? To sum this up:

  • it worked for few hours before AC went too high (about 3% high now)
  • AC is regulated but outside spec, R69 doesn’t have the range to bring it back
  • DC part of the instrument is still spot-on
  • seems temperature dependent (much more than it was when output was correct)

INB4 “replace all the caps” – those are still good and will stay until they fail.

UPDATE: Replacing Q22/Q23 pair with modern NPN parts matched to 300uV didn’t help, it actually made things worse! Also, I tested the original transistors (once removed from PCB) and those are matched to less than 2mV – so good enough. The fault must be in the AC stage then, but how could that be?

Then I realized I didn’t check C20 for DC leakage, just for capacity and ESR. And sure enough that bugger has well over 60uA of leakage at rated voltage, and never fully reforms. To make sure I wired a temporary 10uF/50V replacement and the AC reference is properly regulating 1V again. Turns out that current is excessive enough to cause voltage drop on R65 that throws DC stage off balance.

So, one cap has failed after all – but only in a very specific and limited way 🙂

Computer Week-end, contd.

Phoebes are a bit delayed but I will have some in 2-3 weeks. In the meantime I will open DocBrown orders this Saturday, as usual at noon CEST.

As a reminder, I still can’t ship to New Zealand, Malaysia and pretty much most of Africa and Middle East, except Israel. And since some countries might enter another lockdown due to 2nd COVID wave, this could change at any time.

I’ve been busy with some neglected personal projects that have nothing to do with ODEs or even computers. I don’t think those fit on this blog but there is no other content right now, and dire lack of photos lately, so I could maybe show some of those. I know some people who would be interested but at the same time I don’t want to confuse the general public. What do you think?

Computer Week-end

I was very busy this past month but that’s mostly done now so I can take orders again. On Saturday at 12:00 CEST I will have some Rheas for sale. This is again a smaller batch but the previous one went pretty well, with no drama, I hope this one will as well.

There’s probably going to be another batch of DocBrowns a week after that, I’ll keep you posted.

Please keep in mind I’m still unable to ship to quite a few places, most notably Japan, Australia, New Zealand, Malaysia, and pretty much all of Africa (including Reunion). I’m very sorry but I will not accept orders from these areas at this time.

UPDATE: DocBrowns ordering window will open on Saturday 2020-08-08 at 12:00 CEST. As usual actual Marty owners will be asked to provide serial numbers and served first.

UPDATE 2: Seems like Australia and Japan are back on the shipping list – for now anyway. I’ll try to ship all the backlogged stuff soon. Also, orders from both of these will now be accepted as well.

UPDATE 3: GDEMU ordering window will open on Saturday 2020-08-29 at 12:00 CEST.

UPDATE 4: I can now ship to Taiwan. There’s a few waiting orders – I’ll try to get those processed soon.