AT Computer ephemera

I scored a 486 in a full size AT case for $50 at VCF 2024, and it inspired me to go through all my AT x86 gear in the hopes of building up a system. I found the following motherboards in my collection

486 DX 33

This is the motherboard that came with the case. Not winning any speed races for sure. It seems to work OK. There’s some corrosion where the CMOS battery was (which was removed), and I’ve got some CR3032 battery holders on the way to solder in place. Not sure what it’s ultimate fate will be.

486 DX4 100

A PC Chips M912

This I got at a VCF swap-meet last year. It’s a VLB motherboard, and I got a VLB video card & IDE card to go with it. Unfortunately, it appears to be dead. I picked up a POST card and it just shows dashes on the display, and nothing else. Not sure if it’s a dead CPU or bad mobo; I also tried the DX33, but that was before I got the POST card. More investigation required.

Socket 7 w/ Pentium 120

First of all, I completely forgot I had this; I have no idea where it came from, but I found it in one of my retrocomputing boxes. Thanks to The Retro Web’s awesome website, I was able to determine this is a PC Chips M530. Mine says ‘V1.6A’, while the one on the site is ‘V1.6B’. The only visible difference being the presence of a CMOS battery holder, which mine lacks.

And this lack of a CMOS battery holder is proving to be a bit of a pickle. I spent a while going over the documentation and can’t see anywhere this would wire up to. There are a few pins lifted on the realtime clock, a VIA VT82885. It’s a standard DIP, and doesn’t have a battery in it. After looking at a close-up shot I took, I finally noticed a ‘+ ‘ and ‘- ‘ marked above two of the pins.

The pin marked negative is called out as not connected in the data-sheet, but sure enough, giving it 3.3V across these two terminals clears the CMOS battery error and seems to hold the BIOS info. Right now I’ve got it precariously connected to a bench supply; once the battery holders show up, I’ll probably just solder one across these pins. I would like to know what the original arrangement was though. The pins are not only bent out of the way, but are also broken, so I couldn’t reinstate the original config without getting a replacement chip (which a cursory search on eBay didn’t turn up).

There’s a jumper, J7 that’s called ‘External Battery Selector’, and it’s to be left open by default. If closed, I’m just not sure where the external batter would be attached. I suppose I could go probing around to see if any of the jumpers or unlabeled pads land at the Vbat pin on the RTC socket, but without a replacement chip, it’s a moot point.
Is the venerable Dallas RTC chip a drop-in replacement? The pinout of a DS1287 looks similar. I wonder if that’s what used to be in here what I’m seeing is a hack to work around a Dallas chip that had gone bad. For now I think the best course of action (and certainly the least expensive) is to bodge a battery back on to those two pins.

I was able to install DOS 6.22 onto a CF card using a CF to IDE adapter. The card was 8GB, which is recognized by the BIOS, but in DOS it only shows up as 2GB (I seem to remember this… ) and I believe it’s a FAT16 format. So a few questions:
1) How can I multi-boot and have access to the whole 8GB
2) Related, in the BIOS, there are 3 different ways I can set this drive:

  1. Normal – 15525 Cylinders & 16 Heads
  2. Large – 7762 Cylinders & 32 Heads
  3. LBA – 974 Cylinders & 255 Heads

All those options have 16 Sectors. Auto defaults to the same settings as Normal. ‘LBA’ Stands for ‘Large Block Addressing’, but that’s all I remember.

Back to the RTC battery – I got new battery holders, but managed to snap one of the pins trying to solder to it. I even managed to expose some copper with a grinder, but ended up breaking that too. So yeah.. Gonna have to get a new replacement.
According to this Vogons entry from someone who appears to have the same board, the Dallas DS12887A RTC is compatible enough with the VIA82887 / HT12888A. For $6, I can get a replacement from China. For $25, I can get a drop-in replacement board that has a replaceable battery.
This machine is turning into a money pit.

BIOS Update:

I was able to file away a little more of the chip to expose just enough copper to solder to. The key is gluing these copper pads on top of the chip so you can attach to the pins via fine wire and keep any strain from transferring to them from the larger wires.

It “works”!

Jumping around a bit here – here’s what I think the pinout is for the clock display. A collection of pinouts can be found here


Thanks to this page, I was able to get the network card working with little fuss

Floppy Drives:

I got a gotek as drive A: & the original 3 1/2″ drive as B:, but I’d like get the 5-1/4″ drive working as well. I found this multi-function card in one of my retrocomputing boxes, and I’d like to enable only the floppy port at first. I’ve found two references to similar cards that use this chip, but not the exact one.

One page suggests this pinout, except I’ve only got 9 pins instead of 11.

This entry in lists 12 jumpers, but in both cases, the floppy enable is the first one, so I’m going to assume that’s a pretty safe bet.

No idea whatsoever if this is going to interfere with the built-in controller.

Weeeeell this might be a dead-end. Even with all the jumpers in the disabled position, the machine hangs at the enter setup prompt.

Networking & Soundcard conflicts:

Both the network & soundcard wanted to be at IO 0x220. I could change the IRQ in the SBLASTER line in Autoexec.bat, but changing the IO would shoot an error. Using the network card’s utility, I was able to set it from PnP to Jumperless, and set the IO to 0x240.
With that resolved, I can get the machine to boot with both cards installed, and they both appear to work. They do, however, both share IRQ 5 at the moment. Not sure if that’s going to be a problem. I should also think about a battery hack for the NVRAM on the network card.


I had an Adaptec 2940U kicking around – after a bunch of trial and error, what ultimately worked was calling ASPI8DOS.SYS in Config.sys (following the driver instructions were fine, except for which of the ASPIxxxx files to use).

Sencore LC102

My downstairs neighbor Ben loaned me his Sencore LC102 so I could start troubleshooting my newly acquired Tektronix 576 Curve Tracer that I suspected had some bad caps (spoiler alert, it had many bad caps). I’ve typically slummed it with my Heathkit IT-28 to check PS caps, but that’s not really as useful for lower voltage caps, nor will it tell you anything about ESR.
I’ve tended to ignore Sencore equipment on the count of it’s membrane buttons and non-backlit LCD displays, but damn this LC102 is useful. Ben just so happened to have both a working and non-working unit, and let me hang on to both for a bit.

The bad unit will fire up, but won’t test a cap. It will also lock up if you hit the ‘lead zero open’ switch. The other telltale symptom that something is amiss is a distinctive whine coming from the unit’s power supply, specifically this transformer:

We thought at first the noise was coming from the SMPS responsible for generating the test voltages, but this one handles the +/- 5V rails. The rails measure OK, but this circuit is clearly working overtime to make that happen:

For reference, here’s the good unit:

Here’s the output of the PWM driver on the bad unit:

And here’s the PWM output on the good unit:

Note the frequency readout on my 7854 is being generated by my new-to-me 7D15 plug-in, sitting in the ‘B’ horizontal slot, getting the signal from the trigger pick-off. I know any basic scope can do that these days, but that was cutting edge back in the early 80s.

Also note that the Circuit description says this frequency should be around 27KHz, but this is closer to 16KHz.

So there’s one of two things going on:

  1. Something in the rest of the circuit that uses the +/-5V rails is taxing the power supply, causing it to work harder to maintain these voltages.
  2. Something is awry in the power supply circuit itself.

Given how hot the circuit gets, I don’t want to run the bad unit for too long. I think the best place to start is to measure DC resistances from ground to the voltage inputs on the main board.

Another thing I learned while making these measurements:

  1. tables in wordpress is wet, hot garbage (at least WPTableBuilder is)
  2. The chassis is 100k away from ground (?!) there’s actually 3 different grounds – chassis, circuit & input.



good unit

bad unit


P4 - 5




P4 - 4




P4 - 6




P4 - 7



So right away, that discrepancy on the -5V rail is a big red flag. Lets investigate. Where does the -5V rail show up? A lot of places:

  • It’s on the supply of every opamp
  • It’s a part of the offset null circuit on some opamps (a feature I’m not familiar with)
  • It feeds a pair of 4051 multiplexers
  • It feeds a MC14433 A/D converter
  • It feeds something called the “Ringer” circuit.

Here are all those locations in the schematic (click to get the PDF):

I’m going to rule out that ringer circuit for now since the first thing that the -5V rail hits is a 10k resistor, and I’d be real surprised if that was shorted.

Actually, scratch that – the next thing to do is look at the main board with a thermal camera.

Here’s the good one

and here’s the bad one

A clue! What’s getting really hot is IC27, an LM319 in the capacitance measuring portion. IC22, a TL084 opamp that’s a part of the current source circuit is also getting warmer than expected. The other half of that dual opamp is in the ESR measuring circuit.

I measured the output of IC22 into the LM317, and it’s -1.8V. On the bad unit, it was +3.3V. And here’s where I’m going to say a bunch of things that are probably wrong: With -1.8V on the regulator, it’s essentially shut off – I would expect this since it’s a higher range; and you wouldn’t have a higher range enabled during power up.
Something is causing the regulator to be on in the bad unit. Is it an erroneous signal from the microcontroller, a faulty opamp, something with it’s surrounding analog circuitry, or a problem with the LM317?
I expect pin 12, the input from the microcontroller, to be low. It is on the good unit, but on the bad unit, it’s at like 1V, which is odd, and wrong. I wonder if the next move is to yank the LM317.

Ok, yanked the LM317. With that out of the picture, The SMPS is no longer angry; it’s not buzzing, nor is it getting super hot. The LM319, however, is still getting getting very, hot, theres now +2V coming out of IC22 pin 14, and .4V on the input terminal pin 12. I can’t make that make sense to me.

I mean, fuck-it do I socket IC22 & IC27 next? I feel like they may not be the problems and are just collateral damage, but I’m not sure what to do next.

— The Next Day —

IC22 & IC27 have been socketed. With both removed, and the LM317 reinstalled, the power supply doesn’t whine anymore. With IC22, the TL084 quad opamp reinstalled, the whine returns. I threw in a TL074, and the whine stopped. I measured the resistance between the positive & negative terminals on both devices; on the TL084 I pulled out of the unit, I measured 340 ohms. On the new TL074, I measured 3 Meg, which is closer to what I’d expect. Reinstalling the LM319 Comparator made the whine come back. Measuring between the rails on that chip yields a similarly low value. I’m waiting on replacements. Fingers crossed this is it.

— Parts are in —

I replaced IC27, the LM319 Dual Comparator with a new part I got off eBay. Thermal camera looks good, but I’m still getting the same issue:

  • Open Test, with input open: I get one progress dash on the screen and the unit hangs.
  • Open Test, with input shorted: same thing.
  • Short Test, with input shorted: Error 4
  • Short Test, with input open: Test runs and says ‘Open’ (is this correct?)

Note that the chip labeling looked a little suspect at first, so I whipped up breadboard set-up just to check it out. The Power Designs triple output supply is new to me, and I’m a fan.

Testing some suspicious looking chips from eBay

Something is still fishy in this neighborhood though –

Here’s a few measurements from the good unit.

Pin 10, the input of the Comparator IC27
Pin 9 input of the comparator IC27 – the high limit

I’m going to post a few more pics of signals on that comparator, but I think it might be a red herring. The input signals on pin 5 of IC27 are driven by a pair of MOSFETS, which in turn are driven by two pins on the main processor. Those signals are different on the good unit vs the bad unit, so either there’s EPROM bit-rot (possible but unlikely) or much more plausible, the CPU isn’t getting the right signal from somewhere else to set those pins correctly.

Looking for a new lead, I’m turning again to the thermal camera.

Here’s the bad unit, on for about 45 minutes. IC23, another dual op-amp, looks unusually hot.

The bad one

Here’s the good one, on for maybe 25 minutes (yes yes, I know, it’s not the same amount of time, bla bla bla…). IC23 is warm but a good 20 degrees F cooler. The hottest of the lot though is IC3, the regulator that’s responsible for the 12V reference.

The good one

There’s already a 12V line from the power supply, so we’ll need to do some sleuthing in the schematic to see where this reference line goes. The fact that it’s dead cold in the bad unit could be another clue. I may also desolder IC23, though I think I’m fresh out of quad op-amps at the moment.

I checked resistance between the positive & negative supply inputs of that opamp in circuit (oddly +12 & -5) on both good & bad units, and they both read 3.3k, so I’m no longer suspecting a shorted IC.

Well, things are getting weirder. I double-checked the rails on both the good one and the bad one – they’re all as expected (but note that +12V means more like +13.8V). The 12V reference on the bad unit is bang on at 12.00V. The 12V reference on the good unit is only like 11.7V, so I wonder if even the good unit is harboring some gremlins. Also, another dead lead.

I’m not admitting outright defeat, but I’m going to retreat to other endeavors for now.

Analab Type 1120 Oscilloscope

I generally try to focus on Tektronix scopes, but this showed up locally and looked too interesting to pass up.

The orange screen piqued my interest, as I suspected it might have long persistence P12 phosphor. The longest setting on the timebase is 50 seconds (full screen, not per division) and there’s a setting to use an external capacitor for an even slower sweep.

I spent a lot of time just cleaning all the knobs and faceplate, but I think it was worth the effort.

And there it is, that nice yellow-orange trace.

The manual was available on eBay, so I purchased it and scanned it.

More on this later (maybe), but for now I just wanted to get this post up to share the manual & photos.

Tektronix 568 & 230

This has been on my to-do list for way too many years. I got the 568 as part of a lot the better part of 10 years ago. After that, I found a 230, then another. It took another little bit of time to cobble together a pair of working sampling plug-ins, and the special interconnect cable.
Armed with all the pieces, I just couldn’t for the life of me to get the 230 spring to life. With 2 units of cards, the best combination I could get was some cursors moving on one channel, but nothing else.

Starting from the top, I checked the rails first. All was within tolerance. Next we start with the Buffer Card, where all the incoming signals from the scope land. In order for the 230 to start making measurements, it needs the horizontal sweep (the ‘low speed’ sweep), the sweep gate, and the clock pulses (one per sample). It also needs the mV & time settings on the vertical & horizontal plug-ins, and of course, the signal to be measured, but it doesn’t need those to spring into life.

All signals coming into and out of the Buffer Card were as expected. I started poking around at the Clock & Synchronizer Cards, because those generate a whole host of timing signals & gates that actually trigger the measurements. I honestly starting losing my place, because there are a lot of signals that need to be in the right state for the MEASURE signal to fire. I got tied up looking at the !END signal (! meaning compliment, shown with a line over the signal name in this documentation), which was needed to to start, and it started seeming like an infinite loop of sorts.

Oh, and a word on logic levels. This predates TTL logic, and uses active low logic, where a True state is represented by a low level between 0 – 2V, and a False state is represented by a high level between 6 – 12V. The convention in the manual is ‘High’ and ‘Low’, where High is False and Low is True. so for example the MEASURE signal goes Low for a measurement, and the !END signal goes High to indicate the end of a measurement cycle.

To make things even more confusing (and forgive me if I explain this incorrectly), negative logic changes the meaning of NAND and NOR gates, and they’re drawn differently in the schematics. See the NAND gate below. Electrically (shown in the second column), it behaves as a NOR gate, but in negative logic, it behaves as a NAND.

All this to say, there are a few things that make this a difficult machine to troubleshoot. So I cheated. I bought a 3rd, working unit. It arrived exceptionally well packed (so very grateful for that), with a nicely made interconnect cable (I already had a factory one), and it worked as advertised. Finally, I was able to see this beauty in action!

But I said I was going to fix a 230, so I set about starting to swap cards, this time, with a known good unit.

Swapping the Clock & Synchronizer Cards made no change. This supported my suspicion that perhaps they weren’t getting the right signal from the memory circuit. Swapped the memory, no change. The Start / Stop comparators send signals back to the clock, so I tried those next.

It’s always the last thing you try. With the B channel Memory & Zone Generator, I swapped the A Zone Generator with the known good card, and the unit spring to life. Through mechanisms I’m still trying to understand, if you have a bad Zone Generator Card installed with a good memory card, it can lock up the unit into the state I’d been seeing it in. And as luck would have it, I had three bad zone generator cards.

The Zone Generator Card has (2) logic ICs, and 36 transistors, a mix of NPN & PNP, all individually socketed.

The sockets are a blessing and a curse; It’s easy to swap transistors, but the sockets themselves also become a point of failure.

Sometimes, you want to sit down and dig into a schematic, come up with a hypothesis on why it’s not working, and start probing around to prove or disprove your theory. Other times you just want to brute force a problem, because it’s 11:00 PM, and manual labor is sometimes easier than critical thinking. So I set out to test every transistor in one of the bad cards to see if I could coax it back into the living.

I believe these to be unremarkable transistors, since the application is mostly simple low speed switching. In my model there was a mix of Tek model #s and manufacturer #s:
NPN: 2N3904 / 151-0190
PNP: 2N3906 / 151-0188
I decided to test them on my 575 Curve Tracer at their operating voltage, fearing that perhaps the chinesium battery powered one might miss a fault, since these are operated across 50V rails.
I found 2 culprits:

And with those replaced, the unit sprung to life!

So now we have 2 Zone Generator cards that at least let the unit run measurements, but both have problems. The Zone Generator card generates the areas on the displayed waveform where measurements are made, and show those at highlighted portions on the trace. A zone can be .5cm, 2cm, 4cm or 10cm long (or 12cm, but I’m not getting into that now). When the zone is .5cm, the average of that zone is returned; when it’s longer, the peak is returned.

The first card had an issue where the 0% zone would jump around, and not get to all the locations. We can see in the schematic, there’s a binary coded input coming from the zone position control. Because of the way it was jumping around, I suspected either the 2 or 4 bit, corresponding to Q864 or Q884. Q864 was bad, and replacing it gave me a fully functioning card.

The second card had an issue where the entirety of the 100% zone would be highlighted, so I could only see a movable cursor when only the 0% zone was selected.

This trace shows 2 good ZONE signals, and the incoming SWEEP GATE on the bottom, which I’m triggering from (thank god for 4 channel scopes). The negative pulses represent the position and width of the two zones.

The trace below shows a bad ZONE signal.

I’m seeing this signal coming from M938, but also going into it. Moving upstream, I check Q933. That’s OK. I pull and re-seat M938, and that seems to fix the issue. Now I have a cursor, but it has the same skip issue as the other card. The 100% zone transistors in question are Q964 & Q984. Both of these are good, but re-seating them fixes the issue. Like I said, those sockets are a blessing and a curse.

So now I have 2 more working Zone Generator cards, and a 2nd mostly functioning 230. It’s still wildly out of cal, and there are some quirks with the Nixie display, and cross channel time measurements, but I’ll get to those eventually.

Tektronix 111 Pulse Generator

This’ll be a quick one. The 111 is a pulse generator with ~ 500pS risetime. it has a pre-trigger output, which is adjustable from ~30nS to ~250nS. This makes it very easy to use an external trigger, and get the pulse centered on the screen.

Mine had 2 problems:
1) the pre-trigger delay wasn’t adjustable
2) the rate wasn’t adjustable, being fixed at ~200kHz or so.

Always check the rails first. The -15V line was down around -9V. I did a quick test of the Zener diode, which had to be done with a power supply, out of circuit.

The multivibrator portion of the circuit was adjusting, but the avalanche section that generates the sharp pulse* pre-trigger generator was oscillating in free-run, not taking it’s cue from that multivibrator.

Next I decided to check Q40, which turned out to be bad, showing up on my chinesium tester as 2 diodes.

I decided to just jam some basic bitch PNP transistor in there to see if it would work. To my surprise, it did!

* Edit: Q40 is not running in avalanche, that’s Q84. It would have been very lucky if some random transistor was capable of generating a sharp, predictable avalanche pulse.

Now I’ve got adjustable pre-triggering at 500pS, which has been useful for testing out some of my sampling plugins.

HP 8016A Word Generator

This is a continuation of an effort started a few years back. I had purchased one of these off eBay, but found that only every other bit would get stored. The troubleshooting steps in the manual indicated that this meant either a bad A3 buffer or A4 controller card. Although the schematics are included, there are no board-level troubleshooting instructions, or detailed theory of operation. Furthermore, HP says to not even return these boards for repair, simply discard them and they will send replacements. I because they’re made up of inexpensive 74LSxx series chips, even back in the late 70’s when the device was built, it would not be economically viable to have a technician spend their time diagnosing and repairing these boards. Yet that’s what I’m going to attempt.

When I first dug into this, I soldered dozens of little jumpers of magnet wire to various pins so I could probe the inner workings while the card was inserted. This got me nowhere, and so I shelved the project.

A year or two later, I saw another one come up on eBay, and one it for a surprisingly reasonable sum. It was listed “as-is, for parts”, but I figured I’d take the risk. Although in worse physical shape, this unit actually was fully functional. So now armed with a set of working cards, I was able to troubleshoot the bad unit.

Swapping cards, I was able to confirm that A3 was the problem. Fortunately, this card is accessible in place if you remove the HPIB (GPIB) card, which will not hinder operation. This finally let me probe the device in action.

The A3 board is essentially the edit buffer. Bits are read in, displayed on the front panel, edited, and then placed back into memory. The read & write is done serially, so there’s a shift register made up of 7474 flip-flops, and 7451 And & Nor gates that allow for the bits to be modified. The circuit is split up into even & odd sections, each with their own clock, serial in & out lines, and a few other control signals. (note the odd symbol for a NOR gate in the schematic below).

I first suspected the the flip-flops, so I unsoldered all of them, and installed sockets. I tested them in a little chinesium chip-tester, and one of them was bad, but the problem remained. I turned my attention to the and / nor gates, which unfortunately my little tester didn’t support. I wired up my 16500 Logic Analyzer to the output of each of those gates, and found that U32 was glitching. After replacing that I had what seemed like two working units.

Maybe this happened during repair, or maybe it was an issue that I didn’t catch at first, but now on the second unit (the one I thought was perfect), The 16th bit now mirrors the 2nd bit, on all words. This problem seems to follow card A4, which is not accessible when installed. It’s also a more complex circuit.

A4 card, partial schematic

I flipped the unit on it’s side, so I could start probing the edge connectors. On the analyzer, I’ll designate the last pin on the pod as PROBE, so I can move it around looking for interesting signals. When I find one, I’ll add a grabber to the next free line on the pod, and add it on the trace. I have it in continuous capture mode, to trigger on the clock signal (which only runs during a load or fetch). I turned my attention to J1-8 & J1-5, which are odd & even lines out to the RAM card. I expected them to look similar, but they don’t.

The odd line, which is working, seems to be in phase with it’s clock (this is showing no bits active), while the even one looks different.

I expected to see a similar pattern as the odd line on both lines of the good unit. I was wrong.

It looks totally different. Not shown here, but when there are bits selected on the bad unit, I’m seeing short glitch pulses on both the odd & even lines. Those are not evident on the working unit.

So without being able to dig deeper into the circuit while it’s working, I’m at a bit of a loss. For the time being, I’m going to be content with one working unit and one parts unit.

I decided the next step would be to build an extender card so I can freely probe both good & bad units while in operation. The edge connectors are 3.96mm pitch (?!), and I found 4 on eBay at a reasonable sum. They’re right angle connectors, but I’ll make ’em work. The bigger annoyance is there are jumper cables between cards along the top, so I both need to make an extender for those to connect to card A4, and I need to accommodate cables that need to pass across A4, without having to extend a bunch of other jumpers. I think I’ll design this card with a large enough hole in it. It’ll be mildly annoying, and I think a job left to Future Paul (screw that guy…).

Tektronix 564: Continued weirdness – SOLVED

2 years ago, I attempted to troubleshoot and repair an intensity modulation issue on my 564, but ended up giving up after a few unsuccessful attempts. I had replaced a few power supply caps, and all voltages & ripple were within spec. I thought maybe it could be an issue with the CRT? Last month, a spare CRT popped up 30 miles away for $40, so I grabbed it. I made the swap, and the issue remained.

I have since changed out the remainder of the electrolytic caps in all ‘low voltage’ supplies (all but the -12.2V – I don’t have a good way to mount new caps, and the ripple is really non-existent). I swapped the vertical and horizontal plugins, and never saw the issue, but I think this is a red herring, as when I put just the vertical plugin in the horizontal slot and feed it a 60Hz ramp, I do see the issue. I did this 2 years ago, and confirmed it today. The other thing I did to eliminate my suspicion of the horizontal plugin was to pull the transistor that drives the intensifying signal, Q294. When I pulled it, the issue diminished a little bit, but was still present.

Other things that didn’t work:

Disconnecting the intensifying pulse input – had to leave R838 & R839 connected, otherwise the beam intensity was unstable.

Maybe there was some heater to cathode leakage in the HV regulator tube? I swapped it out with a 12AU7 (similar pin out, close enough performance), the same issue occurred.

The ripple of on the primary of the HV transformer seems negligible, and in line with what I’d expect on a simple unregulated supply. Allowable ripple isn’t listed on the manual like it is for the other rails.

May be on to something: I’m seeing at least 5V of ripple on the grid of V800.

I thought the ripple on the 300V supply was in spec, but I just noticed a filter cap I missed on the other side of the regulator, that’s tucked away in the back of the scope. No change after swapping out that cap, but I think we’re getting closer. When I see problem, I’m also seeing ripple measurements jumping around. When the trace is solid, the ripple is rock steady at less than 5mV.

Things to suspect next:

  • V674, the 6AU6 error amplifier
  • V677, the 6AS7 series pass tube
    Damn, this was looking like the culprit at first, but the issue returned not after not too long. Swapped the old tube back in and issue eventually returned.
  • C670 blocking cap – worth a try?

Worth noting that we can still see the ripple on the 300V rail even if we pull the plug-in.

Turning our attention back to the HV section, looking at the input of the HV regulator error amp, before R816. It’s bang-on at -106V. The problem has been intermittent, and hasn’t happened for a few minutes since I gave that area of the HV supply a blast with cold air (compressed air upside down). Yep, OK, it’s back and the ripple is evident here as well – still not clear if we’re getting closer to the cause, or are just seeing the effect on the +300V rail. OK, this kind of sucks, but I’m getting to the point in the program where we just start swapping out caps. Also waiting on a 6CZ5 to swap out V800

  • C841 – .047u – no change
  • C849 – .001u – no change
  • C801 – 0.2u ? just a disc cap
  • C807 – .001u – says it needs to be 600V, but there’s only ~500V across it. Gonna risk a 600V for now.

SOLVED. It was V634. The 6DJ8 in the -150V supply. The supply that has NOTHING TO DO with any of the HV Circuits, and which exhibited NO RIPPLE. F*&k, this was a hard-earned win.

Side note – I nuked Q294 while I was troubleshooting – it runs the intensifying pulse, and I was yanking it and stuffing it back in live. I put in some basic bitch NPN transistor and it works fine.

Tektronix Analog Sampling

I’ve been putting this off for a while, but after a series of easy wins, I thought maybe I was up for the challenge. I have had in my possession for a number of years, a decent collection of analog sampling plugins and accessories from Tektronix, including 2 digital readout units, which allow for automated voltage and time measurements. Getting those readout units working is the goal, but first I have to start with a reliable sampling system.

I’m doing all this on a tektronix 568 mainframe, which has all the connectivity for the readout and automation systems, but these plugin pairs would work just as well on a 561 or 564 if you just wanted basic sampling functionality.

First I’d like to catalog the state of all of these plugins. I think the majority of them don’t work, so I want to find the units that work the most, and start from there. If I have a working pair, it will help evaluate the other plugins, as sometimes it’s hard to tell if the fault lies in the timing unit or the sampling unit.

The plugins

3S76 – Dual Sampling #2443
This one seems to work on the A channel. I have had it successfully send a trigger to the timing unit, and was able to position a signal on screen. The B channel is not working. After some time, I do get a sweep on screen for it, but I can’t position it. Getting the B channel working might be a good step in understanding how repair other sampling units. More on the repair below.

3T77 – Sweep #2810
This seems to work. Needs trigger sensitivity up pretty high, and it’s not what I’d call rock solid, but so far it seems like the best timing unit of the lot.

3T6 – Programmable Sweep #110543
This also seems to work well. External trigger provided on BNC J123 on the rear of the 568, via the gremar connector on the plugin

3T77 – Sweep #2250
Doesn’t work.

3T77 – Sweep #0881
Doesn’t work.

3T77 – Sweep #2246
Doesn’t work.

3S5 – Programmable Dual Sampling #40199
This seems to work on the A channel with the 1 known good S2 sampling head I have. The B channel is not working
Side note: the channels are lettered on these early sampling scopes, and numbered on traditional scopes. No idea why, but it holds true on the 661, and all of the 560 series plugins. On the 7S14, it’s back to numbers.

3S5 – Programmable Dual Sampling #30151
A channel works, with internal triggering. The B channel is not working.

3T2 – Random Sweep #30320
Doesn’t work.

3S76 – Dual Sampling #1209
Doesn’t work. With normal/invert switches set in the middle (a troubleshooting step in the manual). I get a sweep that can be moved vertically on Channel A, but not on Channel B, suggesting there are a few things wrong with this unit.
The sample diodes seemed to pass a multi-meter diode test, for what that’s worth.

Update: I stole a Nuvistor V1073 & transistor Q2163 from this to repair #2443

3S2 – Dual Sampling #40267
Both channels work, although I wasn’t able to get internal triggering working. Unfortunately I only seem to have one working sampling head at the moment.

3S76 troubleshooting

Set up with plug-in extender
Waveform at test point 4
Waveform at test point 6
Measuring bridge voltage on channel A – all good.
Measuring bridge voltage on B channel – no good
Sampling diode bridge

Everex 386is

Aaaand, I let another year go by without posting. I suck.
Next on the bench – an old 386 computer. I’ve been in the mood to poke at one of these for a bit; last time I had one was squarely in the 90’s and I was running linux on it (probably slackware, maybe redhat).

It boots successfully into BIOS, and has a great front panel character display. I’m going to disassemble it to get part #s off all the components.

Motherboard: EV-1806 Rev G
Turns out this is a 286 motherboard, and what makes this a 386 is the Everex InSTEP daughtercard.

The daughtercard plugs into the the 80286 & 80287 sockets. It has a 80386SX-16 soldered on, and a PLCC socket for an 80387 (which is annoying, since most I’m finding on eBay are PGA).

There is a 5 1/4″ and 3 1/2″ drive, a Teac FD-55GFR and FD-235HF respectively. A cursory search seems to indicate that the 3 1/2″ drive is High Density, so I’m going to make a plain old 1.44Mb DOS bootdisk.

The Drive Controller is An Everex EV-346M. It supports (2) floppy drives from 360kB to 1.44MB, and 2 MFM hard drives. It says only ST506/ST412, but I’m wondering if that’s really the case. I’ve got a pair of ST 225 drives that I might want to try out. Worst case, I’ve got a few other controllers. I was surprised to find an MFM controller; I thought IDE drives were commonplace by the mid 90s.

The Video Card is an Everex E3EEV-628. Not really sure there’s anything to say about it?

I made a DOS 6.22 boot disk, but the way it’s set up, the 5 1/4″ was the A drive. Just flipping the cables so the 3 1/2″ didn’t work -the disk light came on, but still got the classic ‘non system disk or disk error’.

So lets look into the dip switch settings on the 3 1/2″ Ah – there’s a DS0 & DS1, and DS1 was jumpered, so lets switch to DS0. No apparent change. I really need to brush up on floppy drive wiring, but I’m going to beat on this blind for a bit more. I don’t actually know if this drive is any good, or if I’ve even made a bootable disk.
Yeah, so I’m an idiot – I tried just copying the files over onto a floppy disk (I’m on a mac), but in doing so, you have no way of stipulating what ends up where; the BIOS starts executing from sector 0.
It was able to make a bootable disk in a Win10 VM, and it got me to an A: prompt, but a DIR failed. I’m not entirely surprised. It turns out making an old school DOS boot disk is a pain in the ass in 2022. I found bootable images at Winworldpc, and am using a trial of WinImage to write the image to a floppy. I was able to boot into the DOS 5.0 setup disk, but it wanted me to set up to a hard-drive. I also couldn’t exit out of setup and resume it, or even run a DIR command. I need to find a good bootable, single floppy OS.

There’s something funky going on – I grabbed a 6.22 boot image from here, but I’m still not able to get a proper boot – it tries to load a CD rom drive and fails, and after that it tells me it can’t load COMMAND.COM.

I also tried an earlier version that looks like it’s a single 720k disk. I was able to write it using DD directly in MacOS. Mac even recognizes it as a 720k disk, and sees it’s contents, but it won’t boot.

Fuckit, let’s try linux? Trying Fdlinux.
I’m not super crazy about seeing
dd: /dev/disk3: end of device
2881+0 records in
2880+1 records out
Seems the latest version expects you to do some tricks to format a slightly larger drive, but the previous version seems to fit on a standard disk.
I finally at least got a meaningful error from LILO (remember LILO?!):
Error 0x80. this is a disk timeout, suggesting an issue with either the drive or the media. So it’s time to try either another drive, or a damn gotek, I guess.
I appreciate floppies from a nostalgic perspective, but I’m remembering just how much they suuuucked.

3/3 Update:

Yeah, the floppy drive was bad. Goteks (with flash floppy) are great for permanent install, but I’ve got a Lothartek HxC SD that’s great for troubleshooting. I got this with an Atari ST a few years back, and it’s an old model; the only thing to watch out for is that the SD card has to be formatted FAT16. This is easily accomplished on Mac OS with:
sudo newfs_msdos -F 16 /dev/disk3

I was able to boot into DOS, and hooked up a Seagate ST-225 20MB MFM drive. Fdisk was unable to add a partition, however a low level format using Speedstor seemed to do the trick. It’s a great utility, and honestly I need to refresh my memory on what a low level format is.

4/2 Update:

Hard Drive
I was excited to find that I’ve got a working MFM drive, but decided to save it for a machine that might really need it. I had in my stash, a no-name IDE & Floppy drive controller, so I decided to see if I could get that to work with a CF card. Because the BIOS is so decrepit, I wasn’t able to get it to work. Enter XTIDE – an alternative BIOS that supports more modern drives. I was able to burn the 8k AT version onto an EPROM, and install it into a no-frills network card. Through a combination of dumb luck and wizardry, it actually freakin’ worked: After the built-in BIOS runs, XTIDE runs and lets the machine see the CF card. DOS was able to format it and install on to it. And since it’s just a FAT16 formatted volume, I can yank the CF card and put it in a reader to do bulk copies of software.

I replaced the bad 3 1/2″ drive with a Gotek running FlashFloppy. There’s a bunch of different flavors of “goteks” out there, and they have different methods of flashing the software. For this one, I needed an USB A to A cable, and had to run some utility in Windows. Works fine now.

Math Coprocessor
I installed a 387 Math Coprocessor. Linux now gets a little further into booting, but still hung. I’ll play around with this some more some day.

BIOS Battery
I got tired of having to go through setup every time, so I cobbled together a CR3032 battery mount for it.

Next Steps:

Network Card
Can I find drivers for it? It’s a Racal InterLan, and thanks to this page, I was able to figure out how to use an EPROM with it. It claims it to be an NI5210 card, and I found what looks to be drivers that match on, but I haven’t figured out how to install them.

A soundblaster AWE64 Value is what I’ve got – it’s not the best card for this setup, but it works OK. Installation went fine, you just need to install the S64Basic Driver, and make sure that you’ve already expanded the CTCMBSS folder, which contains the plug-n-play drivers that the installer will ask for.

Tektronix 564 Revisited

I recently picked up a somewhat rare 3A5 auto-ranging plugin for the 560 series, and decided to fire up my 564 to test it out. Since it had been over a year since I powered up my 564, I tried it first in it’s current config, a 3A72 Dual Trace plugin, and a 3B3 Timebase. Two problems were readily apparent:

  1. The varying intensity – I strongly suspect this has to do with AC ripple, since it holds steady when line triggered
  2. The trace is about 2 CM short.

Here’s a table showing measured supply rail voltages & ripple. All voltages are within spec, but the ripple on the -100 V, 125 V & 300 V lines are out of spec.

RailDC VoltageRipple
-100 V-100 V120 mV
125 V125.5 V40 mV
300 V301 V120 mV
-12.2 V-12.21 V2 mV
355 V357 V
420 V435 V1.5 V
475 V493 V280 mV
-3,300 V-3,290 V8 V


  • -100 V: C640, a combo 340uF & 10uF 200 V Electrolytic
  • +125 V: C642, a combo 340uF & 10uF 200 V Electrolytic
  • +300 V: C644, a combo 340uF & 10uF 200 V Electrolytic – this is only a 200V cap since it’s low side is off the 125V supply.
  • +475 V: C646 2x 40uF

So that’s a rebuild of 3 cans. C642 & C644 should take care of the ripple on the +420 V unregulated rail, which feeds the high voltage supply.

Using a fluke 6kV probe into my 7854 scope, I was able to measure about 50 V of high frequency ripple on the 3,300 V supply, however I’m not 100% certain that measurement was accurate. I could pick up about that much just by having the probe within a few inches of the HV supply, so I’m not sure how much of that was inductive pickup through the probe, or even the cable of the probe? I’d love someone to set me straight on that.

Following some recommendations on this old post on the tekscopes group, I gave the ceramic binding posts a thorough cleaning with IPA & cotton swabs. So far, I have not seen the intensity modulation problem return that didn’t seem to eliminate the problem, as it returned the next day, albeit not immediately, but after 30 or so minutes of operation. I did discover that with the cover reattached over the HV transformer & diodes, I was no longer able to detect the HF ripple when measuring high voltage. The AC ripple still disappears as soon as it’s measured.

Replacing C342 & C344 greatly reduced the ripple in the 125 V & 300 V supplies, but did nothing for the ripple on the 420 V unregulated, and hasn’t fixed this intensity modulation issue (I was pretty sure it wouldn’t though). I have confirmed that the intensity modulation is at 60Hz, and isn’t related to any errant signals from the horizontal plug-in by running the vertical plug-in in the horizontal plug-in’s slot, and feeding it a 60Hz ramp. The issue reoccurred.

As for the sweep length on the 3B3 – For now I was able to get the trace to the correct length using the sweep length adjustments pots, however, I had to crank them both (the normal and delayed sweep) fairly far clockwise. Once adjusted to the correct length, I do get sweep waveforms as documented in the manual, so I don’t suspect that I’m compensating for a problem further downstream.

Beam Intensity

There are, I believe 3 distinct different methods to modify the beam’s intensity:

  1. J21-14: Intensifying Pulse – changing the intensity of the beam by altering the ground reference of the control grid supply
  2. J21-13: Unblanking Pulse – deflection blanking
  3. J11-24: Chopped Blanking Pulse –

Continued here, 2 years later…