VAXmate Flyback Transformer

Recently I fixed the power supply of my VAXmate, but found that there was a problem on the monitor board, which may be what caused the power supply to fail in the first place. My suspicion immediately fell on the flyback transformer, which is a common failure in vintage screen displays.

I removed it and tried doing a ring test on it to see if there are any shorted coils. Here is a picture of it, with the pins numbered, as close as I can tell, according to the description in the Figure 14-4 of the Technical Description.

Flyback Transformer

VAXmate flyback transformer from the monitor board with pins numbered

My probing with a multimeter suggests that pins 1-2-3-7 form one winding, and pins 5-6-8 form a second winding. This does, however, correspond to the pin numbers shown in the Technical Description.

My ring tests showed good ringing between the following pin pairs:

  • 1-2
  • 1-3
  • 2-7
  • 3-7
  • 5-6
  • 5-8

Where the ringing looks like this:

Flyback Good Ringing

Good ringing from the flyback transformer

I also saw some poor ringing between the following pin pairs:

  • 2-3
  • 1-7
  • 6-8

Where the ringing looks like this:

Flyback Bad Ringing

Poor ringing from the flyback transformer

I can’t be completely sure that the poor ringing means the transformer has failed because I don’t know how the transformer has been wound, and it could be a low number of turns in the particular part of the winding.

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MicroVAX 3100 Model 95 Power Supply

I have a MicroVAX 3100 Model 95 that I acquired some years ago. At 32 VUPs it is one of the faster MicroVAXen.

MicroVAX 3100 Model 95

From the front with RX33 and TZ30

MicroVAX 3100 Model 95

Showing 32MB of installed memory

Some time ago I stopped using it because memory modules would suddenly stop working. At the time, I checked the power supply and some of the voltages seemed to be a little out of tolerance, in particular the output marked as 5.1V seemed to be producing 5.3V. I put the machine to one side because at the time I had little knowledge of repairing power supplies.

Recently I decided to take another look at the power supply with a view to getting the machine up and running again, given it is such a comparatively fast machine. I checked the ripple on the outputs but what I saw were spikes. The spikes seemed to reduce in amplitude after a few seconds. It turns out that I was measuring the ripple with poor technique, but I don’t know what the original ripple was now, because I replaced the majority of the electrolytic capacitors before realising my error.

I decided to replace any vaguely suspect electrolytic capacitors, which included the two big smoothing capacitors as one had a high ESR and the other appeared to be bulging slightly. I discovered that one of these would not fully discharge, which I found when I tried to measure its ESR after having had it powered on. Thankfully the meter had some protective diodes, which now need replacing.

After replacing the suspect capacitors I excitedly put it all back together, only to find that it would not power on. It turned out, that following some other sparks I managed to get from the not fully discharged smoothing capacitor I had ended up frying the UC3842N pulse width modulator. Fortunately I had a spare on hand (UC3842AN), replacing it fixed the power supply, and also seemed to resolve the problem of the capacitor not discharging.

Sadly, I also managed to break the connection to the power LED out to the front of the PSU, so now I won’t have that working.

After resolving my measurement mistakes the ripple on the 5V and 12V outputs was about 20mV and the voltages seemed to be right. So I put the machine back together again.

Unfortunately it seems that one of the memory modules is not quite right because the firmware reports an error:

KA51-A V2.6, VMB 2.1
Performing normal system tests.

? Test_Subtest_40_06 Loop_Subtest=00 Err_Type=FF   DE_Memory_count_pages.lis


16 MB RAM, SIMM Set (0A,0B,0C,0D) present
Memory Set 0: 00000000 to 00FFFFFF, 16MB, 32768 good pages, 0 bad pages
Error: SIMM Set 1 (1E,1F,1G,1H)
 SIMM_1E = 16MB ??   SIMM_1F = 16MB     SIMM_1G = 16MB     SIMM_1H = 16MB
Memory Set 1: 01000000 to 01FFFFFF, 16MB, 0 good pages, 32768 bad pages

Total of 32MB, 32768 good pages, 32768 bad pages, 112 reserved pages

Tests completed.

However, VMS seems to think everything is OK.

$ sh mem
              System Memory Resources on 10-MAY-2020 13:24:53.80

Physical Memory Usage (pages):     Total        Free      In Use    Modified
  Main Memory (32.00Mb)            65536       21316       42491        1729

So I am hoping the machine will now work reliably.

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VAXmate H7270 PSU Fixed, But No Video

In previous posts I have been describing my attempts to find a fault in the H7270 power supply of my VAXmate. After a long time studying and testing it I finally found the fault. It was a shorted diode (marked D24 in my reverse engineered schematic) on the secondary side that was part of the +28V supply to the monitor board.

The reverse engineered schematics are here. Note that they probably still contain errors, especially on the secondary side.

The pictures with the part labels are here.

The problem was that the power supply did not appear to start at all. There was no blip of the fans, no clicking from repeated attempts to start etc. However, with an oscilloscope it was possible to tell that it was attempting to start, but shutting down after only about 20ms, with the SCR (D19) on the primary side detecting an overcurrent. The trace below was taken by lifting R32 and powering the UC3842N (E3) from a bench power supply with the mains still coming in through the AC inlet. It shows the SCR triggering.

Primary Side Shutdown Detail High Resolution

Primary Side SCR Triggering. Ch1. 555 timer. Ch2. D19 Anode. Ch3. D19 Gate. Ch4. Q1 Source.

However, I am not clear why the SCR was taking so long to trigger, in the trace the 15V peaks on the Q1 source have been going for about 15ms, so quite why it triggers the SCR only after that time is not really clear to me. It seemed to me that the peaks on Q1 source were not causing the SCR to trigger, but it turns out that they must have been the cause.

There didn’t seem to be any problem on the primary side, so on the advice of several members of the classiccmp mailing list I checked the secondary side more carefully. It was clear that the secondary side crowbar was not getting triggered, because this is the trace:

Primary Side Shutdown - Secondary SCR Detail

Secondary SCR. Ch1. D19 anode. Ch2. D19 gate. Ch3. Q2 anode. Ch4. Q2 gate

I desoldered and tested the rectifiers on the secondary side, and they tested fine. However, the +28V supply for the monitor board uses a rectifier constructed of discrete diodes (D23 and D24), and it turned out that D24 was shorted. I have replaced both of them to be safe.

It was a bit of a problem to get to the failed diode because it is under the large heatsink. I had to remove the heatsink, but I ended up doing it twice and the second time ended up damaging the vias a bit, so I didn’t completely remove it.

I also replaced capacitors C54, C55 and C56 in the secondary side as they had a high ESR, two of these are part of the +28V supply. Along with a few others that had a marginal or high ESR.

The complete list of replaced parts on the PSU is in the list below:

  • C10, C33, C50, C51, C54, C55, C56, all because of a high ESR.
  • D24, the shorted component.
  • D23, just in case as it is paired with the failed D24.
  • E3, a UC3842N, replaced with UC3842AN, but only due to errors damaging the original.

I was a bit concerned as to why D24 had failed. As it drives the monitor board I examined it. I could not see any visible damage, and it does not appear to present a short circuit. I checked the electrolytic capacitors on the video module and I found 5 where the ESR is marginal. They were all 15uF 16V parts, and they are marked in the picture below. Number 4 has a better ESR but I decided to replace it anyway since all the others have become marginal.

VAXmate Monitor Board - Replaced Parts

VAXmate Monitor Board With Replaced Capacitors Marked

I reassembled the machine and powered it on. It made some reassuring beeps and a few lights came on, but there was no image. A few moments later I noticed a burning smell, so I quickly powered it off. It seemed to me that the smell was coming from the monitor board, so I disconnected it and powered the machine on again. It seemed to work (apart from the video display of course), the diagnostic LEDs did not indicate an error, the floppy disk drive was accessed and it seemed to react to keypresses. There was no more burning smell.

It is clear that something has failed on the video module that caused the power supply to fail. It seemed as if the burning smell may have been coming from the flyback transformer. If that is the case then I suspect that this machine will never work again as I am unlikely to find a new flyback transformer. I took the monitor board out again and another physical examination does not show anything visibly wrong. I wasn’t completely sure that the EHT lead was making good contact with the anode of the CRT though.

I would like to acknowledge the help of all of the following people from the classiccmp mailing list for helping me to find the problem in the power supply. In alphabetical order they are:

  • Matt Burke
  • Rob Doyle
  • Mattis Lind

Plus others on the mailing list who preferred not to be named, but they know who they are.


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Partial Re-cap of H7109-C PSU for a VAXstation 4000 VLC

Recently I had occasion to dig out one of my VAXstation 4000 VLC machines. Before using it I decided it was prudent to check the power supply, a H7109-C, to make sure there were no leaking or bulging capacitors.

It was a good job I did, because one of the capacitors had leaked. So I took the opportunity to check all the electrolytic capacitors and found several that had a high ESR. I have now replaced them all and cleaned up the leak. I also noticed that a ceramic disk capacitor was split (see photo). This capacitor did not measure a stable capacitance, so I replaced that too.

In the photos below I have numbered the replaced capacitors and give their values here:

  1. 39uF 16V
  2. 330uF 25V
  3. 330uF 25V
  4. 470uF 25V, this is the one that leaked
  5. 330uF 25V
  6. 330uF 25V
  7.  3.3nf Ceramic Disk, this is the one that is split.

After replacing the capacitors I powered on the machine and it worked fine. The ripple was about 50mV on both the 5V and 12V outputs.

Here is the machine with the cover off:

VAXstation 4000 VLC (VLC3)

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Memory for a DECstation 2100

Recently I bought a set of 8 memory modules that were advertised as “DEC VS4000”. They had a part number of 50-19464-02. This is one of the modules:

50-19464-02 Memory Module

50-19464-02 Memory Module

I had assumed this was for the VAXstation 4000 VLC, but it turned out not to be the case, the VLC takes modules that are physically narrower. However, they did fit physically in a VAXstation 4000 Model 60. Unfortunately they did not work in that machine and seemed to make the power supply think there was a short circuit.

On suggestion from the classiccmp mailing list, I dug out my DECstation 2100. I discovered mine had 10 of these exact modules installed, out of a possible 12. So now I know that this memory is for a DECstation 2100, and each module is 2MB. I tested the memory in the 2100 and it all worked fine. So now I can have 24MB in my 2100.


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H7874 Power Supply

The H7874 is the power supply  used in the BA4xx and R400x enclosures, such as several machines in the VAX 4000 series. I have a VAX 4000-500 in a BA440. I had some problems with this power supply that seemed to get fixed after replacing some leaked electrolytic capacitors. I still have intermittent problems with the power supply shutting down occasionally, particularly if the machine has not been powered on for a while.

It is an astonishingly complex power supply and very hard to disassemble too. A post on the classiccmp mailing list requested information about this power supply. I have partially reverse engineered the schematic of the 12V output board so I am posting it here. It is unlikely to be correct either.

12V Output Board

H7874 Power Supply – 12V Output Board


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Further Analysis of the VAXmate H7270 PSU Failure

I have been doing further analysis of the failure of my VAXmate’s H7270 PSU. To start with the schematics are now greatly improved. I am still not sure how correct they are, but here are the latest and greatest:

I am fairly sure that the problem is that the primary side is detecting an overcurrent situation. What I am less sure about is whether the overcurrent is real or not. Below is an oscilloscope trace that I think shows this. In this trace the channels are set up as follows:

  1. Ch1: NE555 output, trigger for the one-shot on the negative edge.
  2. Ch2: Vcc (pin 7) of the UC3842 PWM.
  3. Ch3: SCR gate of D19, between R14 and R15.
  4. Ch4: Source of Q1

Primary Side Shutdown

Clearly the gate of the SCR (Ch3) goes high and the 555 stops oscillating. I am a little unsure if I should really be seeing so much spiking on channels 3 and 4.

I decided to look at how the UC3842 PWM power supply is behaving. Accordingly I set up the channels as follows:

  1. Ch1: NE555 output, trigger for the one-shot on the negative edge.
  2. Ch2: Drain of Q1 (pin 2)
  3. Ch3: Cathode of D6
  4. Ch4: Anode of D7

This is what I got:

Primary Side PWM Supply

I am seeing a lot of large spikes on the drain of Q1. I wonder if they are enough to make R13 detect an overcurrent? I don’t know enough about switched mode power supplies to know if this is expected behaviour.

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Possible Cause of VAXmate H7270 PSU Failure

Now that I have acquired a DSO (a Rigol DS1054Z), I have been doing more work to understand the failure of the H7270 PSU on my VAXmate that I first blogged about here. I still don’t fully understand the problem, but I now have an improved insight into what is going on and a possible explanation. The schematics I refer to in this post are:

H7270 Primary Side

H7270 Primary Side

Below is a trace from the oscilloscope during startup of the PSU. What you can’t see here is that Ch1 was steady at about 5V for 15ms before the oscillations start, and this corresponds to a point when voltage on Vcc to the UC3842 has finished ramping up.

H7270 Fault Primary

CH1: Vcc for NE555 and Vref for UC3842 CH2: Base of switching transistor Q1 CH3: Gate of SCR D19 which shuts down on overcurrent CH4: Current sense resistor (R13)

It looks like after the 15ms that the UC3842 starts to switch the transistor. This also corresponds to when Vcc on the NE555 starts to oscillate. The transistor switches for about 25ms and then stops. I have not been able to work out why it stops. I had been thinking that it was because the SCR (D19) was being triggered, but Ch3 of the trace shows it getting pretty much the same signal throughout the period and there is no change at the time the PSU stops, so I am not sure if the SCR is being triggered or not. After all, the voltage across the current sense resistor is not varying much either.

I was puzzled though as to why Vcc to the NE555 starts to oscillate a lot. My guess was that it is because the output from the transformer has not settled yet. I don’t understand why it is quite so spiky though, I imagined a capacitor would smooth it, but I am not sure. I found one capacitor that looks like it is supposed to smooth Vcc (C11). I took it out of circuit to measure it, nominally it should be 220nF, it measured 335nF initially, but as I left it connected to the meter it dropped in about 5 minutes to 300nF, and kept dropping slowly over time.

I decided I needed to improve my schematic. To help me detect the connections correctly removed the transformer (T1). This is it here:

H7270 Transformer

H7270 Transformer

While I had the transformer out of circuit, I decided to carry out a ringing test on it. Unfortunately, this is where I think I may have found the problem. Four of the windings ring correctly, but one of the windings does not ring, it is the one shown as P1 in the schematic, and it is the two pins on the left in the picture above. However, the Technical Description says that one of the primary windings is operated in flyback mode to provide bias voltage for the pulse width modulator. I don’t know if that has any implication for the ringing test though. Here is the scope trace

H7270 Transformer P1 Ringing Test

H7270 Transformer P1 Ringing Test

Another curiosity is that the pin on the right in the picture does not seem to be connected to any other pin on the transformer. I suppose this might mean that a wire has broken somewhere perhaps. I don’t know if transformers can fail with a nasty smell and no outward signs of damage, but I fear it has failed, and that it will be very hard to replace. I am also concerned that if I could replace it some other fault would damage the replacement too.


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VAXmate PSU (H7270) Failure

While I was using my newly working VAXmate the other day, it suddenly failed. I had left it running for a few minutes and when I came back there was a smell and the machine was not running. I wondered at first if it might be the filter capacitors in the power supply, but if these fail the power supply continues to work.

I took the machine apart and examined the H7270 PSU and other boards for any obviously blown components. However I could not find anything. I have since looked multiple times and still cannot see any obviously blown component.

VAXmate H7270 PSU

VAXmate H7270 power supply

My first step was to see if the input is being rectified. I found 300V across the two big smoothing capacitors, so, yes, the input is being rectified. The next step was to see if the switching transistor on the primary side was switching, and to see if it is working. I found that it is not switching, but after desoldering it and putting it into my Peak Atlas Component Analyser, it seemed to be working, so the transistor does not seem to be the problem.

My attention then turned to the NE555 timer that drives the switching of the primary side transistor. It is not doing anything and I discovered that it is not getting any voltage to operate.

This means that the current sense circuit on the primary side is detecting an overcurrent condition, which is shutting down the primary side. The causes of this can be multiple, but one is that the crowbar circuit is detecting an overvoltage and forcing a short on the secondary side that triggers the overcurrent condition which then causes the power supply to shutdown.

I have been testing the PSU with no boards attached, and an old IDE disk acting as a load on the 5V and 12V supplies. According to the Technical Description only these supplies are monitored for overvoltage. Any other overcurrent would be a short in one of the other supplies.

So, I have a PSU which won’t start, despite there being no visible damage that can account for the bad smell when it failed, and despite not being connected to any other boards, and only to a known good hard disk drive. The cause seems to be that a problem is being detected that shuts it down. I only have an analogue oscilloscope, and I can’t use it to diagnose a transient condition like this one. I am going to have to get a Digital Storage Oscilloscope that can be used for transient conditions like this. I want a more modern oscilloscope anyway, so I am going to get a Rigol DS1054Z after a friend recommended it to me and after seeing a positive EEVblog review. When I have that I hope to be able to investigate further.

For reference I have reverse engineered the PSU and done a couple of partial schematics. They are probably not drawn logically as I am not an expert.

The component numbers are assigned by me as they are not marked on the board, and I have posted images of the board with the components labelled. A few component numbers are actually named in the Technical Description and I have used those numbers.


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I acquired a VAXmate earlier this year. The VAXmate was the successor to the Rainbow. It is PC compatible and runs MS-DOS. It was the first commercial diskless PC, although mine came with an expansion box containing a Seagate ST225 hard disk. The machine uses an LK250 keyboard, that looks very similar to the LK201.


VAXmate running Sokoban

When I bought the machine I was told that although it had the keyboard, it did not have the cable to connect the keyboard to the machine. The cable uses a 6-way Shielded Data Link (SDL) connector at the machine end, and an RJ12 on the keyboard itself. I hoped to find another keyboard or to make a cable, but I ended up leaving it for a bit.

The first job was of course to inspect it, before powering anything on. The machine is not easy to work on though. The boards are all installed around the outside of the case and interlock so that it is not possible to remove just one board at a time. In the end I found the rear and left hand (as viewed from the rear) boards could be removed as one by pulling the connectors out of the right hand board and pushing it away a bit. The capacitors all looked OK. I also had to remove all traces of the rubber feet on the main unit as they had turned completely to sticky black goo. The pictures below show the innards of the machine.

My first attempt to power on the machine went well, it powered up and tried to access the floppy disk. I didn’t have a floppy disk though. The display showed an error (60), which is presumably some kind of keyboard error.

I took the disk out of the expansion unit with the intention of trying to using the David Gesswein MFM emulator to image it. It turned out that the disk is bad and all attempts to read it failed miserably. I opened the disk up to see what is going on and it looks to me like it can’t find track 0.

I took the machine to DEC Legacy in November and was able to test it with the keyboard that belonged to someone else. This confirmed that the machine works and that the keyboard I got with it also works.

I then managed to obtain a keyboard cable for an IBM Model M keyboard, as this has the SDL connector on one end. Unfortunately it has a PS/2 connector on the other end and my initial plan was to butcher the cable and put an RJ12 on it. But I decided that I should not do this to a relatively rare cable. So I got a more run-of-the-mill PS/2 cable with a female connector and decided to butcher that instead as an extension plugged into the IBM cable. This failed to work. I thought that this was because it turned out that the IBM cable only had 4 wires out of 6, however I later discovered a different problem and then found that my home-made cable is in fact fine.

In the end an LK250, with the correct cable, came up on eBay, so I bought it. Sadly this keyboard did not work, nor did my original keyboard with the cable. Having checked the cable was OK I smelt a different problem.

Testing the RJ12 end of the cable with the cable plugged in, I realised that the 5V supply to the keyboard was not working. I took the machine apart and traced the 5V supply, eventually reaching a fuse (marked F1 on the board, rated 2A). This fuse was open circuit, so the fuse had blown. At this point it dawned on me that my earlier probing of signals in order to make my own cable may have caused an accidental short and blown the fuse. The position of the fuse is marked on the picture below. It was hard to desolder the fuse because one end must be connected to large 5V plane that sucks all the heat away from the soldering iron.

VAXmate Board Keyboard Fuse Marked

VAXmate Board With Keyboard Fuse Marked

Having now replaced the fuse, both keyboards work and the machine works fine. I ran Sokoban on it, although I don’t seem to be able to remember how to get it to exit from the game cleanly.

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