Sunday, 12 July 2026

Commodore PET No Video Output Repair - 3032 #1

This is not the post I had planned to write.

This is not the board I had planned to repair.

But here we are, let's get on with it.

This is a Commodore PET 3032, sometimes badged a 2001N-32.

The N is important, this is the "New technology" 2001, an almost complete redesign of the PET 2001.

(not to be confused with the 4032N where the N was "Normal" keyboard, and the 8032B was "Business" keyboard)

This has dynamic RAM, 16x 4116 DRAM chips giving 32K.

This has 7x normal 24 pin mask ROMs, 4 or 5 for the OS (BASIC 2 or 4 respectively) and 2 expansion sockets.

(both replacing the rather unfortunate 6540 ROMs and 6550 ROMs of the original 2001).

Or at least it would have those DRAMs and ROMs if they hadn't been removed, along with the 4x 40 pin 65xx chips.

I am afraid I am unsure of the origin of this board. It was in the "untested" pile (actually untested, rather than ebay "untested").

It was either an ebay purchase, a donation, or a part-exchange, but it was a long time ago, so the details elude me.

Oh dear, that's not good.

The electrolyte has leaked out of that capacitor. That doesn't normally happen. I rarely replace caps on boards of this vintage, as they are generally large reservoir capacitors on linear supplies. They only have to deal with 100Hz or 120Hz, rather than the more strenuous tens of KHz in switch modes.

I have not seen one just give up and decide to leak one day. I wonder if it was over voltage?

Musings on a Possible Cause?

I was sent some screenshots of an ebay listing once, a PET for sale in the UK with a US label on the back stating the voltage as 110V. That's fine if you use a step-down transformer, but in the listing in question, the 2 prong US mains plug was fitted with a travel adapter to plug direct into a UK 230V socket.

Nice.

Apparently it was working, although not sure for how long. The transformer would have been getting twice the input voltage, so would be giving twice the output voltage. The 9V that feeds the 7805s to generate 5V would now be more like 18V, and the 16V for the 12V rail more like 32V. The regulators would be cooking, and the capacitors would be over their rated voltage. This one was rated 16V and on the -9V rail, so if it got more like -18V and the regulator next to it was putting out a lot of heat, I can see why it might have given up the ghost and let out the magic juice.

Capacitor Replacement

Yes, that's not good.

It's not like leaky batteries, this generally cleans up fine.

There is a nasty puddle of it under the capacitor.

That cleaned up quite easily. I was trying not to flood the board with IPA as I didn't want to lose the hand-written serial number from 1979.

Speaking of which, 1978 and 1979 must have been tricky years for voltage regulators. That is a 7905, a -5V regulator. It also says 7810, but that is the date code, 10th week 1978, and not a +10V regulator. That same thing must have happened with 7400 series logic 5 years earlier.

The original was 470µF 16V. I have replaced it was 470µF 25V, a nice Nichicon. It never hurts to go for a slightly higher voltage rating. They tend to be physically larger and should last longer.

That all looks quite neat. Right, now why was I looking at this board?

Well, I was in the middle of another repair, and I wanted to check some of the signals on another board, and this one was the same revision.

The only thing was, it also had the same problem.

Well, to be precise, it had the same symptom, but not necessarily the same cause.

Both boards had no video output, no pixel data or vertical sync.

Back to the Other Board

Oh well, never mind, I put this board to one side and went back to the first one.

I eventually found the problem on the first board (more on that in it's own post to follow), but it had several other issues, every time I fixed one thing, I found something else broken.

I decided to leave that first board for a while and thought I may as well have a go at fixing this one.

Back to This Board Again

The video signals on this era of PET were generated from a lot of discrete logic chips, dividing down the 16MHz clock to generate the 15.625KHz horizontal sync and 60Hz vertical sync (all PETs of this era were 60Hz, even the UK ones).

Most of this half of the board is related to generating the video and DRAM refresh timing signals.

The horizontal sync output was correct, but the vertical sync was stuck high.

With something like this, it is best to start at the output and trace back until you find a chip with valid looking inputs, and invalid looking outputs. (or sometimes go the other way, trace the signal from the clock through all the dividers and combinational logic until the signal goes flat).

In this case, there wasn't far to go.

Vertical sync is generated from two halves of a 74LS107 JK flip-flop, via a 74LS00 gate.

I checked the 74LS00 and it's inputs were flat.

I checked the 74LS107 and it's inputs were also flat.

Onto the next page, the next signal, or in this case it's actually called NEXT.

This is generated from a 74S74 D-type flip-flop (shown as 7474 on the schematic).

The inputs to this were wiggling all over the place, but both outputs were flat.

Bingo.

Checking with the scope, it almost looked like the clock signals were always getting blocked by the clear, so I wired up the logic analyser to be sure.

It is not obvious at first glance, but the top chip, G9, is a 74LS74, but the bottom one, G8, the problem chip is actually a 74S74 (Schottky rather than Low-power Schottky).

There must have been a performance gain on the 74S chip that outweighed the extra cost over using another 74LS chip.

The 74LS74 needs a minimum pulse length of 30ns, but the 74S74 fitted only needs 6ns

It did look like the rising edges of the clock were going to be immediately cleared by the falling edge of /CLR, but it was at least 100ns later, so it should definitely have changed the Q output before it was cleared again.

I removed the suspect IC and fitted a socket, since the board already had quite a lot of sockets. It seems to have had a lot of work before. Some a little dubious as there are some bodge wires presumably repairing damage caused with the 74LS153s were all replaced.

Testing the chip in the Mini Pro / T48, and it shows a fail. That's always good to see when you get proof you found the fault (or at least one of them).

I didn't have a 74S74, so I fitted a new 74LS74. It should be fast enough, but I could potentially use a 74HCT74 if it proves not?

I wired it back up to the logic analyser, and there was the Q output, changing when it should.

Yay, it works.

Time to Test The Video Out

I don't like using PET CRT monitors when there are video problems, I don't want bright lines or spots potentially damaging the CRT, so I am using a PET Video Out board to generate NTSC composite video to display on an LCD monitor.


I am expecting a screen of random characters.

Hmm, that's not right.

It is normally random characters with any of 256 values, but there is a distinct pattern there. No inverted characters, no numbers or graphics. Hmm, I think the RAM chip that handles the highest 4 bits is bad. It is a bit unusual, or at least unlikely, that it would randomly pick 1000 values that were all 0-15 from a range of 0-255 (1 in 16 ^ 1000 ?)

Looking at this, the white sockets are original (awful, but original), but I don't think either of the video RAM chips are. The soldered chips are mostly dated early 1979. The MOS 2114 is possibly original, early 1980, but the Fairchild part is from 1983. It is also F7, the one which handles bits 4-7, the one possibly at fault.

A simple test is to swap them around.

Ah, there you go, it is now the lower 3 bits that are stuck low.

If the problem had not changed with the chips swapped, then I would have been looking at the associated buffers and latches (or probably just blaming the white sockets).

It is difficult to find a matching set of 2114 chips that will work fast enough for video RAM, so instead I removed both chips and fitted a 2114 video RAM replacement.

That's better. That is what it is meant to look like.

Testing the Board

Since the ROM and RAM were absent, I fitted a PET ROM/RAM and enabled both.

I also fitted two new WDC W65C21N and a WDC W65C22N, and set it up for testing with a keyboard and datasette and the PET Video out.

Ready for instruction.

A quick test loading from tape.

All looks good.

It initially failed to load from SD2PET, that might be as simple as the edge connector needs cleaning or one of the MC3446 drivers may have failed.

I will address that and do some more testing on this one later, make sure everything is stable etc. But for now the video problem has been fixed and I can leave it running memtest for a bit.

Side Note: 9V PET

It is worth noting, that the power supply on these PETs is two AC supplies, one about 16V, one about 9V. These are rectified, smoothed and feed linear regulators that generate 5V, 12V and -5V. Only the DRAM needs 12V and -5V. The rest of the PET is 5V only.

(other than the datasette which is powered from 6V that is derives from the unregulated 9V rail).

This one has had the DRAM removed and socketed. I presume it didn't come from the factory with a mixture of 16 pin sockets and pairs of 8 pins when the 16 pins ran out.

When you have a PET like this which has the DRAM removed, and you are using a PET ROM/RAM and a PET Video Out, it opens an interesting possibility. You can run it from 9V DC.

I have a PET 2001 board I use for testing sometimes, and I run that from 9V DC, as it never had DRAM.

This board could be used the same, it only requires 0V and 9V on pins 3 and 2 of the power supply connector.

I use a cable I made with a DC jack socket, centre negative obviously.

The bottom 5 pins of the 9 pin connector are the same as the PET 2001. The other pins can be ignored here. (I did make a 9 way version of this cable, but I will not find it until shortly after I put this post live, when it will probably fall off a shelf and hit me)

A setup like this draws about 1.2A at 9V.

You just need to remember to not fit any DRAM chips, but then again, how often are you going to have sixteen working 4116 chips going spare and looking for a home?

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The PET Video Out, 2114 video RAM replacement and PET ROM/RAM are available form my Tindie Store.

Or, if your PET board has more faults or missing parts than these two I have been looking at, you can always take the easy route and get a Mini PET 40/80 drop in replacement.

If you don't want to use Tindie, you can always contact me using the link at the top of the page and let me know what you want and which country you are in, and I will get you a price if you want to buy direct.

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Sunday, 5 July 2026

Testing not-quite-a-ZX-Printer on not-quite-a-ZX81

The Sinclair ZX Printer is another of Sinclair's clever minimalists designs.

A small, cheap printer to go with a small, cheap computer, the ZX81.

It connects to the expansion bus, but does not use the full bus, only one address line and five data lines.

It is mapped to any address where A2 is low, the code uses $FB, where all the other bits are high.

This follows the pattern used for the ZX80 IO, any address where A0 is low, with the code using $FE, again the address where the other bits are all high. The ZX81 added an NMI generator that was disabled by a write to any address where A1 was low, using $FD following the same pattern.

It is a very minimal design, just paper feed, and a moving single pixel head. This head burns tiny holes in special metalised printer paper (yes, really).

I have picked up quite a few of these other the years, usually as part of bundles with ZX81s.

Unfortunately, none of them work.

They seem to have two Achilles heel's, first the circuitry inside is based around a Feranti ULA, and like many of those in most of Sinclair's products, these tend to fail in time. Another victim of time is the drive belts, these test to degrade over time and fall apart.

(there is a third, which only applies if you actually get past the first two hurdles is getting hold of the special paper it uses)

At some point I need to go through all the ones I have and try to find one good ULA and one good belt (or get one of the 3D printed replacements) and get one running.

Until then, I am afraid all I can do is take photos of what it would look like if you connected a ZX Printer up to a Minstrel 3.

Ah, you say, but what if it is the Minstrel 3 at fault and the printers are fine.

Well, I have tested all of these several times over the years on ZX81s and Minstrels various and they have sadly never worked.

Alphacom 32

Another way to verify this is with an Alphacom 32.

This is a ZX Printer compatible printer, also sold as the TS2040 to go with the Timex/Sinclair range of computers like the TS1000 and TS1500.

This has several advantages over the ZX Printer.

Firstly, it has the words "Alphacom 32" written across the front in large, friendly, letters.

Secondly, they generally work, and I think you can actually still buy new-old-stock units, the last of the Timex stock (from the same place that had all the unassembled ZX81 kits).

Another advantage is that rather than the "burning holes into foil" approach, this is an early example of a thermal printer, a technology which is widely used these days.

Pretty much every till receipt and the labels on all the parcels you receive use a thermal printed label.

This means you can buy rolls of paper to drop straight into your Alphacom 32. (update - I just did)

The reason these are used for till receipts etc. is they are very simple and reliable, and require no ink cartridges or toners or anything other than till rolls.

Testing the Alphacom 32 on a Minstrel 3

The Alphacom 32 plugs into the back of your ZX81, or in this case, Minstrel 3.

It responds to the standard LPRINT, LLIST and COPY commands in ZX81 BASIC.

LPRINT is the equivalent of PRINT, it just sends the output to the printer.

LLIST is the equivalent of LIST, and as a bonus, it just keeps going, it doesn't stop after one screen full. Handy for those long listings.

COPY prints out a copy of whatever is on the screen to the printer.

That all seems to be working nicely.

Testing the Alphacom 32 on a Minstrel 4th

Wait? the Minstrel 4th doesn't have a ZX81 style edge connector.

Very true.

But it does have an RC2014 bus slot. And several people have been looking into running a ZX Printer on the RC2014 bus.

George Beckett looked into this a while ago, and ordered some PCBs to Robert Price's design.

Thanks to George, who kindly sent me one of his spare PCBs so I could build it up and test it out.

There is a socket on the right for a 9V DC input, the pads are too small for the sockets I have, so for the moment I will leave that empty. It is only required for the ZX Printer, the Alphacom 32 has it's own supply (rather oddly 24V AC).

Rather than the "any address with A2 low" Sinclair approach, Robert has fully decoded the address, allowing it to be selected from a DIP switch.

I have set this to address $FB (1111 1011), the one used on the ZX81. It also servers as a visual representation of "an address where A2 is low", A2 being the third bit from the right (A0, A1, A2 etc.)

With that all connected up, I can run ZX81 BASIC on the Minstrel 4th, and talk over the RC2014 bus to the Alphacom 32, using the standard BASIC commands.

And there we go, a not-quite-a-ZX81 driving a not-quite-a-ZX-printer.

You might have guessed, I am here because I also wanted to try out the new Lambda 8300 BASIC for Minstrel 4th.

I am pleased to say that also worked fine.

The printout is not very readable, blue on white thermal paper dating back maybe as far as the 1980s.

I did try printing on a 6"x4" postage label from my label printer.

That's more readable, not quite feeding right, so I am sure the proper printer roll will be even better.

That was ZX81 BASIC and Lambda 8300 BASIC. It should be possible to use it from the other ROMs, it would need some code writing to pixelise what you want to print and send it to the printer. (there is an example on Robert Price's page, linked above)

As a final test, I did try the ZX Printer on the Minstrel 4th, if only to get the photo of that combination.

Not quite ideal due to the bodge wire I added for the 9V supply (I knew it wasn't going to work, otherwise I would have made more effort).

As expected, that doesn't work, not because of anything in the BASIC port, the Minstrel 4th hardware, or the interface card, simply a faulty printer. In this case, it just line feeds whenever you try to print.

Update

The new printer rolls have arrived.

2026 printer rolls are smaller than part used 1980s rolls. Who would have guessed?

That's looking good with the printers built in test pattern.

I repeated some of the tests.

It's much clearer, with the new paper.

The quality is pretty good, certainly the best quality of any vintage printer I have.

It is also by design pixel for pixel with the screen font, so none of the scaling issues you get with modern printers, no need for anti-aliasing or anything like that.

Handy for printing out listings for your programming masterpieces (of which that is not one)

Or screenshots of your favourite games.

I am impressed with the usability and quality of the output from the Alphacom 32, I suspect I will be using this more, especially as the only consumable (the printer roll) is still widely available.

I plan to follow this with a more in-depth look at the hardware of the ZX Printer, and repairs or rebuilds to get one working, but that will have to follow another day.

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Including the Minstrel 2, Minstrel 3 and the newly updated Minstrel 4th

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