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Commodore PC-1 repair

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MindWalker

Experienced Member
Joined
Jul 23, 2019
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72
Location
Helsinki - Finland
Hi.

I bought a non-working Commodore PC-1, told to be untested due to a missing power supply. It wasn't too expensive and I was hoping for just a simple power supply -remake (there are several write-ups of this as the orignal PSU seems to be rather fragile and there is a MeanWell RPT-60B which is very much a drop-in replacement).

I own another working PC-1 so I have some points of reference. Testing the new machine with the known good (recapped and tested) power supply I have power LED, but absolutely no other signs of life. There is not even a blip in the picture while powering on (either from the 9-pin display out or the RCA monochrome output, I have verified my display setup/DIP switch settings with the working motherboard) so I presume there is not even the display sync.

With the working motherboard I was able to test that all socketed ICs are good: Paradise System PPC1 and PVC2, MOS 5720, Faraday FE2010, 8088, BIOS and Character EPROMs.

For some reason half of the RAM is socketed - in my working machine the stock 512k is all soldered in, with only four sockets on the right for the expansion from 512k to 640k. At least one of the socketed RAM chips had a bent pin, one has a rusted pin and three of the chips tested bad on my Retro Chip Tester. I am not sure if all the RAM needs to be in, I have tried both ways and no difference.

I even have a PC-1 to ISA -adapter and I tested plugging in an ISA POST-card. It stays at 00 10 (I know this shouldn't really work on XTs but tested it anyway). The LEDs on the card show that the RESET line works (IRDY turns off while reset button is held), CLK and FRAME are lit all the time. Only the 8088 and the (programmable logic chips? The ones on bottom-right with white 3188xx-01 labels) get warm.

Depending on how the original power supply died, there could've been over-voltage (perhaps killing the RAMs?) but as the big chips tested good I am somewhat hopeful.

I was thinking of making a copy of Ruud's diagnostic ROM, but I am not sure if that will help, as there is no display?

I have a logic probe, and later on I can get my scope from storage so I guess I should start looking at the address and data lines (I've been watching Adrian's digital basement and I can follow his troubleshooting but I am somewhat insecure on where to start looking) etc.

All ideas are greatly appreciated :)
 

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I'm a PC-1 fan as well and still busy disassembling its BIOS. In short: I don't see why "my" BIOS (but the honor goes to Modem7) should not work. The video is initialized after the beep. If the BIOS doesn't get to the beep, the video won't be initialized either and therefore no blip.
The good thing: the BIOS outputs its own POST code on the LPT port. I attached my disassembly. Look for "Coldstart:" and the parts labeled "POST point".

Good luck!
 

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Thanks Ruud.

I did little more testing. I wrote a 27128 EPROM with your diag ROM. I tested it on my working machine and it does give the short-long-short beep but doesn't seem to initialize the video (9-pin or composite out).

The good news is that plugging the diag ROM into the dead motherboard also gives the initial beeps so it's not totally dead then :)

I guess the video side of the PC1 is unusual enough to not work. Probably the Landmark diag ROM wouldn't work any better in that regard?

I don't have a parallel port display card, but I guess I'll need to get myself one (y)
 
I wrote a 27128 EPROM with your diag ROM.
Click to expand...
I am reading this and I am confused. What diag rom are we talking about?

Ruud's attachment is just a partial disassembly of the actual PC-1 rom, or the rom you already have, so I assume you're referencing a PC diagnostic rom I can't seem to find.

edit: I guess this is the one? https://minuszerodegrees.net/ruuds_diagnostic_rom/Ruuds Diagnostic ROM.htm

I don't have a parallel port display card, but I guess I'll need to get myself one (y)
Click to expand...
I assume the value stays lit on the port. There aren't that many bits to check, so I'd do it manually bit by bit.
 
Last edited:
MindWalker said:
I guess the video side of the PC1 is unusual enough to not work. Probably the Landmark diag ROM wouldn't work any better in that regard?
Click to expand...
One idea was to plug in a (clone) MDA card in your expansion but then I realized that Ruud's diagnostic ROM does read the video memory and that could cause bus clashes with the on board video memory. If nothing else works, it is worth a try.

I forgot to ask: what kind of PC-1 to ISA -adapter do you have? Just curiosity, I created one myself.
 
Little more testing:
  • Ruud's diag: short-long-short beep, ISA-adapter + modern POST card shows 80. Could this point to "checkpoint 8D (hex), then display "Test of video RAM failed.", then halt". The machine indeed halts, pressing the reset-button produces the short-long-short beep again, the code doesn't change.

  • I made an EPROM with the Supersoft diag. I get a series of beeps and while having some difficulty counting them, I believe I identified at least all these errors:
    • 5 hi/lo beeps + 9 short beeps: "cannot initialize monitor"
    • 1 + 4: 8253 timer channel 1
    • 1 + 5: 8253 timer channel 2
    • 1 + 8: 16k critical memory region
    • 3 + 4: System memory at address 00000
I did these tests with the second bank of RAM (socketed) removed. As there already were three bad RAM chips, perhaps I should suspect there is more? I have desoldering gun so it wouldn't be enormous amount of trouble to desolder and socket all of those (and test on my Retro Chip Tester Pro).
 
MindWalker said:
Ruud's diag: short-long-short beep, ISA-adapter + modern POST card shows 80. Could this point to "checkpoint 8D (hex), then display "Test of video RAM failed.", then halt".
Click to expand...
No. Because:
1. On the POST card's display, 8D is displayed very differently to 80, because the D is displayed in lower case.
2. If your modern POST card is like the modern example shown at [here], the button on it allows one to scroll back through all codes received. Per [here], you would expect to see the code history of 33, 00, 02, 03, 04, 06, 08, 09, 0A, 0C, 0D, 8D

I would not be surprised if a modern ISA POST card does not work in your PC-class computer. Hopefully, you have a parallel/LPT type of POST card on the way.

MindWalker said:
I made an EPROM with the Supersoft diag. I get a series of beeps and while having some difficulty counting them, I believe I identified at least all these errors:
Click to expand...
Be wary of those beeps. Be very wary. It is easy for that diagnostic to lead one astray.

If we assume the beeps to be accurate, then a failure of channel 1 of the timer chip has RAM implications, that channel typically used as part of the RAM refresh mechanism.
 
Thanks, modem7.

The POST card is identical to your to your article and right, it seems it's not of use in this case.

I know that the PC1 is CGA on paper, but it doesn't seem to be totally compatible as certain games that won't detect it as CGA. Ruud's diag wouldn't initialize the video on my working PC1 either so at least something in the PC1 is different enough.

Parallel port card is now on the way 📦, perhaps I'll try socketing and testing the rest of the RAM while waiting!
 
MindWalker said:
Parallel port card is now on the way 📦,
Click to expand...
If cheap enough, perhaps also get a replacement 8253A timer chip. If untested, maybe two in case one is faulty.

MindWalker said:
perhaps I'll try socketing and testing the rest of the RAM while waiting!
Click to expand...
I did that (add sockets for RAM) to one of my IBM 5150 motherboards, but primarily so that I could properly test new versions of Ruud's Diagnostic ROM on an IBM 5150.
 
MindWalker said:
I know that the PC1 is CGA on paper, but it doesn't seem to be totally compatible as certain games that won't detect it as CGA. Ruud's diag wouldn't initialize the video on my working PC1 either so at least something in the PC1 is different enough.
Click to expand...
I will take a look at Ruud's disassembly of the motherboard BIOS. It might be easy to enhance Ruud's Diagnostic ROM to support the PC1's unique CGA video.
 
Glanced at Ruud's diagnostic rom.

No support for UART output? That's surprising.
 
Little update (very little progress too, I haven't really had the energy to work on this for a while now)

I desoldered the first bank of RAM from the motherboard. All chips tested good and I re-installed it with sockets.

I have the four-digit LPT post card now, with it I get:
  • Stock ROM: stops at 50 71. Pressing reset it looks like the sequence is 71, 00, 11, 30, 50, 71, 00.
  • Supersoft ROM: -- 00 (and the same error beeps as before)
  • Ruud's ROM: init beeps and 20 00
 
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