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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 15th Apr 2022, 1:05 pm   #41
ajgriff
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Default Re: Perth PET

Oh dear Colin, it looks like we now have at least two separate faults to contend with. You’ve ruled out the CPU (6502 rather than 6503 I would guess) in either case so it’s worth doing the same with UC7 (6520 PIA). I can’t remember whether or not you have a spare 6520 but in any event I’d try running the PET without UC7 but with the Tynemouth RAM & ROMs activated. The machine should boot without UC7 but there’ll be no cursor at all and the keyboard won’t work. Please post a photo of the screen display.

Alan
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Old 15th Apr 2022, 3:45 pm   #42
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I didn't even know there was a 6503.....typing error.

So - RAM enabled on Tynemouth board with UC7 removed gives the attached photo. No functioning keyboard.

I swapped over UC6 and UC7 6520s and the problem persists as per the photo in post 38.

Colin.
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Old 15th Apr 2022, 6:19 pm   #43
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Bearing in mind that UC9 has already been replaced and that swapping the PIAs makes no difference I suspect that this has something to do with the keyboard. Could even be something as simple as a sticking return key. Anyway by operating the machine without UC7 in place for the time being it should be possible to investigate the RAM problem as originally envisaged.

As you’ll remember from the epic thread I’d advocate replacing the sixteen 4108s with eight socketed 4116s but Sirius will no doubt favour an approach aimed at identifying the faulty chip or chips (RAM or logic).

Alan
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Old 15th Apr 2022, 6:34 pm   #44
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I have the Daver2 and the Slothie EPROMs on the way kindly from Retromit. I'll start with them to further diagnose what's going on shortly.

Colin.
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Old 15th Apr 2022, 6:54 pm   #45
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Not sure if I remember correctly but did you not have a way to test the ram chips?
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Old 15th Apr 2022, 7:00 pm   #46
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Yes but I seem to have misplaced to testing EPROMs, so I've been lucky enough to find someone on here who is lending me theirs.

Colin.


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Not sure if I remember correctly but did you not have a way to test the ram chips?
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Old 15th Apr 2022, 7:07 pm   #47
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If I remember correctly the EPROMs proved that all the RAM chips were working fine at the time. We're now facing a different scenario whereby at least one chip (or the supporting logic) has failed.

Alan
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Old 15th Apr 2022, 7:16 pm   #48
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I thought you had a separate tester to test the chips out of the board.
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Old 15th Apr 2022, 7:52 pm   #49
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Nope. Not here.

Colin.


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I thought you had a separate tester to test the chips out of the board.
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Old 15th Apr 2022, 8:13 pm   #50
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Tim (retromit) who is sending Colin the EPROMs is the one with an out of circuit tester.

Alan
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Old 15th Apr 2022, 10:31 pm   #51
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Default Re: Perth PET

I am mostly off grid at the moment but AJ is right that I favour a more diagnostic approach, the two test EPROMs have different strengths, Slothie's halts and continually retries on the first failure to write / read back from RAM which will give good steady state waveforms to look at, Daver2's code tries to indicate the nature of the error by showing the character which was read back, where it does not match the character which was written. Both of these behaviours are useful. Given the remarkable survival of all of the system RAM first time around, plus the inexplicably high fail rate of logic ICs, I am inclined to hope that another of the logic ICs has fallen over.
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Old 16th Apr 2022, 9:25 am   #52
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In truth I think the diagnostic approach is the right way forward this time given Perth PET’s history with regard to faulty logic chips. Also I wish I could think of some logical reason why so many of them might have fallen over without the major chips being similarly afflicted.

Alan
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Old 16th Apr 2022, 2:08 pm   #53
ScottishColin
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OK - thanks to retromit, I have a DaverR2 ROM and a Slothie ROM. Any preference where to start?

Colin.


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In truth I think the diagnostic approach is the right way forward this time given Perth PET’s history with regard to faulty logic chips. Also I wish I could think of some logical reason why so many of them might have fallen over without the major chips being similarly afflicted.

Alan
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Old 16th Apr 2022, 2:46 pm   #54
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OK. I think we can all stand down now.

I tried DAVER2 and got a blank screen. I then tried Slothie's EPROM and got a zero page fault. I looked back on my previous thread and that led us to replace UE10. I happen to have a spare chip for UE10, so I swapped that out and it has all sparked back into life so we're all good now.

I'm letting DAVER2's RAM tests run for a while to make sure we're all good there, but I'm back up and running now.

Thanks everyone (especially retromit).

Colin.
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Old 16th Apr 2022, 3:11 pm   #55
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OK - don't go away just yet. Now I'm on a completely blank screen after reboot. I think I'll check the continuity on the UE10 pins on the socket and see if something has happened there.

Colin.
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Old 16th Apr 2022, 5:30 pm   #56
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Ok. I've worked on UE10 and it's happier. I also had a dodgy monitor cable socket from the motherboard. I've sprayed a liberal amount of Deoxit into the socket and that seems to be more settled. So I'm running the EPROM tests as a soak test and I'll take it from there.

The strange reverse cursor has gone too.

Colin.

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OK - don't go away just yet. Now I'm on a completely blank screen after reboot. I think I'll check the continuity on the UE10 pins on the socket and see if something has happened there.

Colin.
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Old 16th Apr 2022, 6:50 pm   #57
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Good news! Glad Perth PET is up and running again.

Alan
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Old 16th Apr 2022, 7:53 pm   #58
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That’s great news you’ve got the Pet working again, Colin.

Tim.
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Old 16th Apr 2022, 11:50 pm   #59
ScottishColin
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Right. I concede defeat for tonight. After a long EPROM testing of the RAM, I rebooted and now cannot get past the garbage screen at boot.

I hate intermittent problems.

Please can someone suggest where to start? I still have my scope and my meter if they are the right tools.

Frustrated of Perth.
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Old 17th Apr 2022, 2:15 pm   #60
ScottishColin
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So. Slightly different displays depending on whether I use the Tynemouth board or not.

If I use my 6502 plugged in directly, I get the PET boot garbage screen - attached.

If I use the Tynemouth board with my 6502 set to BASIC 2.0, I get a screen that is mostly chequerboard (hex FF if I remember rightly) but not completely.

I have removed and re-seated video RAM chips (B3/C3) to no effect.

I still have my scope and meter.

Colin.
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