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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 7th Apr 2021, 6:08 pm   #1
John Earland
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Default Commodore PET working intermittently

Hi folks,
Youíve been so helpful in the past with my various projects so I hope you can help again.
I have a PET 3016 machine. It has no ROM chips so instead I use a ROM/RAM board from Tynemouth Software which works great. The PET has been working well over the past year but it has suddenly developed a new problem and has got increasingly worse. When I switch the pet on it sometimes works fine but more often I get a screen load of random characters. Sometimes when I press on the motherboard and switch it back on it works and when it does work it works really well. However, every time I switch it on at the moment Iím getting a screen full of random characters.

Previously, when it developed a fault, you were able to help me locate a problem with two of the chips which I replaced and advised me to replace the sockets of those ICs which are socketed. All of the sockets were replaced save one. It has been performing well as I say until recently when it began to run intermittently. I then decided to replace the one remaining socket but it has not resolved the issue. I can rule out ROM/RAM because Iím using the Tynemouth board. It may be a problem with my soldering of course on this last socket, the others were replaced by someone better at it than I am. As I say, when it does work itís brilliant. It runs all software I can throw at it and I can even get on BBSs.

Any advice would be very welcome as to how I can check my soldering or the efficiency of any lines etc. I have a multi-meter and an oscilloscope (which Iím afraid to say I donít know how to use!!).

Best wishes
John
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Old 7th Apr 2021, 6:41 pm   #2
Mark1960
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Default Re: Commodore PET working intermittently

You could post a picture of your soldering, but maybe better to use the multimeter to check the continuity of each pin on the IC to both the solder pad on the bottom of the board and also to any trace from each pin on the top of the board. This would confirm that you haven’t damaged the through hole plating when you removed the socket.

Edit: also check to make sure you don’t have any short circuits between adjacent pins.
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Old 7th Apr 2021, 6:56 pm   #3
SiriusHardware
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Default Re: Commodore PET working intermittently

To get a feel for the previous history of this machine and a better idea of John's capabilities - which are better than he makes out - it might be useful to skim this thread - don't worry, it's a mere 300+ posts long.

https://vintage-radio.net/forum/showthread.php?t=163589

It sounds like replacement of the one remaining original socket, which was a decently intuitive decision, did not change the fault which by the sound of it had already become permanent before you changed the socket.

If it was working from time to time until you changed the socket and then never worked again after that, then I would say look more closely at the socket you replaced.
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Old 7th Apr 2021, 7:43 pm   #4
John Earland
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Default Re: Commodore PET working intermittently

Quote:
Originally Posted by Mark1960 View Post
You could post a picture of your soldering, but maybe better to use the multimeter to check the continuity of each pin on the IC to both the solder pad on the bottom of the board and also to any trace from each pin on the top of the board. This would confirm that you havenít damaged the through hole plating when you removed the socket.

Edit: also check to make sure you donít have any short circuits between adjacent pins.
Thank you. I will try this first thing. I thought I was careful but....!
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Old 7th Apr 2021, 7:45 pm   #5
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Default Re: Commodore PET working intermittently

Quote:
Originally Posted by SiriusHardware View Post
To get a feel for the previous history of this machine and a better idea of John's capabilities - which are better than he makes out - it might be useful to skim this thread - don't worry, it's a mere 300+ posts long.

https://vintage-radio.net/forum/showthread.php?t=163589

It sounds like replacement of the one remaining original socket, which was a decently intuitive decision, did not change the fault which by the sound of it had already become permanent before you changed the socket.

If it was working from time to time until you changed the socket and then never worked again after that, then I would say look more closely at the socket you replaced.
Thank you for your reply. I switched it on earlier today and it worked but it booted in lowercase! All worked fine though but it should have been in uppercase. I then switched it off and on again and got the dreaded random character screen!
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Old 7th Apr 2021, 9:51 pm   #6
ortek_service
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Default Re: Commodore PET working intermittently

Well, looking at what seems the most accurate PETSCII character set details, here: https://www.atarimagazines.com/compu...ACTER_SETS.php

it seems that the only way it can change from upper case to lower case (without numbers being corrupted), is by A10 input to the character ROM being high (Which selects Alternate character set display) rather than low (for standard character set). And the Char.ROM A10 input is controlled by a latch - driven by writes to 59468 (maybe in one of the 6520 / 6522's?) Bit 1 (of bits 0..7).

So could be a lack of proper- initialisation issue, and if intermittent / changing with pressure on PCB(s), does seem that IC sockets / dirty IC pins could well be the cause.

However, if very intermittent then finding this could be tricky. And the use of alternate diagnostic EPROM's (or maybe even built-in ones) could help
- especially with infamous screen of random characters, which signifies that either the processor isn't executing the instructions from the ROM's correctly, the lower Main RAM pages aren't working, or the processor is not able to write to the Video-RAM. As even if this is intermittent, a good diagnostics EPROM should at least narrow down where the fault is occurring so less to have to try and check.
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Old 7th Apr 2021, 10:14 pm   #7
SiriusHardware
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Default Re: Commodore PET working intermittently

I may be remembering this wrongly (I have not read back through that old thread) but I seem to remember that John could choose from at least two sets of ROMs (BASIC 2 / BASIC 4?) on his ROM / RAM board and this makes me wonder whether it also contains some diagnostic tools which can be swapped in instead of the regular ROMs? It's the kind of thing I would have done if I had designed such an item.

If not, provided the 'normal' PROMs can be switched off by means of links or switches, then I would be happy to programme a copy of one of the diagnostic EPROMs we've been playing with. Although obviously faulty, this machine is starting from a better place in so far as the video RAM and video hardware are obviously working - most likely just a single (intermittent?) failure is causing this problem - I doubt that there will be multiple device failures on this one.

Last edited by SiriusHardware; 7th Apr 2021 at 10:20 pm.
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Old 8th Apr 2021, 12:47 am   #8
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Default Re: Commodore PET working intermittently

Yes, I did think later that there was probably a single EPROM (or hopefully a FLASH-EEPROM these days) on this, that might be able to be read & temporarily- reprogrammed with a Diagnostic ROM, using a PC-BIOS etc. IC programmer etc. (that I think you can acquire cheap online). Or, a bit safer, obtain another IC to swap with.

If it has does have various selectable ROM-image options, then may need to establish where they all need to be (But maybe could just fill one with lots of repetitions of the Diagnostic ROM / different versions, controllable via external selection switches - As with several more recent Multi-ROM Cartridges for C64 / Spectrum etc., using now cheap Mbit FLASH-EEPROM's & DIP-Switches)

However, it does seem that the original PET Kernal did have built-in diagnostic routines, enabled by pulling the Diagnostic line low on one of the connectors.
But it's not that clear how the loopback connectors are used on keyboard & User Port Connectors, as other than wiring for these there doesn't seem to be a great deal of info about this - Although it does seem to be able to report individual memory IC faults, from an example screen shot I found.
And I expect there's a commented disassembly of the PET ROM's, as done for other Commodores.
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Old 8th Apr 2021, 7:14 am   #9
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Default Re: Commodore PET working intermittently

To get back to this specific machine: John, which IC socket was it that you replaced? (Which IC position / circuit number).
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Old 8th Apr 2021, 8:19 am   #10
John Earland
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Default Re: Commodore PET working intermittently

Quote:
Originally Posted by SiriusHardware View Post
To get back to this specific machine: John, which IC socket was it that you replaced? (Which IC position / circuit number).
UF10 was the socket I replaced. You're quite right about the 'test' facility on the ROM/RAM Board. After a few on and off switching I eventually was able to access the tester. It showed that the RAM at least is fine. Here's the thing though, I've just put the ROM/RAM board setting to Basic 4 32k. Upon booting the machine it should all be uppercase but it isn't and it's only showing 16k which also happens to be what my machine has as it's own RAM without the board. Could there be something wrong with the board?

**Just realised that it is showing on board 16k because that's what I had set the ROM/RAM board to! However, this doesn't explain the lowercase that I'm seeing.

Last edited by John Earland; 8th Apr 2021 at 8:37 am.
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Old 8th Apr 2021, 8:25 am   #11
SiriusHardware
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Default Re: Commodore PET working intermittently

I think we'd better go with one thing at a time. The upper / lower case issue could conceivably be a side effect of the UF10 socket replacement since UF10 happens to be the character generator PROM.

If you have not already done so just make sure you didn't accidentally fold one of UF10's pins over when you put it back in, and while it is out meter between all the UF10 socket pins and places on the circuit that they should be connected to - I think you will be able to work out how to do that without more detail but if you need it, just ask.
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Old 8th Apr 2021, 8:28 am   #12
John Earland
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Default Re: Commodore PET working intermittently

Quote:
Originally Posted by ortek_service View Post
Well, looking at what seems the most accurate PETSCII character set details, here: https://www.atarimagazines.com/compu...ACTER_SETS.php

it seems that the only way it can change from upper case to lower case (without numbers being corrupted), is by A10 input to the character ROM being high (Which selects Alternate character set display) rather than low (for standard character set). And the Char.ROM A10 input is controlled by a latch - driven by writes to 59468 (maybe in one of the 6520 / 6522's?) Bit 1 (of bits 0..7).

So could be a lack of proper- initialisation issue, and if intermittent / changing with pressure on PCB(s), does seem that IC sockets / dirty IC pins could well be the cause.

However, if very intermittent then finding this could be tricky. And the use of alternate diagnostic EPROM's (or maybe even built-in ones) could help
- especially with infamous screen of random characters, which signifies that either the processor isn't executing the instructions from the ROM's correctly, the lower Main RAM pages aren't working, or the processor is not able to write to the Video-RAM. As even if this is intermittent, a good diagnostics EPROM should at least narrow down where the fault is occurring so less to have to try and check.
Hi, thank you for this. Where could I get an EPROM from?
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Old 8th Apr 2021, 8:45 am   #13
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Default Re: Commodore PET working intermittently

I can sort you one out, if it comes to that. I was just trying to establish first whether your ROM / RAM board already had that capability, which it seems it does.

Using a conventional test EPROM in UD9 or UD8 socket would mean that you would need to be able to disable the equivalent ROM on the add-on board.
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Old 8th Apr 2021, 8:58 am   #14
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Default Re: Commodore PET working intermittently

The Tynemouth ROM/RAM board is available again and this is the current offering:

https://www.thefuturewas8bit.com/petromram.html

Note the 'TESTER' setting which can be selected using the DIP switches. Presumably John has access to some form of documentation explaining this functionality.

Alan
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Old 8th Apr 2021, 9:53 am   #15
John Earland
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Default Re: Commodore PET working intermittently

Quote:
Originally Posted by ajgriff View Post
The Tynemouth ROM/RAM board is available again and this is the current offering:

https://www.thefuturewas8bit.com/petromram.html

Note the 'TESTER' setting which can be selected using the DIP switches. Presumably John has access to some form of documentation explaining this functionality.

Alan
Hi Alan,
Yes, in test mode all seems to be functioning. I have sorted out the lower case issue - had the 6520 and 6522 in the wrong sockets! However, I am still getting the random character screen so it must be a dodgy connection somewhere. I shall start with my replaced socket. Thank you.
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Old 8th Apr 2021, 9:58 am   #16
John Earland
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Default Re: Commodore PET working intermittently

In desperation, I switched the PET on and got random characters. Without switching off I removed the ROM/RAM Board and reinserted it and I got the Boot screen but no cursor. I've tried this several times and it always does the same - it's almost like a reset but no cursor.
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Old 8th Apr 2021, 10:04 am   #17
NivagSwerdna
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Default Re: Commodore PET working intermittently

Quote:
Originally Posted by John Earland View Post
it's almost like a reset but no cursor.
Try... pressing down on the 6502 socket of the replacement board... also try running your finger around the VIA and around the CPU socket pins...
Weird idea... but try it.
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Old 8th Apr 2021, 10:18 am   #18
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Default Re: Commodore PET working intermittently

Please, never ever insert or remove modules or components when the system is powered on. There is a high risk of damaging not only the thing you insert or remove but any random component it happens to be connected to, which in this case is quite a few.
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Old 8th Apr 2021, 10:28 am   #19
John Earland
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Default Re: Commodore PET working intermittently

Quote:
Originally Posted by SiriusHardware View Post
Please, never ever insert or remove modules or components when the system is powered on. There is a high risk of damaging not only the thing you insert or remove but any random component it happens to be connected to, which in this case is quite a few.
Point taken!
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Old 8th Apr 2021, 11:25 am   #20
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Default Re: Commodore PET working intermittently

Briefly OT : NivagSwerdna, I've finally decoded your handle. Originally I thought you must be from Sweden or something...
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