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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 Yesterday, 8:14 pm   #181
Slothie
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Default Re: Commodore PET 2001 voltage regulators

Quote:
Originally Posted by retromit View Post
One thing I did notice when the PET booted was that there was no flashing cursor. I did try and swap the 6520s around in UC6 and UC7, but on doing this (after powering on the PET) I weas greeted by a blank screen. On swapping the 6520s back again, I had the boot screen and no cursor.

The next test was to run the Slothie EPROM in UD9. On powering up the PET it started to run, but gave the following error (picture also attached):

KB : E0 !

So I guess this is down to the potential issue with the 6520?

Tim.
Well done for checking the processor, lucky you had a spare as that could have been hard to diagnose. If you buy one to replace it be careful as there are a number of variants - the modern WDC 65C02 is not quite pin compatible (if I recall you need to disconnect a pin or something).

I would expect to see the 10 hex scan codes across the bottom, not just one. Also I note the '0' in the top right hand corner is still 0, this should be counting up 0-9 then going round again, at approximately 1 sec intervals. This suggests to me that there is a problem with the interrupts, this could be caused by a faulty I/O chip (6520 or 6522). The interrupt source is the vertical retrace from the video circuitry. It looks like the keyoard scan is getting interrupted, this could be caused by continual interrupts causing the stack to overflow?

Its also possible it is a RAM refresh problem, although BASIC seems to start OK.

If you press a key it is supposed to show which key was pressed. the "!" key would normally show the first scan code as "FE" and all the others as "FF".
A screenshot of the diagnostic with no keys pressed is attached.
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Old Yesterday, 8:32 pm   #182
retromit
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Default Re: Commodore PET 2001 voltage regulators

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Originally Posted by ajgriff View Post
PIA UC7 would be my favourite as it interfaces with the keyboard and is prone to failure. Try swapping the two PIAs.

Alan
I've tried swapping them, but when I do I get a blank screen; then when swapping them back I get the normal boot screen with no cursor.

Tim.
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Old Yesterday, 8:39 pm   #183
ajgriff
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Default Re: Commodore PET 2001 voltage regulators

Quote:
Originally Posted by retromit View Post
I've tried swapping them, but when I do I get a blank screen; then when swapping them back I get the normal boot screen with no cursor.
Sorry Tim I got confused. What happens if you put UC6 in the UC7 socket and leave the UC6 socket empty?

Alan
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Old Yesterday, 8:58 pm   #184
retromit
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Default Re: Commodore PET 2001 voltage regulators

Quote:
Originally Posted by ajgriff View Post
Quote:
Originally Posted by retromit View Post
I've tried swapping them, but when I do I get a blank screen; then when swapping them back I get the normal boot screen with no cursor.
Sorry Tim I got confused. What happens if you put UC6 in the UC7 socket and leave the UC6 socket empty?

Alan
Okay, the current confoguration of UC6 and UC7 gives the boot screen, but no cursor. Following your suggestion:

- Removed UC7 and put UC6 in UC7 socket (with UC6 then empty). This produced the boot screen and flashing cursor; good news!

- The new UC7 removed, old UC7 inserted (UC6 still empty). This produced the boot screen with no prompt.

I then tried the first scenario on UC6 in UC7, then put the original UC7 in the UC6 slot and powered on the PET - it only gave a blank screen.

Incidentally, when the first scenario was in place (when we had the cursor) the keyboard looked to be working as well. I must admit I thought I might have had issues with the keyboard not working.

Tim.
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Old Yesterday, 9:00 pm   #185
retromit
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Default Re: Commodore PET 2001 voltage regulators

Quote:
Originally Posted by Slothie View Post
Quote:
Originally Posted by retromit View Post
One thing I did notice when the PET booted was that there was no flashing cursor. I did try and swap the 6520s around in UC6 and UC7, but on doing this (after powering on the PET) I weas greeted by a blank screen. On swapping the 6520s back again, I had the boot screen and no cursor.

The next test was to run the Slothie EPROM in UD9. On powering up the PET it started to run, but gave the following error (picture also attached):

KB : E0 !

So I guess this is down to the potential issue with the 6520?

Tim.
Well done for checking the processor, lucky you had a spare as that could have been hard to diagnose. If you buy one to replace it be careful as there are a number of variants - the modern WDC 65C02 is not quite pin compatible (if I recall you need to disconnect a pin or something).

I would expect to see the 10 hex scan codes across the bottom, not just one. Also I note the '0' in the top right hand corner is still 0, this should be counting up 0-9 then going round again, at approximately 1 sec intervals. This suggests to me that there is a problem with the interrupts, this could be caused by a faulty I/O chip (6520 or 6522). The interrupt source is the vertical retrace from the video circuitry. It looks like the keyoard scan is getting interrupted, this could be caused by continual interrupts causing the stack to overflow?

Its also possible it is a RAM refresh problem, although BASIC seems to start OK.

If you press a key it is supposed to show which key was pressed. the "!" key would normally show the first scan code as "FE" and all the others as "FF".
A screenshot of the diagnostic with no keys pressed is attached.

I've just posted #184, where one 6520 is present in the PET and this gives the boot screen and prompt. I then tried the EPROM again at this point, and it gave the same screen that you supplied (so looks good). On pressing the keys, they were represented on the screen and there was also the counter on the top right of the screen.

So it looks like it's working correctly bearing in mind just the one 6520 in place.

Last edited by retromit; Yesterday at 9:01 pm. Reason: typo
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Old Yesterday, 9:09 pm   #186
ajgriff
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Default Re: Commodore PET 2001 voltage regulators

Faulty PIA originally in UC7 from the looks of things. There are quite a few reasonably priced equivalents that will work if you need to buy one. Can advise if needed. Some interfaces won't work without UC6.

Alan
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Old Yesterday, 9:15 pm   #187
retromit
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Default Re: Commodore PET 2001 voltage regulators

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Originally Posted by ajgriff View Post
Faulty PIA originally in UC7 from the looks of things. There are quite a few reasonably priced equivalents that will work if you need to buy one. Can advise if needed. Some interfaces won't work without UC6.

Alan
Hi Alan,

It certainly seems that. I was just reading about why the PET had two 6520s, and although it was for an 8032, the document stated that one 6520 was for the keyboard and speaker, with the other being used for the IEEE-488 interface. I take it that this would be the same for the 2001-N?

If that's the case then if you could recommend any replacements which I can get, then that would be great.

Thanks,
Tim.
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Old Yesterday, 10:18 pm   #188
ajgriff
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Default Re: Commodore PET 2001 voltage regulators

Yes I think the PIA architecture was much the same in all the PETs. The Western Design Centre WC65C21N was the best buy I could find when repairing my 2001-8 and it worked perfectly. I got it here:

https://www.ebay.co.uk/itm/164581033...wAAOSwjVhf19ZU

Alan
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