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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 23rd Oct 2021, 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 23rd Oct 2021, 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 23rd Oct 2021, 8:39 pm   #183
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Default Re: Commodore PET 2001 voltage regulators

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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 23rd Oct 2021, 8:58 pm   #184
retromit
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Default Re: Commodore PET 2001 voltage regulators

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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 23rd Oct 2021, 9:00 pm   #185
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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; 23rd Oct 2021 at 9:01 pm. Reason: typo
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Old 23rd Oct 2021, 9:09 pm   #186
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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 23rd Oct 2021, 9:15 pm   #187
retromit
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Default Re: Commodore PET 2001 voltage regulators

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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 23rd Oct 2021, 10:18 pm   #188
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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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Old 24th Oct 2021, 8:31 am   #189
SiriusHardware
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Default Re: Commodore PET 2001 voltage regulators

A lot of progress has been made since I last swung by - a nice bit of luck for it to be socketed parts which have failed, and ones which are not too difficult to get replacements for.

Why it would be those parts which would fail when the +5V went down is anyone's guess. Ours not to reason why, I suppose.
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Old 24th Oct 2021, 8:55 am   #190
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Default Re: Commodore PET 2001 voltage regulators

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Why it would be those parts which would fail when the +5V went down is anyone's guess. Ours not to reason why, I suppose.
Old MOS chips which didn't take kindly to an intermittent supply rail combined with frequent on/offs during fault finding?

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The Western Design Centre WC65C21N ....
Should say W65C21N of course. Spurious 'C' introduced by incompetent late night typist.

Alan
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Old 24th Oct 2021, 9:03 am   #191
SiriusHardware
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Default Re: Commodore PET 2001 voltage regulators

I would tend to think (as indeed you originally suggested) that the ICs which run on multiple rails, ie, the RAM, would be the first to object to one of those rails going AWOL.

Assuming the W65C21N will solve the PIA problem, this still leaves Tim in need of a replacement 6502. Does anyone have one in their back pocket?
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Old 24th Oct 2021, 9:18 am   #192
Slothie
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Default Re: Commodore PET 2001 voltage regulators

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Originally Posted by SiriusHardware View Post
A lot of progress has been made since I last swung by - a nice bit of luck for it to be socketed parts which have failed, and ones which are not too difficult to get replacements for.

Why it would be those parts which would fail when the +5V went down is anyone's guess. Ours not to reason why, I suppose.
Anecdotal evidence suggests the 6520 was not a particularly reliable chip in the first place. It may just have self-destructed with the passage of time, not an uncommon event with MOS/CSG chips...
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Old 24th Oct 2021, 9:20 am   #193
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Default Re: Commodore PET 2001 voltage regulators

At least 6502 CPUs are fairly easy to obtain if Tim needs to buy one. This is just one example picked at random:

https://www.ebay.co.uk/itm/174965990...cAAOSwApBhXJo9

Alan
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Old 24th Oct 2021, 9:27 am   #194
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Default Re: Commodore PET 2001 voltage regulators

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It may just have self-destructed with the passage of time
Looking back over the whole thread it's difficult to know whether the 6520 was or was not faulty before Tim acquired the machine, as it had a more obvious screen RAM fault which he did fix. I don't know how much of a chance he got to test things out before the +5V went down.

What does seem clear is that the 6502 was working until that point and wasn't from then onwards, so I would be inclined to think that the same trauma which took out the previously working 6502 also damaged the 6520 at the same time.
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Old 24th Oct 2021, 11:22 am   #195
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Default Re: Commodore PET 2001 voltage regulators

Hi Tim - I have a spare (new) 6502 - see photo - which I have popped into my 3016 PET and checked that it works which you're welcome to if you want.

I also have a spare 40 pin turned-pin socket if you need it. Let me know.

Colin.
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Old 24th Oct 2021, 1:08 pm   #196
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Default Re: Commodore PET 2001 voltage regulators

You could always add an accelerator to it https://blog.adafruit.com/2021/10/13...tagecomputing/
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Old 24th Oct 2021, 3:07 pm   #197
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Default Re: Commodore PET 2001 voltage regulators

More conventionally, I wonder if Tim has any kind of storage for the machine, like a datasette cassette unit. If not, I understand there are Arduino-based units which aim to emulate such things and there's also the 'SD2Pet' SD card interface but the latter requires a working IEEE interface.

ScottishColin was considering one or other of these a while back but had to put it on the back burner at the time due to the expense involved in fixing the machine itself - not sure if he went ahead with something like that in the end, as he did have a working cassette unit for his.
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Old 24th Oct 2021, 3:42 pm   #198
retromit
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Default Re: Commodore PET 2001 voltage regulators

Quote:
Originally Posted by ajgriff View Post
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
Hi Alan,

Thanks for the link. I've just ordered one of these, so hopefully it will fix the issue and the PET will be back to a full complement of 6520s!

Tim.
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Old 24th Oct 2021, 3:50 pm   #199
retromit
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Default Re: Commodore PET 2001 voltage regulators

Quote:
Originally Posted by SiriusHardware View Post
Quote:
It may just have self-destructed with the passage of time
Looking back over the whole thread it's difficult to know whether the 6520 was or was not faulty before Tim acquired the machine, as it had a more obvious screen RAM fault which he did fix. I don't know how much of a chance he got to test things out before the +5V went down.

What does seem clear is that the 6502 was working until that point and wasn't from then onwards, so I would be inclined to think that the same trauma which took out the previously working 6502 also damaged the 6520 at the same time.
Yes, I'm not too sure on the 6520. When the screen RAM was fixed, the PET was briefly working. I did try the keyboard at this time, but with nothing being displayed on the screen (from keys pressed) I did wonder if the keyboard needed fixing. After that I didn't have too much time to test as I was then hit with the +5V issue. It could have been that the 6520 was already gone (with the keyboard not working, and with the 6520 in UC6 the keyboard looks to be okay), but the +5V took out the processor (and maybe the 6520?). It's just fortunate I had the spare from the VIC-20 to be able to test the processor.

Hopefully with the replacement processor, and also the replacement 6520, the PET should be in a better place.

I do have a quick question about the daver2 test EPROM. It starts to run on power up, but I've left it running so it cycles around the 32k RAM test. Does it just keep cycling, or is it waiting for input in order to perform more tests?

Tim.

Last edited by retromit; 24th Oct 2021 at 4:01 pm. Reason: typo
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Old 24th Oct 2021, 3:57 pm   #200
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Default Re: Commodore PET 2001 voltage regulators

Quote:
Originally Posted by SiriusHardware View Post
More conventionally, I wonder if Tim has any kind of storage for the machine, like a datasette cassette unit. If not, I understand there are Arduino-based units which aim to emulate such things and there's also the 'SD2Pet' SD card interface but the latter requires a working IEEE interface.

ScottishColin was considering one or other of these a while back but had to put it on the back burner at the time due to the expense involved in fixing the machine itself - not sure if he went ahead with something like that in the end, as he did have a working cassette unit for his.
When I bought the PET it did come with a datasette and a selection of tapes. I've added a few pictures to show these (as well as a couple of books). There were also a number of other tapes, but these don't have labels, etc., so not sure what could be on them.

I haven't had a chance to go through any of these due to the issues faced. So I'm hoping to get something tested to see if any of the tapes load. I'm also thinking to put the BASIC toolkit back into UD5 as well to make sure it works in the PET - with the leg being repaired I'm hoping it would be okay, but would it be better (safer) to potentially burn a new EPROM for this?

Now that it seems to be functioning, it will be good to get some further tests carried out.

With you mentioning the SD solutions, I have seen one advertised from The Future was 8 bit. Has anyone any experience of the SD2PET?

Tim.
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