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PET 2001 Booting Issues

ryanhayward

Member
Joined
Jun 14, 2022
Messages
42
Location
St. John's Newfoundland and Labrador Canada
Greetings All,

I have recently become the proud owner of a 2001 and I am having strange issues with it. At first we thought it was a simple failed CPU or bad video ram as I was getting sing characters filling up my screen. So a nice fellow named Mark over on the PET facebook group sold me a few solid upgrades which I had planned on doing anyway and would likely help fix my issues, two birds stoned in the bush at once or something to that affect. Anyway after some trouble shooting I have come to find that the video rom appears to be working fine, as is the CPU (tested in an apple ii) so the only replacement part I have installed is the video ram upgrade. I will post some photos below. When the PET is first turned on, I get the standard random character screen but the CPU doesn't appear to reset and it just hangs on the character screen. Any thought on how I should proceed? I have been in IT for well over 20 years but I am not an electronics expect but I can manage.

Thanks Everyone!

Cheers,
Ryan
20220614_104524.jpg20220614_104639.jpg20220614_104648.jpg
 
Hi Ryan,
Welcome to the Vintage Computer Forum.

It seems to me that the displayed screen is not random characters. You say the 'video ROM' is OK. If you mean the character generator ROM, I would not think so. Do you have an adapter to try a 2716 EPROM to replace the 6540 character ROM?

Or can you temporarily remove the Video RAM replacement board you have installed, and with no Video RAM, if the character ROM is good, you should see a checkerboard pattern on the screen.
 
He's also got a tynemouth ram/rom board with the PETTESTE2KV04.bin rom on it.
(It had the wrong roms for a non crtc, so I whipped up a replacement rom)
 
Hello and welcome. As stated the 6502 CPU is the wrong way round and that's not a random character screen. The first image below shows the correct orientation for the CPU and the second image is an example of a random character screen.

Alan


PET 320008 Layout.jpgPET Start.JPG
 
Hi Ryan and welcome to VCFED.

Yep, the CPU in backwards won't help I am afraid...

Don't forget to add your location to your profile - so we know roughly where you are in the world.

You will also be under moderation - so you will need to get 10 posts under your belt before your posts will appear without moderation.

Since you potentially have the CPU in backwards - it would be sensible to double-check everything else before proceeding?

Yes, that is a strange 'non random' screen image you have there. There is something suspect with the hardware somewhere (even if you put the CPU in the correct way round and it still works).

Also note that the Tynemouth card DOESN'T have my version of the PETTESTER on it (as of today at least).

Do you have any test equipment (e.g. multimeter, logic probe, oscilloscope etc.)?

Dave
 
Last edited:
Hey All, thanks for the responses!
And it looks like your cpu is in backwards.
I had the CPU in the way it was when the computer landed in my possession, switching it around doesn't seem to change the screen at all.
Hi Ryan,
Welcome to the Vintage Computer Forum.

It seems to me that the displayed screen is not random characters. You say the 'video ROM' is OK. If you mean the character generator ROM, I would not think so. Do you have an adapter to try a 2716 EPROM to replace the 6540 character ROM?

Or can you temporarily remove the Video RAM replacement board you have installed, and with no Video RAM, if the character ROM is good, you should see a checkerboard pattern on the screen.

Dave removing the video ram does create the checkerboard pattern screen, so I assume that means the character rom is good.
Hello and welcome. As stated the 6502 CPU is the wrong way round and that's not a random character screen. The first image below shows the correct orientation for the CPU and the second image is an example of a random character screen.

Alan


View attachment 1242472View attachment 1242473
Thanks Alan, that is a great schematic

Hi Ryan and welcome to VCFED.

Yep, the CPU in backwards won't help I am afraid...

Don't forget to add your location to your profile - so we know roughly where you are in the world.

You will also be under moderation - so you will need to get 10 posts under your belt before your posts will appear without moderation.

Since you potentially have the CPU in backwards - it would be sensible to double-check everything else before proceeding?

Yes, that is a strange 'non random' screen image you have there. There is something suspect with the hardware somewhere (even if you put the CPU in the correct way round and it still works).

Also note that the Tynemouth card DOESN'T have my version of the PETTESTER on it (as of today at least).

Do you have any test equipment (e.g. multimeter, logic probe, oscilloscope etc.)?

Dave
Thanks Dave, I updated my profile, switched the CPU around (No change) removed the video ram got the checkerboard screen, I have access to a few tech tools, mostly an ancient Oscilloscope and I have several multimeters.

Any other suggestions, I also removed all but the 4 ram chips in J 1,2 L 1,2 as well as the ROM chips with the property switches flipped on my Tynemounth

Thanks guys, this group is super helpful!

Cheers,
Ryan
 
I just wanted to make sure that people didn't read the post and assume that it was present on the card as 'standard' when purchased.

I wanted to mention this for a reason...

Dave
 
The question is - now that the CPU has been plugged in the wrong way round - is it toast? Can you test this elsewhere again to make sure it is still working?

One rule I have (and I am working through this at the moment with some cards) is that "whoever had it last time didn't know what they were doing"!

Test (with your multimeter) the output rails from the voltage regulators to make sure they are in spec.

We can test if the CPU is running with a logic probe or an old-and-ancient oscilloscope...

The 'consistent @@ and other patterns' implies that there is something not well with the video display hardware somewhere. It is either a counter for the character on the screen or an address multiplexer I would guess at this point. I will have a look at the schematic to see where we would go next.

Dave
 
The question is - now that the CPU has been plugged in the wrong way round - is it toast? Can you test this elsewhere again to make sure it is still working?

One rule I have (and I am working through this at the moment with some cards) is that "whoever had it last time didn't know what they were doing"!

Test (with your multimeter) the output rails from the voltage regulators to make sure they are in spec.

We can test if the CPU is running with a logic probe or an old-and-ancient oscilloscope...

The 'consistent @@ and other patterns' implies that there is something not well with the video display hardware somewhere. It is either a counter for the character on the screen or an address multiplexer I would guess at this point. I will have a look at the schematic to see where we would go next.

Dave
Hey David

Thanks! Yes the CPU is still good, works fine in my Apple II, I checked all other chips and they all seem to be the right way around, as well I swapped out the Video Rom for a replacement unit and it still give the same type of screen not sure if that helps at all

Cheers,
Ryan
 
Thanks Alan, that is a great schematic
That board view does give the correct orientation of the IC's, but be aware that there are mistakes in part nomenclature in that drawing. It is laid out in vertical order rather than in grid coordinate per the schematic. For instance on the board view the 6502 CPU is shown as F1 as it is the first IC in the 'F' column, while in the schematic it is shown as F3, the closed grid location. It makes troubleshooting a little more difficult. Apparently Commodore never fixed that drawing.
 
That board view does give the correct orientation of the IC's, but be aware that there are mistakes in part nomenclature in that drawing. It is laid out in vertical order rather than in grid coordinate per the schematic. For instance on the board view the 6502 CPU is shown as F1 as it is the first IC in the 'F' column, while in the schematic it is shown as F3, the closed grid location. It makes troubleshooting a little more difficult. Apparently Commodore never fixed that drawing.
Ahh yes I see what you are saying. These PETs are cool old rigs for sure. Can't wait to get mine working to start using it for my Computer Science Class I will be teaching in September.
 
Yes and PIA2 is labelled MPS- 6320 instead of MPS- 6520. Shoddy work by someone at the time.

Alan
 
What happens if you fit the Tynemouth board with the switches set to take over all the RAM and ROM functions?

Alan
 
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