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Commodore PET 2001-8 stuck on garbled screen

Follow Nivag's advice and try the ROMs one at a time in the socket he recommends.

If you get one that results in the "random screen of death", remove that ROM, put it to one side as "dead" and carry on with the others.

We can then determine which ROMs appear to be good and which ones not.

We can repeat a similar test (later on) for the RAM. Get a set of RAMs that work (as now) and replace ONE of the good RAM devices with ONE of the unknown ones and test with PETTESTER. If that test fails - the RAM is duff. If that test passes, swap that same device for another unknown one. Any that fail the test are obviously duffers. We can take the rest forwards and test those further with PETTESTER.

Dave
 
Ok rom test done.
H1 e85f ok
H2 31fc ok
H3 ca92 ok
H4 garbage sceen
H5 Ic100 which I guess is faulty as no checksum match?
H6 i349 ok
H7 e88e also faulty?

So 4 seem to be ok in that they show a checksum.
I have written on there checksums with pencil.
 

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Assuming you have a typo you could populate H1, H2, H3 and H6. H2 and H6 form a pair so that might make the D checksum match daver2's documentation.

Looks like the rest are dead... You need to go ROM shopping.

Over to daver2 for RAM testing....
 
I would suggest redoing your test for H5 and H6 (and H7 while you are at it) - as your checksum doesn't appear to be correct for these parts.

Dave
 
BTW... I think E85F is probably quite rare it... It is the original Basic 1 ROM with all the initial release bugs! My PET has the E7CB variant.

A true collector would probably appreciate the bugs!
 
That's a bit disappointing... I don't think that is the correct D answer... Maybe clean the legs and contact clean the sockets?

Being Checksum-16 you can add 31FC+1E49=5045

BTW... These checksums are quite weak with respect to errors... It is a suggestion they are correct rather than a definitive statement they are correct. You should dump them and compare every bit to be super confident but this isn't a bad start.
 
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Never thought I'd see my favourite game running on this pet. Using the Tynemouth. Got the tape drive working on J6. Swapped out via A3 6520 for a 6522 from my other pet and it worked. Looks like J3 will need some work. Probably those transistors dave mentioned.
 

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That's a bit disappointing... I don't think that is the correct D answer... Maybe clean the legs and contact clean the sockets?

Being Checksum-16 you can add 31FC+1E49=5045

BTW... These checksums are quite weak with respect to errors... It is a suggestion they are correct rather than a definitive statement they are correct. You should dump them and compare every bit to be super confident but this isn't a bad start.
I will clean up the contacts on the legs and sockets and try again.
 
Checksums are weak - but in the opposite way... If the checksum is correct the ROM contents may still be wrong (no address dependency is included in the algorithm). So, for example, the ROM may be coded 'backwards' and you would still get the correct checksum - but it won't work!

Just triple-check which sockets you are really plugging the ROMs into...

Confused (post #311) ... A VIA is a 6522. The 6520 is a PIA?

Space invaders rule!

Dave
 
Checksums are weak - but in the opposite way... If the checksum is correct the ROM contents may still be wrong (no address dependency is included in the algorithm). So, for example, the ROM may be coded 'backwards' and you would still get the correct checksum - but it won't work!

Just triple-check which sockets you are really plugging the ROMs into...

Confused (post #311) ... A VIA is a 6522. The 6520 is a PIA?

Space invaders rule!

Dave
I guess that's why it didn't work. I thought they were the same chip but 6522 was a later version with more features.
Roms, I guess the reality is that I won't find out if the four that actually read correctly will work until I get replacements for the other three.
I plan on reading their contents at some point in the future and comparing them too the bin files.
Going forward I do not want to be using the tynemouth board.
I want a complete set of basic 1 roms.
 
Someone asked that very question not so long ago on a Commodore PET thread that I answered... Some of the pins have different functions between the 6520 and 6522. Fortunately the power pins are the same, otherwise you could have done some damage.

I am guessing that they won't work because the 'D' checksum is not correct for the H2/H6 ROM pair. I have just tried to work out if you have accidentally paired the wrong ROMs, but you haven't...

I subtracted each individual ROM checksum from 50D6 to see if the result matched up to another ROM...

It would be interesting to see if the ROM checksum for that pair is constant during the countdown.

Dave
 
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Someone asked that very question not so long ago on a Commodore PET thread that I answered... Some of the pins have different functions between the 6520 and 6522. Fortunately the power pins are the same, otherwise you could have done some damage.

I am guessing that they won't work because the 'D' checksum is not correct for the H2/H6 ROM pair. I have just tried to work out if you have accidentally paired the wrong ROMs, but you haven't...

I subtracted each individual ROM checksum from 50D6 to see if the result matched up to another ROM...

It would be interesting to see if the ROM checksum for that pair is constant during the countdown.

Dave
Hi Dave
I was going to start helping but it looks like you are making progress. I was thinking of doing more intensive work with the noop and two scope probes. The video clearly looked like the video ram was getting multiple write enables that didn't belong. The note about the 74154 was likely this original failure and an attempt to fix it without a complete check using the noop test.
The bad ROM is a serious issue. I'm not sure which PET this is but most of them have partial address decoding in the ROM, making a simple EPROM replacement unable to fix it.
As I recall, only the edit ROM can be directly replaced with an EPROM.
That doesn't mean an EPROM can't be used, it only means on had to make an adapter to include the missing decoder. This can be easily done with some 74xx parts.
I was wondering what the vertical clearance was in the ROM area. One could easily use some vector board to include the logic and EPROM if it were mounted sideways, sticking up.
One could even make it extend over the other logic and not raise it too high but I think putting it in at right angles to the board would be the simplest ( just my opinion, we all have one ).
Anyway, I just wanted to say this because many get too discouraged when they find a bad ROM. I would never let that stop me.
Dwight
 
I should also note that having two adjacent ROMs bad means that the vector board could be flat over both ROM and use a single decoder with a single EPROM.
Dwight
 
This is an original 2001 PET, so has the strange 6540 ROMs. There are plenty of 6540 to 27xx adapters available...

Dave
 
I should also note that having two adjacent ROMs bad means that the vector board could be flat over both ROM and use a single decoder with a single EPROM.
Dwight
My ROM replacements can do pairs as suggested...

1674035340704.png

using exactly the decode logic you are talking about. It really depends on the aesthetics you are after... some people like only originals, some people like adapters, some people like every socket populated. My personal take, is that making your own enhancements with contemporary parts is part of the game... that's what I did back in the day!

Using an EEPROM or UVEPROM with a simple adapter is definitely, by far, the cheapest... looks ugly to me ;o) If you can tolerate empty sockets then you don't need much decoding as you have 4K boundaries; for complete population 2K (and inversion of select in some sockets).
 
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