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Thread: New PET 2001, garbled screen.

  1. #1
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    Default New PET 2001, garbled screen.

    I was finally able to acquire a blue surround/blue label PET 2001. It has a couple of interesting mods including an upgraded (I think) 320351 16k motherboard. It has D416C-1 RAM chips and I am getting a garbled screen on start up. I've re-seated all of the socketed chips and got it to this point (it started up with a blank screen before). I am assuming it's bad RAM.

    Blue PET - front.jpg

    Since the RAM is soldered in, can I piggyback a RAM chip to see if it helps? Here's what the screen looks like:

    Blue PET - garbled screen.jpg

    More pictures here showing some of the mods. It includes a fan and what looks to be an internal S100 slot?

    Any help is much appreciated (including what the mod is).

  2. #2
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    The little board by the PET power supply unit looks like another dc supply including rectifier diodes and a smoothing capacitor.

    I would agree about the extra socket - it looks interesting! My question is: is there something on the PET motherboard for the connector on the flying lead to plug into? It looks like a large-ish edge connector to me; with no matching tongue to mate with. I wonder if this means that there was an adapter board on the original PET mating with this connector that you don't have?

    By all means try the piggybacking trick with the RAM. If it was me though I would start by checking the power rails, reset pin, clock pins and interrupt pins first. Also remove some of the "non essential" socketed ICs - as their failure can stop the CPU from executing correctly.

    There are plenty of things to check first before suspecting a RAM fault (that are easier to check and repair).

    Just a thought...

    Nice acquisition though!

    Dave
    Last edited by daver2; October 10th, 2017 at 11:48 PM.

  3. #3

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    Don't early PETs have an edge connector on the right hand side of the board? I don't have one, but I thought they did, and one that looks just like it would mate with that connector.

    My only one, a later board, has a pin header there, but the cutout in the case makes more sense for a card edge.

  4. #4
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    That's what I thought - but the main board PCB on the site that snuci linked to didn't appear to have the edge connector I was expecting to see - however, I had just woken up when I checked so I could have been half asleep at the time !

    EDIT: Just checked on-line and (yep) the (original) PET 2001 had an edge connector in place of the IDC headers for the memory expansion.

    If it doesn't have an edge connector - I wonder if the main board has been swapped out and it is not the original?

    I can't check the photographs out from work - as file sharing sites are blocked.

    Dave
    Last edited by daver2; October 11th, 2017 at 01:29 AM.

  5. #5

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    I've got the same issue here, can't see them except on my phone where the page won't let me zoom.

    That must be the same board I have, which according to mine is a "2.0 Upgrade board" or something to that effect. In that case, the RAMs are 2114 (I think) and I'd shotgun replace them before doing any other diagnostics, only because they most likely are bad.

    If the missing board was a memory expansion, it would make sense to remove it when upgrading the main board.

  6. #6
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    Here's a picture of the original PET motherboard taken from http://www.dasarodesigns.com/wp-cont...ardLabeled.jpg (I am posting these as I assume you can see pictures here?)

    PET2001BoardLabeled.jpg

    You can see the edge connector on the bottom of the board. The 80 pin connector on the mod that I have DOES fit into the slot on the side of the PET 2001 case and I think there's enough clearance to close the case as well so I believe we have a good theory here as it should fit on the edge connector. I haven't measured the voltage out of the extended power supply components but maybe it provides additional voltages that an S-100 memory card needs? The fan is situated to the left of the s-100 card (if inserted component side up) where the voltage regulators normally are so this makes sense why there is a fan. HUH Electronics did have an S-100 expansion unit for the Commodore PET noted here http://chiclassiccomp.org/docs/conte...UH_Catalog.pdf but it was a whole unit with power supply. I believe it connected to the same 80 pin card edge slot. Things seem to line up that the modifications may be to add an S-100 memory board to the original 4k or 8k PET board perhaps?

    In terms of this 320351 motherboard (what I had mentioned was probably upgraded), I did have 8 TMM416D-4 chips that I thought might work so I tried them. I first piggybacked one chip onto each memory chip and turned on the computer in between chip fittings and I always got the same screen. I then piggybacked a full 8 chip row and did this with both rows and got the same result. I was going to try the video memory chips next but I'm not sure I have 2114s. On occasion, if I wait a while to start it up, I get a blank screen. At one point, I piggy backed one chip and then got the garbled screen. Removed the chip and continued to get the garbled screen.

    I'd like to fix this board and swap it with one of my other 2001s. I do have another 2001 (let's call it PET 2) with original board shown above but it also has issues but I should separate that into another post. It just shows a segmented line on screen (vertical collapse with bars that look like dashes because of the collapse).

    I'll try the video memory next on PET 1 if I can find the memory chips.

    Thanks for the replies guys.

  7. #7
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    That makes sense now...

    I (personally) wouldn't bother with the video memory at this stage of the game. Extremely unlikely that this will be the problem.

    I would go back to checking to see if the CPU is doing anything sensible...

    Dave

  8. #8
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    Quote Originally Posted by daver2 View Post
    I would go back to checking to see if the CPU is doing anything sensible...
    Hi Dave,

    I will check power first according to the schematics. I'll have to do a little research on how to check the reset, clock and interrupt pins.

    Thanks.

  9. #9
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    If you look at my posts on The PET threads you should find the information you require.

    If not, let me know and I will describe it for you.

    Have you an oscilloscope?

    When checking the voltages, check them across the pins of the rows of ICs after checking the voltage rails from the regulators. I have seen the odd occurrence where most of the ICs were being powered - but one row wasn't! That threw me a bit... Admittedly, not a frequent fault - but worth testing whilst you have the multimeter/oscilloscope probes in your hands!

    Dave

  10. #10
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    Quote Originally Posted by daver2 View Post
    Have you an oscilloscope?
    Yes, we fixed one of my other 2001-8's in the past here: http://www.vcfed.org/forum/showthrea...s-for-testing/ and I haven't used it since, to be honest. While I have a scope, I don't necessarily know how to use it beyond the very basics.

    I will check voltages across various ICs as well as the rails.

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