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Thread: Commodore PET 2001 RAM adapter?

  1. Default Commodore PET 2001 RAM adapter?

    I've got an early Commodore PET 2001 that needs a few parts. The Ram/Rom had gone bad, and I replaced those with one of the Rom/Ram replacement boards from Tynemouth Software (https://www.tindie.com/products/tyne...commodore-pet/) which got it booting.

    However, it still has an issue. The letters appear and react to key presses, but they flicker and change what characters they are constantly. It looks like my video RAM is bad and needs to be replaced.

    I know a RAM adapter was made previously by D'Asaro Designs (http://www.dasarodesigns.com/product...1-ram-adapter/), but they appear to no longer be manufacturing them.

    Does an adapter still exist? Or perhaps specs to manufacture one? I have to believe there's some solution to deal with bad video RAM that I just haven't run across.

    If there is still an adapter for individual RAM and ROM chips out there, I'd definitely be interested. I'm fine with using the replacement motherboard, but if a product exists, I'd be even happier to replace all the chips individually.

  2. #2
    Join Date
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    It may be the video RAM, it may not be.

    It may be the character generator ROM. It may be other things.

    Any chance of posting a video of your problems?

    You can also download a copy of my PETTESTER and see if you can get it burnt into a 2716 EPROM. This replaces the EDIT ROM in your machine. It will test the video RAM, ROM and DRAM for you.

    Dave

  3. #3

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    Quote Originally Posted by Dubis7 View Post
    I've got an early Commodore PET 2001 that needs a few parts. The Ram/Rom had gone bad, and I replaced those with one of the Rom/Ram replacement boards from Tynemouth Software (https://www.tindie.com/products/tyne...commodore-pet/) which got it booting.

    However, it still has an issue. The letters appear and react to key presses, but they flicker and change what characters they are constantly. It looks like my video RAM is bad and needs to be replaced.

    I know a RAM adapter was made previously by D'Asaro Designs (http://www.dasarodesigns.com/product...1-ram-adapter/), but they appear to no longer be manufacturing them.

    Does an adapter still exist? Or perhaps specs to manufacture one? I have to believe there's some solution to deal with bad video RAM that I just haven't run across.

    If there is still an adapter for individual RAM and ROM chips out there, I'd definitely be interested. I'm fine with using the replacement motherboard, but if a product exists, I'd be even happier to replace all the chips individually.
    I've repaired quite a lot of PETs (many 2001 with 6550/6540), it may or may not be the video RAMs, have you tried to swap them? Have you tried to use some other chips? I my experience it isn't usual that ALL the 6550 go bad. In my own 2001 for example, 50% of the
    chips were bad, but the other 50% is working fine.
    There's an adapter for sale here:
    https://www.arcadeshopper.com/wp/?pa...0&sort=nameAsc

    That's designed by me. If you want to make your own (4 x SMD small chips under the socket), I can provide the gerber files to order your PCBs.
    It uses 2114 RAM chip to replace a 6550 chip.

    Frank IZ8DWF

  4. Default

    I grabbed a video to show what I'm talking about. https://youtu.be/IQ6ptCQrzbM

    I think my character ROM is fine because you can see the characters being generated, but I could be mistaken.

    I realize I was mistaken. I thought all the RAM chips in the early PETs were 6550, but if I'm understanding correctly the video RAM is a standard 2114 and doesn't need an adapter?

    Regardless, I'm definitely interested in picking up some standalone adapters for my main RAM and ROM chips, and it looks like you have both. Do you also sell the EProms to go in the ROM adapters, or will I need to find someone to burn a new set?

  5. #5

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    Quote Originally Posted by Dubis7 View Post
    I grabbed a video to show what I'm talking about. https://youtu.be/IQ6ptCQrzbM

    I think my character ROM is fine because you can see the characters being generated, but I could be mistaken.

    I realize I was mistaken. I thought all the RAM chips in the early PETs were 6550, but if I'm understanding correctly the video RAM is a standard 2114 and doesn't need an adapter?

    Regardless, I'm definitely interested in picking up some standalone adapters for my main RAM and ROM chips, and it looks like you have both. Do you also sell the EProms to go in the ROM adapters, or will I need to find someone to burn a new set?
    First of all: from the video, I think that defect might be really either the ROM, the VRAMs or any associated logic and even a combination
    of more than one fault. Sure it looks like the Char ROM addresses aren't stable, but that might be caused by bad RAMs (you should
    really try to swap the two VRAM's positions and see if the flickering chars change shape) or might be caused by unstable VRAM addresses
    (the '157 multiplexers could be bad, or dirty/bad sockets or bad solder joints) or even bad/floating address decoding inside the char ROM.
    There's always need for good troubleshooting to avoid just blindly swapping parts.
    I've showed quite a few PET repairs on my youtube channel:
    https://www.youtube.com/iz8dwf
    If you have a 2001 revision with 6550 RAMs, then also the VRAMs are 6550, if you have a 2114 based 2001 PCB, then also the VRAMs are 2114. I've never seen any hybrid board (Though I can't exclude that they actually exist).
    The adapters for sale on that site are my own design, but I'm not the owner of that site. So I'd need to contact him to see if he could program the EPROMs. I live in Italy and of course I could do adapters and EPROMs but shipping overseas would cost much more than
    typical shipping inside the same continent.

    HTH
    Frank IZ8DWF

  6. #6
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    First thing. Is your character generator ROM in a socket? If so, can you pull it and tell us what you see on the screen (or post another video) please.

    Second thing. Are your video RAMs in sockets?

    Dave

  7. #7

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    Quote Originally Posted by daver2 View Post
    First thing. Is your character generator ROM in a socket? If so, can you pull it and tell us what you see on the screen (or post another video) please.

    Second thing. Are your video RAMs in sockets?

    Dave
    All 2001 boards I've seen have all RAMs and ROMs socketed. If he removes the char ROM we'll see the solid screen I'm sure, since characters on some screen position (or some characters out of the ROM) are good, so all logic *after* the ROM output is already working good.
    If he removes the VRAMs we might see flickering stuff (that points to some bad ROM or bad socket/solders) or still some solid screen and that means bad VRAMs and/or bad addressing logic to the VRAMs.
    I've already asked a few times if he could simply swap positions to the two VRAMs.

    Frank IZ8DWF

  8. Default

    I've created another video to try and answer all your questions. I show my display with and without the video ROM chip. https://youtu.be/U5ikSsLh_bI

    This isn't a hybrid board. It has 6550 RAMS throughout. I got turned around before because everyone kept mentioning 2114 chips, but if my understanding is correct I still need that adapter board to use the 2114 chips in my unit. I've ordered two, one for each VRAM. At the very least I'll more or less future proof it that way.

    I did try swapping some of my standard RAM chips in for the VRAM chips. That didn't appear to have any impact, so perhaps the VROM is my issue? If so, where would could I get a new EPROM burned? I know I'll need to grab an adapter board for that as well.

  9. #9
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    I have adapter boards that convert 6550 to 2114 i think they are dated 1980. Theae are not for individual ics but for yhe primary ram banks . Pm me if interested. These are the adapter boards http://mikenaberezny.com/hardware/pe...4-ram-adapter/

    iZ8dwf. I have a 1977 6550 2001 board and a 1978 2001 2114 board. There are NO boards that mix the two types of ram.

    Dubis my 2001 which had 6550 ram had bad video ram. Tynemouth sold me a ram board for my video ram.
    Last edited by VERAULT; June 14th, 2020 at 05:42 PM.

  10. #10

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    Quote Originally Posted by Dubis7 View Post
    I've created another video to try and answer all your questions. I show my display with and without the video ROM chip. https://youtu.be/U5ikSsLh_bI

    This isn't a hybrid board. It has 6550 RAMS throughout. I got turned around before because everyone kept mentioning 2114 chips, but if my understanding is correct I still need that adapter board to use the 2114 chips in my unit. I've ordered two, one for each VRAM. At the very least I'll more or less future proof it that way.

    I did try swapping some of my standard RAM chips in for the VRAM chips. That didn't appear to have any impact, so perhaps the VROM is my issue? If so, where would could I get a new EPROM burned? I know I'll need to grab an adapter board for that as well.
    I don't recall what size the 6550 RAMs are. When you swap them around, pay attention to how the flakiness looks exactly the same of differs. If moving them around doesn't change the address, you may be looking at a slight difference. Thinks like ti was change some letter from as A to B. but after moving is around it was changing F to Z ( no real values here ). It is still is a difference. The point is, it may have changed. What the letters change from to is a hint as to what may be the problem. Don't expect a bad RAM in location A moved to location B to fix the problem. What is important is exactly how the problem changed or remained exactly ( I really means exactly ) the same.
    Dwight

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