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Thread: Commodore PET 2001 IEEE-488 interface

  1. #1

    Default Commodore PET 2001 IEEE-488 interface

    Hello!

    I've moved on to solving problems on my 488 interface. I know that I have at least one failed MC3446 because one leg was disintegrated. The other two are unknown... However, I purchased a pair of MC3446N chips and tried swapping between them... The GPIO diags that Frank referred me to come back even worse with the new chips in place. With the old chips, the drives will light up and seek, but I get weird stuff at the top of the display and disk access is definitely not successful.

    I can't seem to find anything definitive. Can anyone tell me if the MC3446 chips are MC3446N or MC3446AP in the original? The schematics just have "MC3446" on them

    Apparently the difference is internal termination. Of course, with old parts, it's also entirely possible that both of the "new" chips are bad.

    Thanks!

    ps- Before you ask, yes, I've tried a number of different 6520s. No difference.

  2. #2

    Default

    Quote Originally Posted by dhoelzer View Post
    Hello!

    I've moved on to solving problems on my 488 interface. I know that I have at least one failed MC3446 because one leg was disintegrated. The other two are unknown... However, I purchased a pair of MC3446N chips and tried swapping between them... The GPIO diags that Frank referred me to come back even worse with the new chips in place. With the old chips, the drives will light up and seek, but I get weird stuff at the top of the display and disk access is definitely not successful.

    I can't seem to find anything definitive. Can anyone tell me if the MC3446 chips are MC3446N or MC3446AP in the original? The schematics just have "MC3446" on them

    Apparently the difference is internal termination. Of course, with old parts, it's also entirely possible that both of the "new" chips are bad.

    Thanks!

    ps- Before you ask, yes, I've tried a number of different 6520s. No difference.
    I have recently fixed a TTL PROM chip (out of a Disk II interface card) that kept on losing pins (iron oxide does that, as soon as the tin plating starts having holes), so if the only problem of the old 3446 was missing pins and there's still room to solder new pins, then I would attempt that. I have made a video of the chip repair process, if anyone is interested.
    Having said this, on my 2001N, one of the original MC3446 was shorted inside, and became very hot as soon as it was powered on, so I just got MC3446A from an italian ebay seller (as far as I remember). It worked fine, it doesn't seem there should be any difference between the -A and non -A chip revision.
    Where did you buy your replacements from? The logical explanation is you got fake/bad parts and I would attempt getting money back if I were you.
    Frank

  3. #3

    Default

    Quote Originally Posted by iz8dwf View Post
    I have recently fixed a TTL PROM chip (out of a Disk II interface card) that kept on losing pins (iron oxide does that, as soon as the tin plating starts having holes), so if the only problem of the old 3446 was missing pins and there's still room to solder new pins, then I would attempt that. I have made a video of the chip repair process, if anyone is interested.
    Having said this, on my 2001N, one of the original MC3446 was shorted inside, and became very hot as soon as it was powered on, so I just got MC3446A from an italian ebay seller (as far as I remember). It worked fine, it doesn't seem there should be any difference between the -A and non -A chip revision.
    Where did you buy your replacements from? The logical explanation is you got fake/bad parts and I would attempt getting money back if I were you.
    Frank
    Thanks Frank!

    Unfortunately, the leg disintegrated right at the plastic body, so there's no chance of a repair there.

    I purchased the chips via ebay, from China, so very little chance there for a refund; they were only a few dollars. I ordered a set of 10 from another seller with the thought that a larger proportion of them might still function.

    Also, rereading my first post, it's not clear that the system functions better with the original 3446 chips; with the replacements, it's totally DOA. With the old chips, I get crazy DS$ results, the top of the screen "animates" with junk when I try to format the disk (and locks up until I turn off the drive), the format fails, disk directories are completely blank, saving doesn't result in an error, but it definitely doesn't save, etc.

    Anyway, more when the replacements arrive. Based on the other problems this board has had, I'm planning to follow the traces to each of the 3446 pins just to be sure there's nothing else wrong.

    As far as the 2031LP and the 4040 drives that I have, the 2031LP and drive 0 on the 4040 *act* as though they work, but it's hard to be sure without a reliable PET... The D1 on the 4040 has lights, but it seems the drive motor never actuates. I'm going to end up taking the whole thing apart, I'm sure, but I think I'm going to save that until I move into my new office that has an actual electronics workbench for the FPGA and other circuits that I build.

    Thanks!

  4. #4

    Default

    Quote Originally Posted by dhoelzer View Post
    Thanks Frank!

    Unfortunately, the leg disintegrated right at the plastic body, so there's no chance of a repair there.

    I purchased the chips via ebay, from China, so very little chance there for a refund; they were only a few dollars. I ordered a set of 10 from another seller with the thought that a larger proportion of them might still function.

    Also, rereading my first post, it's not clear that the system functions better with the original 3446 chips; with the replacements, it's totally DOA. With the old chips, I get crazy DS$ results, the top of the screen "animates" with junk when I try to format the disk (and locks up until I turn off the drive), the format fails, disk directories are completely blank, saving doesn't result in an error, but it definitely doesn't save, etc.

    Anyway, more when the replacements arrive. Based on the other problems this board has had, I'm planning to follow the traces to each of the 3446 pins just to be sure there's nothing else wrong.

    As far as the 2031LP and the 4040 drives that I have, the 2031LP and drive 0 on the 4040 *act* as though they work, but it's hard to be sure without a reliable PET... The D1 on the 4040 has lights, but it seems the drive motor never actuates. I'm going to end up taking the whole thing apart, I'm sure, but I think I'm going to save that until I move into my new office that has an actual electronics workbench for the FPGA and other circuits that I build.

    Thanks!
    I have the habit of never buying NOS chips from China (or other old parts), since I've got fakes or bad parts in the past.
    Today I've ordered a MB7052 TTL PROM from a german seller, for a ridicoulus high price. If I'd order the same part from chinese seller it would be like 1/4 of the price but it would probably have problems, so why should I risk wasting my time and actually end up spending more?
    I could have a good deal if the parts were genuine, but in my experience the odds are not many, and surely I wouldn't take a chance on a programmable ONCE part like the one I need this time.
    I could risk on MC3446 probably, but not after reading what you got

    Frank

  5. #5

    Default

    It can sometimes be hard to find genuine vintage old stock IC's, with all the fakes coming out of the far east.

    To get around this I have some trusted suppliers that often have very difficult to get old stock parts, transistors & IC's. Here is particularly good one and the prices are usually quite reasonable:

    https://www.sh-halbleiter.de

    Search your IC and I think they will stock it for the MC part.

  6. #6

    Default

    Quote Originally Posted by Hugo Holden View Post
    It can sometimes be hard to find genuine vintage old stock IC's, with all the fakes coming out of the far east.

    To get around this I have some trusted suppliers that often have very difficult to get old stock parts, transistors & IC's. Here is particularly good one and the prices are usually quite reasonable:

    https://www.sh-halbleiter.de

    Search your IC and I think they will stock it for the MC part.
    Hey, thanks! This is why I've often ended up with a chinese seller.

  7. #7

    Default

    I've "repaired" several chips with pins broken off flush to the plastic by filing away some of the plastic.

  8. #8

    Default

    Quote Originally Posted by KC9UDX View Post
    I've "repaired" several chips with pins broken off flush to the plastic by filing away some of the plastic.
    Hmm.. that is a good suggestion, but that chip went out with the trash a few days ago.

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