Image Map Image Map
Results 1 to 8 of 8

Thread: PET transformer question

  1. #1

    Default PET transformer question

    Hi, I'm new here. After doing many searches for answers to my questions but not finding any, I noticed a common thread -- the folks on here seem to know PETs extremely well. Hopefully you may be able to help me get mine working.

    I recently picked up a PET 2001n-32. I've already done some work on the board and made some positive progress, but when I got stuck I decided to start from the beginning where I should have initially started: testing voltage.

    So, I started with the voltage regulators on the board and they all checked out except VR5 which is a 7812. It's putting out 9V, when I think it should be 12V (but correct me if I'm wrong). I then went to the transformer leads and I found a schematic on zimmers.net so I'd know what the voltages should be. However, and this is where I absolutely have to be doing something stupid, I'm reading zero volts from every connection. There's power going to the board, so I'm extremely confused by this. Any ideas what I'm doing wrong?

    I'm going to be heading to bed in a bit after giving up on this for the day, so if I don't answer right away I do apologize. I'll look in the morning and hope to find some helpful replies.

    Thanks for the excellent forum, and thanks in advance for any help you can offer.

  2. #2

    Default

    Quote Originally Posted by cihs View Post
    Hi, I'm new here. After doing many searches for answers to my questions but not finding any, I noticed a common thread -- the folks on here seem to know PETs extremely well. Hopefully you may be able to help me get mine working.

    I recently picked up a PET 2001n-32. I've already done some work on the board and made some positive progress, but when I got stuck I decided to start from the beginning where I should have initially started: testing voltage.

    So, I started with the voltage regulators on the board and they all checked out except VR5 which is a 7812. It's putting out 9V, when I think it should be 12V (but correct me if I'm wrong). I then went to the transformer leads and I found a schematic on zimmers.net so I'd know what the voltages should be. However, and this is where I absolutely have to be doing something stupid, I'm reading zero volts from every connection. There's power going to the board, so I'm extremely confused by this. Any ideas what I'm doing wrong?

    I'm going to be heading to bed in a bit after giving up on this for the day, so if I don't answer right away I do apologize. I'll look in the morning and hope to find some helpful replies.

    Thanks for the excellent forum, and thanks in advance for any help you can offer.
    7812 should produce around 12V. What pins are you checking? What's the voltage on the Input side of the regulator? Did you check the diodes before the regulator (powered off, multimeter with diode check)? Did you check the capacitor for leakage/high ESR?
    All important voltages on a 2001N board can be checked on any 4116 RAM chip as each one needs +5/-5/+12 and GND connections to work.
    Voltages on the power transformer are obviously AC, after the diodes and capacitors you get DC voltages, so if your meter isn't auto-sensing, you need to switch it. I'm sure you already know this, but I thought I'd mention it anyway.

    Frank IZ8DWF

  3. #3
    Join Date
    Jan 2014
    Location
    Freedom City
    Posts
    5,568
    Blog Entries
    1

    Default

    Yep sounds like you're trying to measure AC with a DC meter. I yoosta have an auto-sensing one at work but upgraded to one that isn't. So I've several times gotten burnt by this, which for me is embarrassing at the least and inexcusable and deadly at the worst.

  4. #4

    Default

    Quote Originally Posted by iz8dwf View Post
    7812 should produce around 12V. What pins are you checking? What's the voltage on the Input side of the regulator? Did you check the diodes before the regulator (powered off, multimeter with diode check)? Did you check the capacitor for leakage/high ESR?
    All important voltages on a 2001N board can be checked on any 4116 RAM chip as each one needs +5/-5/+12 and GND connections to work.
    Voltages on the power transformer are obviously AC, after the diodes and capacitors you get DC voltages, so if your meter isn't auto-sensing, you need to switch it. I'm sure you already know this, but I thought I'd mention it anyway.

    Frank IZ8DWF
    Thank you for your reply! I had a few moments before work this morning to double check. I confirmed that I have my meter set to AC. I was testing between each connecter and the ground that's connected to the case. Again, all read 0 volts. I think at this point I have to assume my meter is broken.

    Your tip about checking the RAM was brilliant. I checked each RAM and they were predictably -5.1V, 12.1V and -4.9V on the corresponding pins. If the RAM chips are getting proper voltage, can I assume my meter is broken on the AC setting and rule out an issue with the transformer or would there be situations where the RAM could be getting proper power but there is still an issue at the source?

  5. #5

    Default

    replying to myself, -5.1, 12.1 and 4.9 (not -4.9)

  6. #6

    Default

    When I first started, I'd have those cheap VOMs. I'd damage them by leaving the meter on Ohms while measuring voltage. It took a couple meters before I learned to check.
    Recently I got did some damage. I have one of those meters that Harbor Freight was giving away. I'm use to having a meter with AC and DC, complimentary scales. I was checking out a failure of a light fixture and wanted to check the AC voltage. I put it on what I though was the AC scale that turned out to be the DC current scale. Luckily it blew the fuse and only one resistor. The meter was still OK, after replacing blown parts.
    I feel silly about it as I clearly know the difference.
    As for the regulator, check that it isn't really hot. If it is, it might mean you have a high resistant short some place. What ever it is will also be getting hot as well. These regulators will shut down with excess current. When they do, they get hot. ( usually more than 100C ).
    If the regulator is not really hot, it is just a bad regulator.
    Dwight

  7. #7

    Default

    Quote Originally Posted by Dwight Elvey View Post
    When I first started, I'd have those cheap VOMs. I'd damage them by leaving the meter on Ohms while measuring voltage. It took a couple meters before I learned to check.
    Recently I got did some damage. I have one of those meters that Harbor Freight was giving away. I'm use to having a meter with AC and DC, complimentary scales. I was checking out a failure of a light fixture and wanted to check the AC voltage. I put it on what I though was the AC scale that turned out to be the DC current scale. Luckily it blew the fuse and only one resistor. The meter was still OK, after replacing blown parts.
    I feel silly about it as I clearly know the difference.
    Dwight
    I likely fried mine in similar manner mostly because I use it for AC so infrequently.

    I wanted to generally thank everyone for their helpful replies. If I don't respond right away it's because I'm new on the forum and my replies are moderated still. I have been at work all day and haven't had time to look at the PET again other than to confirm that the voltages on the RAM chips are correct. When I get home, I'll post more details about exactly what I'm observing with my PET and I'll attempt to include a link to a video of it running a Tynemouth debug CPU.

  8. #8

    Default

    Again, due to me being moderated I can't see what I posted last, so I don't recall what information I have already provided and what I haven't. Some background:

    When I purchased this PET it wouldn't boot. I tracked that down to the reset pin on the 6502 being stuck low. The cause of that was a dead c66 cap in the 555 timer circuit. I replaced that and the machine was able to boot to something resembling the expected boot screen (except with reverse video and diagonal video artifacts on the display). Since then I have tested roughly every IC in the video RAM circuit and the video logic circuit with a logic analyzer and they all appear to act in a sane manner. By this, I mean that provided the given inputs, the outputs have all matched what data sheets say I should expect. What I don't know is in which cases the inputs were potentially not sane.

    I socketed the two video RAM chips and swapped in 2114 ICs from a VIC-20, but swapping them in and out did not change any behavior. I found one visibly burned capacitor at c25 and replaced that, but there was no change in behavior.

    Here's a link to a video of it with a Tynemouth Software debug IC inserted in place of the 6502:
    https://youtu.be/eyOLqxgV2XQ

    I have observed that the video is typically reversed except for the results page of the memory and ROM tests, where everything appears perfect.
    On the screens where it's outputting all PETSCII for business and graphics mode, the characters are completely jumbled on the one screen and only the numbers are jumbled on the other.
    When it's doing the memory tests, you can see how wonky the video gets.

    Here's a link to a video of it with the stock 6502 inserted:
    https://youtu.be/hQL0u8r1mIE

    Reverse video, wonky horizontal artifacts. Most keys work and I can type and get sane responses like syntax error.
    Last edited by cihs; June 15th, 2018 at 04:57 AM. Reason: substituted youtube links so folks won't need to download the entire video

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •