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Thread: Sick SuperPET

  1. #1

    Default Sick SuperPET

    Hi,

    I picked up a SuperPET with some problems. It has a 8032089 Universal Dynamic PET motherboard (click on any of the images below to enlarge them).




    When started in 6809 mode, sometimes I get the language startup menu, with some corrupted characters, and no ability to do or choose anything.



    When started in 6502 mode, I just get a black screen with no startup beep, or sometimes a glowing phospor dot that may fade or get brighter (so I shut it off quickly). So....judging from this I know:

    1. the monitor works (I sometimes see the menu displayed)
    2. the MOS6545 Cathode Ray Tube Controller chip probably works.
    3. the Character ROM probably works.
    4. the SuperPET daughterboard with 6809 probably works (somewhat)

    Because of the unwillingness for the machine to respond at all when switched to 6502 mode, it seems likely there is a problem with the main motherboard. So, I've removed the SuperPET daughterboard, temporarily turning this machine into a stock 8032 PET. I put a spare 6502 into the CPU socket because the original 6502 is on the (now removed) SuperPET daughterboard.



    Turning the machine on like this results in a black screen, or sometimes a green phosphor dot.

    I should mention that there is a strange jumper wire on the underside of the main motherboard. It runs from PIN 35 of the 6502 CPU to PIN 1 of a SN74LS393N at location UD3. Apparently this latter chip is a 4-bit binary counter. The confusing part is that pin-35 of the 6502 is listed as "NC" (not connected), but maybe this line is used to transfer timing from the counter, up the ribbon cable that normally sits in the motherboard CPU socket, to the SuperPET daughterboard for use by the 6509? Really I have no idea as I'm out of my league on this.



    Not sure if a stock 8032 has this jumper. I've heard rumours that there might be a small mod in SuperPETs in regards to where CPU gets its timing from - I'm not sure what this means exactly - but maybe it's related to this? Also, perhaps because of this, a SuperPET won't run as a regular 8032 PET unless this mod is reversed?

    First, I want to test the power supply to see if it's outputting proper voltage. I disconnected the power supply connector from the motherboard at location J8 with the intent to test voltages with a multimeter. However, I cannot find a proper voltage pinout diagram or table for this connector. I've found some confusing information about the 8032's power connector online, but some of these mention an 11-pin connector. The power connector on my SuperPET seems to be a 9 hole connector with one hole (hole 3) blocked. It connects to a 9-pin header on the motherboard ( location J8 ) with pin-3 intentionally missing. I've also downloaded the 8032 and SuperPET service manual (technical guide). It has a diagram of the power supply, but there's nothing on there that makes sense to me or matches the connector I see physically in the machine:



    Secondly, I've heard there are some AC voltages on this connector. Okay....I'm used to dealing with PC/C64/VIC-20 power supplies that are all putting out DC, but I can probably handle testing the AC voltages if I have a pinout.

    Thirdly, I've heard that some of the PET's power supply is actually on the motherboard - I can see the large capacitor and couple of circular voltage regulators on the black metal section, so am guessing that's part of it.

    Once I have determined that the power supply is good, I can move on to the next suspect: a possible bad EDIT ROM - apparently with CRTC video chip PETs if the EDIT ROM is bad the machine will appear black on power up. But first I have to eliminate the power supply as the culprit.

    Can anyone help me out, or guide me through this? I should mention that I have experience repairing old computers, and can do fine electrical work / component replacement. However, I am self-taught. I am no electrical engineer and cannot diagnose problems that are not obvious (i.e. burnt component) or easily measurable with a multimeter / logic probe.

    Thanks!
    Last edited by Ral-Clan; September 13th, 2016 at 04:55 PM.

  2. #2

    Default

    I don't know anything about the 6809 board, but I wonder if you have bad zero page RAM.
    Be polite and I may let you live.

  3. #3
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    Quote Originally Posted by Ral-Clan View Post

    The confusing part is that pin-35 of the 6502 is listed as "NC" (not connected), but maybe this line is used to transfer timing from the counter, up the ribbon cable that normally sits in the motherboard CPU socket, to the SuperPET daughterboard for use by the 6509?
    You are correct, pin 35 of the CPU is not used in the 6502, and so the SuperPet uses it as a tie point to route the 16 MHz clock to the 6809 board. You have a nice computer. The guys here will help you fix it.
    -Dave

  4. #4
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    Quote Originally Posted by Ral-Clan View Post

    Secondly, I've heard there are some AC voltages on this connector. Okay....I'm used to dealing with PC/C4/VIC-20 power supplies that are all putting out DC, but I can probably handle testing the AC voltages if I have a pinout.
    The active electronics for the PET power supply are on the main board. There is only a transformer and a big capacitor off the board. So if the J8 cable is disconnected from the main board, one will see the secondary AC voltages. However once the J8 cable is connected, two full wave rectifiers are hooked up and you will see DC on some pins. Here is a sketch that may show the detail. Study it and ask questions. Sorry for the condition; I always meant to clean it up.

    PET Power.jpg

  5. #5
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    Welcome to VCFED.

    I can't look at your pretty pictures from work - so I will look at your nice machine from home tonight...

    I assume you have found the 'standard' resources over on Bo's site at http://zimmers.net/anonftp/pub/cbm/s...32/index.html? Dave_m has already pointed you at the power supply schematic.

    You are doing the right things so far...

    Disconnect the monitor (power and signal) until you are happy that the 8032 mainboard is working as a standard 6502 PET. Failure to do this could damage your monitor.

    You have already removed the 6809 SuperPET board - so that is good (reduce the problem to the smallest sub-set of components and fault find on that until it is working).

    Dave's already identified the 'modification' that was made to a standard 8032 PET to get the 6809 CPU to work by adding a faster (16 MHz) clock to an unused pin of the 6502 CPU. This modification was subsequently 'rolled' into the PCB's of the later 8032 boards. It does not have any effect on the operation of the 8032 as a standard 6502 PET. It should work just the same with the 6809 SuperPET board removed.

    Check the AC voltages from the transformer - with the PET mainboard disconnected. A simple multimeter set to AC Volts will suffice for this. If you are still a bit confused regarding which pins and what Voltages to expect - let us know and we can rectify that...

    After that - connect the mainboard to the power supply (well - transformer and large smoothing capacitor) and check for the correct DC voltages at the major chips - including ripple voltages. I assume you have an oscilloscope?

    After that it will be looking at the 6502 RESET, NMI, INT and CLOCK pins to see if we have the correct activity on those. There are quite a few threads on VCFED related to fixing a 'standard' 8032 which are immediately relevant.

    Dave_m: Regarding my SuperPET add-on card. I have added a transistor and some resistors for the /NOROM signal (as you recommended) and removed that from the GAL. The 'bow-tie' reset circuit will not work as defined - the signal is not a change-over switch (in the sense required by the 'bow-tie' circuit) and there is the added complication of the programmable CPU changeover from the SYSTEM latch. Thoughts?

    Dave
    Last edited by daver2; September 13th, 2016 at 01:08 AM.

  6. #6

    Default

    Hello folks!

    First of all, thanks for the warm welcome. I have been a forum member over on Denial (VIC-20) and Amiga.org for over ten years, but haven’t needed to join VCF until this PET fell into my hands. I first used a PET about 1979 in my elementary school, so have a lot of nostalgia attached to this machine and would like very much to get it working again.

    Daver2, you asked what equipment I have. I am not an electronics expert, but here’s what I have:

    1. basic soldering iron with 2 temperature settings (yes I said it was basic).
    2. solder sucker iron (which I'm not too bad at using to remove components).
    3. digital multimeter (done lots of DC voltage and continuity testing with this, but not much more).
    4. logic probe (just acquired, haven’t use it yet)
    5. two Tektronix oscilloscopes (read below)

    I was very recently given two old 1970s era Tektronix era oscilloscopes similar to this model:
    http://www.proctorinc.com/images/slab0115022.jpg

    They probably haven’t been calibrated in 20 years, though, and frankly, I have never used an oscilloscope and haven’t had time to learn or check over these to see how reliable they are – although one does power up and shows a flat line on the CRT.

    Again, I have experience replacing components on boards – I’m pretty good at de-soldering and soldering, etc. I’ve done some component testing with a multimeter, but I would need guidance for anything more intricate. Fact is, I’m going to ask a lot of newbie, basic, simple questions as I’m taking baby steps here.

    Dave_m, thanks for the schematic of the power supply. Although I can generally understand what's going on, it’s with a lack of confidence. I find it a bit daunting, especially with the hand-written notes, but think I can decipher it with help.

    One thing that I find confusing about the diagram si that the connector at J8, as listed on the diagram, seems to list the pins out of order. I suppose this is because it’s supposed to be a symbolic representation of the connector, rather than a literal one – the pins are out of order on the drawing because that’s the clearest way to draw the signal paths without having them overlap one another. It's for clarity's sake. Am I close?

    So, assuming I am correct, and I have the J8 power supply connector removed from the motherboard, and have an multi-meter all ready to test the output of this J8 connector, how would I test the power supply’s voltages? This is what I think I can decipher from that diagram:

    Pin 1: Brown, Looks like 22VAC, but might also be 12VAC (hard to read writing)
    Pin 2: Red...seems to connect to the positive terminal of a capacitor at C1 (the big barrel capacitor?)
    Pin 3: Black, no idea...seems to connect both the negative end of capacitor C1 and an branches to an inductance coil (one end of the transformer?)
    Pin 4: Red...also connects to the positive end of capacitor C1
    Pin 5: Brown, again could be 22 or 12VAC (hard to read), connects to the end of an inductance coil
    Pin 6: Black, connects both the negative end of capacitor C1 and an branches to an inductance coil
    Pin 7: Key, not used. A plugged hole to ensure proper orientation of connector
    Pin 8: No colour indicated, 18VAC (Positive? Negative?)
    Pin 9: same as pin 8 above, but opposite end of inductance coil (Positive? Negative?)

    Based on the above, assuming I'm even partially correct, I still wouldn’t know where to start testing this with a multimeter. I'm still not clear what voltages to expect on which lines. I think I might be able to check for 12/22VAC on pins 1 and 5, and 18VAC on pins 8 & 9, but am not sure. Also, where do I get ground?

    Please be honest - if you think coaching me to repair this PET is a hopeless cause, please let me know. I was planning to send the motherboard off to Tynemouth software in the UK, but had hoped I could at least diagnose the power supply before hand (if not more). I’m willing to learn, anyway.
    Last edited by Ral-Clan; September 13th, 2016 at 07:40 PM.

  7. #7
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    Quote Originally Posted by Ral-Clan View Post

    Based on the above, assuming I'm even partially correct, I still wouldn’t know where to start testing this with a multimeter. I'm still not clear what voltages to expect on which lines. I think I might be able to check for 12/22VAC on pins 1 and 5, and 18VAC on pins 8 & 9, but am not sure. Also, where do I get ground?
    You are doing good. On the voltmeter, select AC and either Autorange or 30VAC range. For AC readings, you just have to read the voltage across the pins, i.e., read 22VAC with one lead on J8-pin 1 and the other on pin 5. It doesn't matter which color test lead is on pin1. No polarity or ground reference for AC but you do need to worry polarity for DC readings. You should measure about 18 VAC across the blue wires pin 8 and pin 9.

    If you get those readings, you are good to go for powering off and mating the J8 connector, and with power up for reading DC voltages on the board. For DC readings you can use any bare metal on the case/chassis for ground reference for the black test lead. Use a test lead with an alligator clip if you have one.

  8. #8
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    Ral-Clan,

    People with less equipment and absolutely no experience with electronics have managed to get PETs repaired by following the instructions from this forum! Some of the faults have been real ****** as well! So take heart...

    Let's just do it step by step...

    Follow what dave_m has asked you to do and we will work from there.

    I suspect at some point (since you have access to an oscilloscope - or two!) this piece of test equipment will make diagnosing the more 'difficult' faults easier - so tracking down the user manuals and making sure at least one of your oscilloscopes works would be beneficial. Logic probes are useful - but there are a couple of threads around where the cheap ones just cause more confusion than they solve when looking at clock signals...

    Dave

  9. #9

    Default

    Great! Thanks Dave(s)!

    So I will disconnect the plastic connector that normally attaches to J8 on the motherboard and test for the following on the pin sockets:

    • First across holes 1 & 5 for AC. Dave_m, can you please confirm for me whether you wrote 12VAC or 22VAC here (the first number in your writing looks like it could either be a "1" or a squished "2")? Thanks.
    • Then for 18VAC across holes 8 & 9.


    My multimeter doesn't have an auto setting, so I'll set it to the 30VAC range. I will report back with what I find.

  10. #10

    Default

    22VAC, from memory (which isn't great). The thing is, you'll probably measure the right amount anyhow, and, if not, you'll still be within reasonable range to determine if it should be 12 or 22, or, you'll have any obvious problem anyway.
    Be polite and I may let you live.

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