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Thread: Pet 2001 chiclet display problem

  1. #1

    Default Pet 2001 chiclet display problem

    Hi guys, first post here. I'm restoring a 1978 PET I bought recently and on powering it up today for the first time, was presented with a single bright horizontal line. So no vertical sync. My thoughts are that this could be a CRT problem, but the fact that it's collapsed completely leans me more toward it being an issue on the PCB. I'm waiting for a scope to arrive so I can check.

    Pic here - https://imgur.com/a/7qmTGdY

    Anyhow, I looked with my meter and am seeing about 4.3VDC on pin 1 of the video connector, about the same on pin 3, and about 1.68VDC on pin 5 (all relative to pin 7).

    The cassette drive capstan is also running continuously - my memory of the machine (I had one in the 70s) was that it spins for a few seconds on boot and then stops.

  2. #2

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    I got the scope and can see a healthy looking horizontal pulse, but just flat noisy DC on the vertical pin. It seems to me that according to this diagram - http://zimmers.net/anonftp/pub/cbm/s...1/320008-3.gif - pin 12 is low. Pins 1 and 6 are low on B6, as you'd expect. I'm not sure what I should be seeing at 9 and 12 on B6, but the diagram shows those inputs as two clocks, so I should be seeing something other than 0V there right? So I'm not sure whether I should head over to A1, or C7. I don't have enough experience, I only have basic knowledge of electronics (gained from a few years at college).

    IMG_20191127_173217.jpg
    IMG_20191127_174554.jpg

  3. #3

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    Just so if anyone is interested wants to see, I've asked the same question on a related forum and to save time, here's what's been done so far:

    http://forum.6502.org/viewtopic.php?f=4&t=5847

  4. #4

    Default

    Why discount the CRT? A horizontal line indicates no vertical deflection, not necessarily no vertical sync.
    It makes sense to check the vertical sync of course but a voltmeter won't show what you need to see since it's all happening at a high frequency. You'll need the scope to see anything definite.

    Do you see any pixels or brightness variation in that horizontal line?

    The capstan running indicates the startup routing is not getting far enough to turn the motor off which could be anything. Bad RAM, ROM, CPU, Clock.
    Last edited by Hutch; November 28th, 2019 at 08:51 PM.

  5. #5
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    Default

    The average DC levels seem OK. The horizontal sync signal is used to generate the high voltage in the CRT so that signal seems fine. When you get a scope check the Vertical drive on J7-pin 3 for a negative going pulse of 1.26 milliseconds every 16.66 milliseconds, a frequency of about 60 Hz. If that is OK, then there may be an issue in the video board with a capacitor or one of the several transistors in the vertical section.

    Here are the schematics for the 2001-8 PETs. Note that there are several versions of mainboards and video boards. Try too match yours with the correct schematics.

  6. #6

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    Thanks, all my posts have to be approved which means I haven't been able to update this thread quickly, but I have a scope now and there's definitely no vertical sync.

  7. #7
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    Default

    Quote Originally Posted by PET 2001-8 BS guy View Post
    I have a scope now and there's definitely no vertical sync.
    OK, the four-state sequencer may not be running. Check D8-pins 4, 5, and 6 for pulses (bottom of schematic sheet 3). States are listed at the top of page 3.

  8. #8
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    It is hard to tell the cause from the effects of a stuck sequencer due to the feedback terms. Please scope signals at D8- pins 4,5, and 6 to see if the counters are running. Any of the components in the middle of sheet 3 may be bad.

  9. #9

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    Thanks, I am replying quickly but each post has to be authorised so it's taking a while.

    D8 pins 5 and 6 are both high at about 4.5V, so pin 4 is permanently low. I've been working with a user on the Commodore section of the 6502 forum (I posted this problem to both forums as I'm new to either and wasn't aware of the breadth of expertise on both) and we've worked through the board and got down to E6, D7 and D6, near the bottom of the diagram. Now the user who's been helping me is away from the weekend but I've found that D7 appears to have three healthy clock signals but is not counting. D6, which is an identical IC, has a different tale to tell.

    D7:

    pin 1 = 15.625KHz, 64s, 3.4Vrms
    pin 2 = 0.16V
    pin 3 = 0.08V
    pin 4 = 0.08V
    pin 5 = 0.08V
    pin 6 = 0.08V
    pin 7 = 0V
    pin 8 = 15.625KHz, 64s, 3.56Vrms
    pin 9 = 0.08V
    pin 10 = 0.08V
    pin 11 = 0.08V
    pin 12 = 0.08V
    pin 13 = 15.625KHz, 64, 4.05Vrms (pic attached)
    pin 14 = 5.11V

    D6:

    pin 1 = 15.625KHz, 64s, 3.4Vrms
    pin 2 = 1.78V
    pin 3 = 3.97V
    pin 4 = 3.97V
    pin 5 = 1.7V
    pin 6 = 1.7V
    pin 7 = 0V
    pin 8 = 0.08V
    pin 9 = 3.16V
    pin 10 = 3.97V
    pin 11 = 3.97V
    pin 12 = 15.625KHz, 64s, 3.56V
    pin 13 = 15.625KHz, 64s, 3.48V
    pin 14 = 5.1V

    I have plenty of 14 pin sockets so I was wondering if I shouldn't just socket them both and swap them, to see what the chip currently in D7 does, but I thought it better to wait for further guidance lest I inadvertently damage a pad or something.

  10. #10
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    I finally found all the work you have done on this problem on the 6502 forum, I will plow through all the pages and see where you are.

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