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Thread: PET won't power on

  1. #21


    Start by getting a different multimeter and verifying your measurements. Next, measure the AC voltage at the transformer secondary and tell us what you get.

    It sounds like your clock is running but you had the meter leads reversed.
    Be polite and I may let you live.

  2. #22


    Black lead to the ground pin, red lead to the clock pin, right?

    But yeah, I'll check with a different multimeter. I have a few to choose from.

    Edit: okay, tried a different multimeter and this one shows the readings as more like +/- 4.8V and +11V.

    Which pin on the transformer is the secondary? I admit, my knowledge of low-level electrical stuff is embarassingly limited...
    Last edited by commodorejohn; May 18th, 2017 at 08:25 PM.
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  3. #23


    4.8V is marginal (in my opinion) but generally OK for 5V TTL. I like to see it higher on older things like this, but I wouldn't worry about it at this point. 11V is good enough.

    You can't really go wrong measuring any combination of pins on the transformer with an autoranging meter. The secondary will be the one that isn't 120 or 240V (nominal). I don't know which to tell you otherwise because I'm nowhere near mine, but I'm sure others here will be able to tell you. Either way, you'll find one pair of wires measures line voltage and the other pair is the secondary. At this point, I wouldn't bother, because it's going to be correct given how close your DC voltages are.
    Be polite and I may let you live.

  4. #24
    Join Date
    Dec 2005
    Toronto ON Canada


    Check the voltage across the terminals of the big off-board capacitor; it should be 8V DC or more. The voltages you're measuring are all regulated; odds are better that your meter reads a little low than that all three regulators are off. Check voltages across C14, C16, C24 and C32; they should be very close to what you measured at the RAM chip.

    I'm afraid that it looks like your voltages are OK (they usually are) and the problem is elsewhere; more investigation required. Good opportunity for some practice using that scope

  5. #25


    Next up, check voltage at 6502 signals RESET, IRQ, NMI, and all the A lines, and report back.

    When you check /RESET, it should be 0V when you first power on, and go to 5V after a few seconds. If not, the reset circuit is suspect.

    If /NMI is 5V that's good, else you may have a bad 6502 or a short somewhere.

    If /IRQ is 0V that's bad, pull out your I/O chips (6x2x) and check again. If still 0V, same prognosis as /NMI not 5V.

    What your report at the address lines will give us an idea of what's next.


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