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Thread: Help wanted to Repiar Commodore CBM PET 4032

  1. #11
    Join Date
    Jun 2012
    Location
    Livermore, CA
    Posts
    183

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    My meter auto calibrates scale

    Measure the voltage across C10 (1uF) and let us know what the voltage is. This is the input to the +5V regulator- so it should be somewhere between +7.5V and +9V d.c. Note the polarity of your meter.
    Starts at 1.2v and falls to zero after about .6 minutes and is steady there

    Measure the d.c. voltage across C11 (47uF). This is the output from the +5V regulator. It should be +5V +/- 0.25V.
    steady at 1.2v

    C5 (4,700uF) input to the +12V regulator.
    can't get to it, board still in the case
    ]
    C17 (10uF) output from the +12V regulator. Set you multimeter to read something like 15-20 Volts full scale.
    .1v

    C2 (470uF) input to the the -5V regulator.
    can't get to it, board still in the case

    C16 (10uF) output from the -5V regulator.
    -.6v

    I would also measure the voltage across the very large, external, green capacitor. This voltage should be something like +9V d.c.
    1.2v
    Let us know what all the readings are - and we can take it from there.

    If the input voltages are low to the voltage regulators, then this could indicate a problem with the bridge rectifier diodes or the transformer. If the input voltages are OK - but the output voltages are either wrong or start off correct and then fall over time - this could indicate faulty voltage regulator(s) or a fault somewhere on the main board. However, we will address these issues once we know what the voltages are and what we are dealing with.many voltages start a bit higher then fall to a steady number

    Note that all of the capacitors I am describing are polarised. Thanks, but already known to my limited experiances
    Atari Falcon 030/FX-1 CF & SD 14mb, Atari MSTE, Atari PS3000, STACY 4 /w UltraSatan SD, 4160STE-CosmosEX, Atari PC1-PCH204-2xPCF-551-PCM124, 1040STFM, 1040STF, 520ST(4mb, 16mhz), 400-48K & std kybrd, 800, 600XL, 800XL, 1200XL(256 Rambo,svideo,31-OS,APE upgrade), 130XE, XEGS, matching peripherals for all,
    Apple llGS ROM 3-4mb-CF HD, C64, 4+, IBM XT 5160, IT 99/4+, TRS80-4P, Amiga 500,

  2. #12
    Join Date
    Jun 2012
    Location
    Livermore, CA
    Posts
    183

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    Between pins 1 and 3 I get 8 Volts a.c. (Brown to black on the wire colours on my PET).
    .2v

    Between pins 5 and 3 I also get 8 Volts a.c. (Brown to black on the wire colours on my PET).
    .2v

    Between pins 1 and 5 I get approximately 16.3 Volts a.c. (Brown to brown on the wire colours on my PET).
    .9v

    Between pins 8 and 9 I get approximately 14.8 Volts a.c. (Blue to blue on the wire colours on my PET).
    .2v

    The wire colours are (I think) a bit arbitrary on the PET...

    If you open up the monitor and disconnect the twisted pair power leads to the monitor.
    I do not want to get to the monitor just jet, Lets get the board to measure correctly fist, if possible.

    I hope this sort of detail is helpful to you? VERY MUCH !!
    Atari Falcon 030/FX-1 CF & SD 14mb, Atari MSTE, Atari PS3000, STACY 4 /w UltraSatan SD, 4160STE-CosmosEX, Atari PC1-PCH204-2xPCF-551-PCM124, 1040STFM, 1040STF, 520ST(4mb, 16mhz), 400-48K & std kybrd, 800, 600XL, 800XL, 1200XL(256 Rambo,svideo,31-OS,APE upgrade), 130XE, XEGS, matching peripherals for all,
    Apple llGS ROM 3-4mb-CF HD, C64, 4+, IBM XT 5160, IT 99/4+, TRS80-4P, Amiga 500,

  3. #13
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    2,010

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    I thought I would start simply and find what level you were at...

    I just want to confirm something please before we move on. In the last post you did have your multimeter set to read a.c. Voltage and you were measuring on the pins of the power supply connector connected to the wires that run back to the transformer and not on the pins of the PET main board?

    Reading 0 Volts from the transformer secondary windings indicates a fundamental problem with the transformer but (in this case) you shouldn't be reading anything at all on the main board.

    Dave

  4. #14
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    2,010

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    I am going on a business trip for the week so will not be on VCFED for a while.

    If I assume that you are not getting any a.c. Volts out of the secondaries of your transformer (or is is intermittent) I would next have a look at the primary side of the transformer - see ftp://www.zimmers.net/pub/cbm/schema.../8032051-3.gif for the schematic.

    Check the fuse (!) the switch (!) and that you have some continuity between live and neutral of the plugtop (not plugged in but using your multimeter set to resistance). The resistance should be very high when switched off (> 1 MOhm) and fall to less than 100 Ohms when switched on.

    Warning, you are messing around on the mains side here so be careful. Depending on your multimeter you may get a high voltage build up on the transformer when you disconnect your multimeter - so avoid touching the metal pins of the plugtop with your fingers.

    If in doubt, ask a qualified electrician to check this bit out if you don't feel comfortable.

    If the above tests pass OK, I would be tempted to plug the PET into the mains (but not with the transformer connected to the main board), power it up and measure across the transformer primary winding with your multimeter set to 250 or 120 Volts a.c. (Depending upon where you are in the world and your mains supply voltage) and ensuring you get the correct mains voltage across the transformer primary with the PET switched on.

    If the above is OK, but you are still not getting anything out of the transformer secondary windings - it is likely the transformer is dead.

    Another check is to look at the heater on the rear of the VDU screen. Is it lit (dimly) when the PET is powered up? You may have to look when it is dark.

    A few things to think about and report on...

    I still don't think this is your main problem though if you have managed to measure something on the PET main board with your multimeter. You should be able to read something from the transformer secondaries. Just a daft thought - your multimeter is working correctly when measuring on the a.c. Voltage range isn't it?! Been here, done that got the tee shirt myself!

    Dave
    Last edited by daver2; October 14th, 2018 at 05:33 AM.

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