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Thread: Cbm 3032 power supply short / damaged board

  1. #11


    Quote Originally Posted by Hugo Holden View Post
    In that photo you posted, your line fuse holder is damaged(cracked) it must be replaced.
    Just ordered another one!

    Quote Originally Posted by Dwight Elvey View Post
    Before replacing them, make sure the old diodes are open by spreading the leads. With the transformer disconnected, check CR4 and CR5, that they are not shorted. It looks like they may have been part of a 4 diode bridge. If the two blow, it is probable that the other two diodes are also shorted. If so, you'll likely blow the new ones out.
    I did the test, and CR4 and CR5 aren't shorted!
    I'm still waiting for the diodes to arrive, I will replace them in the weekend hopefully. It's safe to keep CR4 and CR5 on the board or I should replace them also?

    Quote Originally Posted by daver2 View Post
    If you are in Italy, and you are running the PET from a 230 V ac supply, the mains fuse rating should be 800 mA anti-surge (slow blow) and NOT 1.6 A (this size of fuse is only for 110 V operation).

    Has the transformer truly been set for 230/240 V operation before continuing?

    Check the schematic here for a start
    I received today the 0.8A slow blow fuses. And yes, the transformer is working at 230V: before my stupid move that caused the short, the computer was correctly powering on and showing the garbage screen, so I don't suspect any issue on this.

    So I should proceed like this:
    - replace the fuse
    - with main board and crt board disconnected, turn on the machine
    - if the fuse doesn't blow, measure the voltage on the power supply
    - if everything is ok, reconnect the board (with replaced diodes) and check the voltage regulators
    - if everything is ok reconnect the crt board

    Now, before doing any kind of damage again, here is my transformer:


    • 4 and 6 are connected (brown)
    • 7 and 8 are connected - (brown -> to the CRT board - now disconnected for safety)
    • 5 is ground (black)
    • 9 and 10 are connected (cyan)

    which voltages should I expect?
    9 and 10 Voltage should be 15, judging from the schematics, but I don't know about the other ones.

    Also, is this the correct way of measuring the voltage on the windings? I connected a pin of the power supply to the GND of the big capacitor.
    This photo has been taken with the machine powered OFF so don't worry


    thanks everyone!

  2. #12


    I would say the diodes sound OK. You need to measure, with both leads on the transformer terminal leads, for the AC measurements. Use the ground lead for the DC measurements.

  3. #13
    Join Date
    Jun 2012
    UK - Worcester


    >>> And yes, the transformer is working at 230V.

    No problem, just checking...

    If the diodes aren't short circuit - it is unlikely that it is these causing the problem. You have checked all of the diodes in the two (2) bridge rectifiers haven't you? If not, you may as well do that now.

    Just emphasising Dwight's post above:

    Set your multimeter to something like 20 Volts AC full-scale.

    Do not use the BLACK (or RED) leads of the big capacitor. These are only even in circuit if the transformer is connected to the main board (which it isn't if you are measuring open circuit AC voltages directly from the transformer).

    Use your multimeter leads to measure between the following points on the transformer:

    4 and 6 (thick brown wires).

    7 and 8 (thin brown wires).

    5 and 9 (black and blue wires).

    5 and 10 (black and blue wires).

    9 and 10 (blue wires).

    Post the readings for each measurement.

    We will be able to tell you then if they look right before proceeding.

    When the transformer is connected to the main board, you can then use the BLACK wire terminal of the capacitor with the black (negative) lead of your multimeter for DC voltage measurements.

    However, I wouldn't reconnect the monitor just yet. Whatever has gone wrong could have stressed some of the logic on the main board. We have a simple test (voltage measurement) that will check if we have a video and horizontal and vertical drive signals present before we connect the monitor.

    Hope this is clear?

    Last edited by daver2; March 4th, 2020 at 11:37 PM.

  4. #14


    Ok, here is an update:

    I did a continuity test on the diodes, and the only shorted ones the 2 destroyed ones (CR1 and CR2), which of course needs to be replaced.

    About the power supply, here are the measurements, taken with the crt board disconnected and the main board (of course) disconnected.

    My multimeter supports 600V as the smallest value for the AC measurements.

    Here are the values

    4 and 6 - 18v (stable)
    7 and 8 - 17/18v
    5 and 9 - 0v
    5 and 10 - 0v
    9 and 10 - 17v (stable)

    The 0V voltage on 9 and 10 with ground could be because the circuit is not closed when the CRT and the mainboard are disconnected?

    Here are the photos of the multimeter leads placement and setting, just to be 100% sure

    voltaggio_4_6.jpg voltaggio_5_9.jpg voltaggio_5_10.jpg voltaggio_7_8.jpg voltaggio_9_10.jpg

  5. #15
    Join Date
    Jun 2012
    UK - Worcester



    4-6 and 7-8 appear to be OK.

    9-10 is also OK.

    You should have read approximately 8V AC from 5-9 and from 5-10?

    Nope, you are reading directly at the transformer pins. This should not have anything to do with the disconnected monitor and main board. My 8032 reads OK like this.

    The black wire is a “centre-tap”, so should read exactly half the voltage that you read between the two blue wires.

    With the PET switched off and disconnected from the mains supply. Set your multimeter to read resistance and see what the resistance is from the black wire (5) to each of the blue wires (9 and 10).

    I am suspecting a bad connection somewhere.


  6. #16


    The resistance in 5/9 and 5/10 is 1. The other pins is 0.

    So it should be a bad connection. I can't find a way to open the transformer (is this even possible?), but i did a continuity test on the very small section of the wire coming out from the transformer to the pin 5/9/10 pins and it's NOT beeping.

    Check the photo, even if it's a bit hard to see: i put one probe on the pin (using a clip) and the other one on the wire



    I tried the same on pin 4, and it's beeping.
    Should I try to resolder?

  7. #17
    Join Date
    Jun 2012
    UK - Worcester


    No, not possible to open the transformer...

    By ‘1’ I assume you mean ‘infinite resistance’ (i.e. the same resistance as with your multimeter probes open circuit)?

    The copper wire is actually covered in enamel (an insulator) so trying to take a resistance reading from the ‘wire’ to the pin will not work.

    It sounds like the transformer centre tap is faulty. This will be a problem.

    Can you follow the black and blue wires back from the transformer to the connector that plugs onto the main logic board and take the three resistance readings from the relevant pins of the connector itself please? You may have to poke a short length of bare wire into the connector to make a good contact.

    Black - blue#1 resistance.
    Black - blue#2 resistance.
    Blue#1 to blue#2 resistance.


  8. #18


    Looking closely at the photo, of the wire that exits the transformer and wraps round the tag on the pin 5 (center tap) it looks like the soldering there is poor (worth re-soldering).

    But there is another clue, it looks like a single core of wire. Being the center tap, what you should see there are two strands of wire equal in diameter to the ones seen on pin 9 and 10.

    So, this suggests that those two wires are terminated inside the transformer to an in line thermal fuse, and the wire you see connecting to pin 5 is the other wire of the thermal fuse and the tests indicate that thermal fuse is open circuit. (unless its bad soldering).

    If this "theory" is correct, all is not lost, often by removing the surface insulation of the transformer, you can get at the other end of the thermal fuse and replace it.

    (The above is only a valid theory if pin 5 is definitely supposed to be the center tap connection and its not elsewhere)

  9. #19
    Join Date
    Dec 2005
    Toronto ON Canada


    Are you guys looking at the right winding? Shouldn't 5 be the center tap of 4 and 6, not 9 and 10, in which case everything looks good to me.

  10. #20


    Quote Originally Posted by MikeS View Post
    Are you guys looking at the right winding? Shouldn't 5 be the center tap of 4 and 6, not 9 and 10, in which case everything looks good to me.
    I think you are right and the wire diameters around the tags match pin 4 & 5 (even though its only possible to see one of the wires on the pin 5 tag) So everything with the transformer is probably ok.


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