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Thread: Cbm 3032 power supply short / damaged board

  1. #1

    Default Cbm 3032 power supply short / damaged board

    Hi, this is my first post here.
    I was trying to measure the voltages on a CBM3032 and I accidentally shortened pin 4 and 6 on the power supply.
    This caused the fuse to blow, and the explosion of 2 diodes (CR2/CR1).

    I removed the board, changed the fuse (I took one from a C64 board), powered on, and also this fuse blown.
    What should I do? The computer was displaying garbage on boot, so I was about to replace all the regulators and capacitors, but I just made the situation worse.
    Are the power supply (or the board) damaged permanently?
    I can replace the diodes, but I have no clue about what should I do or check with the power supply.

    photo_2020-02-27_01-57-32.jpg
    photo_2020-02-27_01-57-27.jpg

  2. #2

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    Part of this trouble comes from the assumptions made about what needs to be replaced, the regulators & capacitors may well be ok.

    The reality is that you should not assume anything needs replacing until you have performed tests which indicate that the component does need replacing...or not.

    Or else what happens is the multiple interventions can create more troubles along the way and it create a real mess.

    The issue with the garbage on boot may have been nothing to do with the power supply.

    It is easy to see whether the power supply is working. Just check with a meter on the input & output of the regulator IC's. That can sometimes miss ripple on the regulator output, if say the regulator input voltage is troughing below the output voltage +2.5V (78 series regulators need at least 2.5V more than their output voltage as an input). When that happens though, the computer's video output has an obvious hum voltage crawling through it, seen on the VDU.

    So it is not really a wise plan to attack the circuitry on the basis of a hunch about faulty parts and no measurements.

    Still, having said that, it might sound harsh, however, every person servicing electronic gear has made a slip up at some point, accidentally shorted something out and destroyed some components, so you are not alone & don't feel bad about it. Think of this as a learning experience.

    The first move now is to get back to exactly where you were before the short & the damaged parts.

    Replace the damaged diodes, re-power the board, check the existing regulator input and output voltages before considering replacing the regulator. Check the main filter capacitor with a uF meter and an ESR meter, it may well be ok.

    Now you are getting the idea that to be an effective service technician, you will require some test gear, or else how can you test the parts, especially the capacitors. There are "tricks" of course like paralleling another capacitor as an experiment.

    A lot of the time, experienced engineers can get away with a diagnosis and repair on a hunch. So if they set an example that they fixed something just by replacing capacitors and regulator IC's etc, it actually sets a bad example for others learning, who might think that is all you have to know, or do, to fix a broken computer.

    But if you are not experienced and familiar with a systematic type of fault finding, you will fall into about as many pot holes as Bugs Bunny could dig in an afternoon.

    This wisdom was beautifully summed up by the Hair Dresser, Vidal Sassoon who once said : "First you know the rules, then you can break them". Of course it is folly to break the rules if you don't know their intention and what they were designed to protect. The same applies to electronics servicing.
    Last edited by Hugo Holden; February 27th, 2020 at 05:31 AM.

  3. #3

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    The diodes are likely 1N5402 like the others. These are 3 amp diodes. Other PET supplies use 4 amp diodes but these are in a bridge rather then independent. Since I don't know the board or recognize the usage, I can't suggest more.
    It is unlikely that you did more damage so I'd replace these and the fuse to see what it does.
    Hugo is right, replacing part that don't need it is a waste of effort and often ends up doing more damage. Sure, power supply capacitors often fail that just means that is the first place to check for failures, not the first place to replace parts. Power supply failures usually cause multiple faults, such as the other PET that was just fixed on this MB.
    Most often, following what it isn't doing right is the best way to find the problem.
    Dwight

  4. #4

    Default

    Thanks Hugo, you're completely right. I have been already a bit harsh with myself for this, so don't worry

    I tried to check the voltages on the DRAM and I noticed a 7.5V on the ram chips (4116-3) and
    I was afraid that the IC could be damaged. The correct value should be 5V, am I right?

    Basically that's why I decided to check the PSU. I should have been testing it from the regulator output as you suggested! My bad. The next time i will think twice before trying things. I did a quick test with the logic probe tho, and all chips had activity, before the short.

    I can't find the mains filter capacitor under the transformer (check the attachments), I think that it has been already removed by someone else in the past.

    photo_2020-02-27_20-17-48.jpg photo_2020-02-27_20-17-52.jpg

    I ordered the replacement diodes today, let's see what happens. I will report the results of the test in some days.

    @Dwight: the machine is a CBM3032, the board is a 320530 rev E
    http://www.zimmers.net/anonftp/pub/c...N/320351-6.gif

    And I really hope that the damage is limited to the diodes. Thanks for your advice!

  5. #5
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
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    7,290

    Default

    Are you saying that without replacing the blown diodes the new fuse blew anyway? What are the fuse ratings, old vs. new?

    If you did replace the diodes, are you sure they're installed in the right orientation?

  6. #6

    Default

    Quote Originally Posted by MikeS View Post
    Are you saying that without replacing the blown diodes the new fuse blew anyway? What are the fuse ratings, old vs. new?

    If you did replace the diodes, are you sure they're installed in the right orientation?
    On the schematics the fuse, is listed as a 125V / 1.6A. The "original" one i found in the machine was a 250V and this should be correct, because i'm in Italy.

    Yes, the new fuse blew without replacing the diodes. Anyway, I don't consider this as a valid test, because I took one from a Commodore 64 (it was a 250V / unknown Ampere value).
    On monday I should receive a kit of 250V / 1.6A fuses, so i will test again!

    Should I disconnect the crt board for extra safety?

  7. #7

    Default

    Quote Originally Posted by Elder0010 View Post
    Thanks Hugo, you're completely right. I have been already a bit harsh with myself for this, so don't worry

    I tried to check the voltages on the DRAM and I noticed a 7.5V on the ram chips (4116-3) and
    I was afraid that the IC could be damaged. The correct value should be 5V, am I right?
    These early DRAM IC's have three power supplies, (looking at their data sheet) +5V, -5V and +12V. So you could expect to see three voltage regulators on the pcb at least, for these voltages.

    In that photo you posted, your line fuse holder is damaged(cracked) it must be replaced.

    Hugo.

  8. #8

    Default

    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.
    Dwight

  9. #9
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    3,543

    Default

    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?

    Always disconnect the monitor supply (the plug in the monitor housing) until you are happy you have the video and H and V drive signals. This will prevent “cascade damage” to the monitor (that can occur if the main board is not operating correctly).

    Given where you are, I would disconnect the transformer from the main board and test first without any logic connected at all.

    Measure the AC voltage coming from the various windings on the transformer. There are three separate windings. Two windings are “simple” and one has a centre tap.

    If you don’t know what to measure, please ask before making the measurements.

    Check the schematic here for a start http://www.zimmers.net/anonftp/pub/c.../8032051-3.gif.

    Note that there are various transformer manufacturers used inside PETs, so the transformer pinouts may not be identical to that shown, but the connector pins in use should be consistent.

    On power-up, the fuse shouldn’t blow (!), and please report back the AC voltages you measured and where you measured them at. The wire colours from the transformer are usually a good clue (or the pins of the connector).

    Dave
    Last edited by daver2; March 1st, 2020 at 03:29 AM.

  10. #10
    Join Date
    Jun 2012
    Location
    UK - Worcester
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    3,543

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    If the fuse stands up OK - and we are happy with the AC voltage measurements, the next step is to connect the transformer up to the main board (monitor still disconnected) and power up again.

    If the fuse blows this time, we have to look at the bridge rectifiers, voltage regulators etc.

    If the fuse doesn’t blow, you then need to measure the DC voltages from the regulators.

    If you have a digital multimeter, the easiest way is to connect the black/negative probe of your multimeter to 0V/GND. I use the negative side of the big capacitor off-board near the transformer. This should have BLACK wires connected to it.

    I would then use the red/positive lead of the multimeter to measure the DC voltages on both sides of the following diodes:

    CR10, CR11, CR12 and CR13.

    Take the LOWEST voltage measurement.

    For CR10 and CR11 this should be approximately +5 Volts. There are two (2) separate +5 Volt regulators. Both need to be working...

    For CR12 this should be appropriately +12 Volts.

    For CR13 this should be approximately -5 Volts. Ok, this is the highest numeric value rather than the lowest. If you have an analogue multimeter, you will have to swap the probes over.

    Post the results.

    As your posts are being moderated, it may take a period of time for them to appear.

    Dave

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