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Thread: Tried repairing mac classic analogue board but now its blowing fuses

  1. #1
    Join Date
    Apr 2020
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    Default Tried repairing mac classic analogue board but now its blowing fuses

    My mac classic analogue board I'm fairly sure now is beyond repair, certainly beyond my knowledge so for one last go I'm hoping someone out there will be able to help.

    This started a while back when I purchased a mac classic locally for 30, talk about to good to be true! It needed recapped and I was happy to do that but when powered up with all new caps it gave a wobbly checker board. After some discussion that turned out to most likely be caused by electrolytic still on the board causing shorts. I took the decision to remove all components so I could properly wash the board. Its all through hole and single sided (obviously) so easy enough to do. All components were removed, labelled and stored for reuse except diodes DP5, 6, 7 that were crusty so got replaced along with IP1 and QP1. I tested all components I could before reinstalling and all checked out fine.

    The board got a soak in soapy water then a bath in IPA to try and remove all the leaky gunk. It was thoroughly dried before reassembly.

    Now however when powered up the fuse goes instantly with a bright light so must be some serious current running through it. Power supplies aren't really my thing and I'm starting to think removing all the components was a bad idea (all be it I got plenty of soldering practise!) Worth noting there is no obvious sign of damage to any other components.

    To be honest I've more or less given up on it now but if anyone has any wild ideas as to whats happening please let me know. Failing that does anyone in the UK fancy taking a look at it? I'll warn you though the underside of it isn't pretty. The damage from leaked electrolytic was extensive and unfortunately various pads and traces lifted on me while desoldering. About 10 in total and all around the power section of the board. I've repaired it as best I can but I openly admit part of the cause was more than likely me being to heavy handed with the heat on the desoldering station.

    Its a real shame as I had hoped this would be a cheap project box, just a recap and it would be fine. Never seems to work out that way.

    Sorry for the long winded post!

  2. #2

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    Hate to ask, but are you absolutely sure all components are installed back on their correct place and in correct orientation?

  3. #3
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    If the fuse is instantly blowing, then you have a direct short between hot and neutral. I'm going to guess that you either put a component where it shouldn't go, or you put something in backwards. Chances are that said component is going to be dead.

    Use a multimeter in continuity mode and check between hot and neutral past the fuse on the analog board, you'll almost certainly find a dead short. Start inspecting components for being in the wrong place or wrong orientation. If you can't find a smoking gun, start tracing the circuit and pulling components off the board until the short clears. A direct short across the fuse is going to be somewhere on the primary side of the supply, if it was on the secondary side, the supply just wouldn't start up properly, or have weird voltages.

    In the future, you need to take detailed pictures of the board, because it sounds like you just made a list of what component goes to a silk screen label on the PCB, which isn't really enough detail.

  4. #4

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    Quote Originally Posted by GiGaBiTe View Post
    If the fuse is instantly blowing, then you have a direct short between hot and neutral. I'm going to guess that you either put a component where it shouldn't go, or you put something in backwards. Chances are that said component is going to be dead.

    Use a multimeter in continuity mode and check between hot and neutral past the fuse on the analog board, you'll almost certainly find a dead short. Start inspecting components for being in the wrong place or wrong orientation. If you can't find a smoking gun, start tracing the circuit and pulling components off the board until the short clears. A direct short across the fuse is going to be somewhere on the primary side of the supply, if it was on the secondary side, the supply just wouldn't start up properly, or have weird voltages.

    In the future, you need to take detailed pictures of the board, because it sounds like you just made a list of what component goes to a silk screen label on the PCB, which isn't really enough detail.
    I can never understand why people want to make such a mess of this stuff. Gigabite is right. You most likely got a component upside down or a blob of solder where it doesn't belong.
    Dwight

  5. #5
    Join Date
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    Firstly just let me clarify. I took high res detailed photos of everything and as best as I can see everything is in its correct place and orientation. I've triple checked the soldering there are no blobs as you put it.

    Primary section before


    Primary section after


    Doing some further testing this evening there is no straight short but with the meter set to 2K I get a reading of about 1400ohms between live and neutral. Weirdly with the meter set to 20k and 200k I get nothing then with it set to 2M I get a reading of 450k ohms between live and neutral.

    I'm wondering though if DP9 could be at fault here. It is without doubt installed in the correct orientation but it reads short between + and - in both directions. Doesn't seem correct but then again I admit I'm not sure what is correct for this component.

    EDIT: nope testing the diode out of circuit it seems fine. More hunting required.
    Last edited by Glen M; August 9th, 2020 at 10:50 AM.

  6. #6
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    After some more testing I think I might have found the culprits.

    DP2 is shorted as are the middle and bottom pins (looking at the images) of QP2. Edit - checking a data sheet this is pins 2 and 3.

    With DP2 removed the short between + and - on DP9 still exists. With QP2 also removed this short is gone.

    But... as I say I'm by no means an expert with such things, so in your opinion is this likely to be the cause of the problem?

    Surely a good starting place anyway.

    Also, out of interest and just to confirm, can I power this analogue board out of the computer by just connecting say an old HDD for a bit of load? Am I correct in saying that with the motherboard disconnected the HV side of things is dead?

    In my initial power on the analogue board wasn't connected to the motherboard but I did make all connections to the CRT. It would certainly be handier for testing if I could run it out of the machine and not have to worry about the HV side of things.
    Last edited by Glen M; August 9th, 2020 at 12:13 PM. Reason: further information

  7. #7

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    Someone wouldn't have a pointer to a schematic for this thing?
    Dwight

  8. #8
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    Quote Originally Posted by Dwight Elvey View Post
    Someone wouldn't have a pointer to a schematic for this thing?
    Dwight
    You can get the schematics from here

    https://archive.org/download/Macinto...matics.tar.bz2

    This is an archive with various schematics. The MAC class schematic is found at \bomarc\macintosh classic schematic (bomarc).pdf Pages 4, 5, 6 are my analogue board.

  9. #9

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    Quote Originally Posted by Glen M View Post
    Also, out of interest and just to confirm, can I power this analogue board out of the computer by just connecting say an old HDD for a bit of load? Am I correct in saying that with the motherboard disconnected the HV side of things is dead?
    Yes, that is correct. The HV stuff is driven by the mainboard.

  10. #10
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    Apr 2020
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    I've order replacements for DP2 and QP2. I can't help but wonder why they failed. Possibly another component is bad or perhaps it was just their time to go with the stress of desoldering / resoldering causing them to fail. I'm fairly sure from memory that I checked the diode before reinstalling and it was fine though.

    Reviewing the schematic

    https://imgur.com/a/Vfq2biw

    with DP2 and QP2 shorted that is more or less a direct + to - fault. RP30 and RP15 test fine. I'll check the other various components around this also and I suppose there is potential that IP1 might be damaged.

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