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Shamelessly stolen from BobDenford on Reddit.

I recapped my SE/30 but mistakenly reversed the polarity of the two axial capacitors (C2 and C11). Prior to my error, the machine was fully functional with the exception of being unable to produce sound. When I turned the computer on the first time after replacing the capacitors, the startup chime rang. The computer was on its side, so I didn't see whether the display worked. I turned the computer off, attached the floppy and hard drive, put it right side up, and powered it back on. The fans spun up but the computer otherwise did nothing, even after I reset it a few times and let it sit powered on for a couple of minutes, at which point I removed the motherboard, discovered my error, and replaced the two capacitors, the larger of which (C2) was bulging. After doing so, the computer still refused to produce any video or sound. I tried reseating all of the onboard components, removing all but two sticks of RAM, resetting the PRAM, and pressing both the debug and reset buttons on the side of the motherboard several times to no avail. The floppy drive does nothing, even when a disk is inserted, and my scsi2sd flashes a yellow light once upon powering the machine on, then remains inactive. There is no obvious physical damage to the board, aside from the now-replaced bulging capacitor, and there is no indication that any of the components are missing their magic smoke (that is to say, I never saw, smelled, or heard any).

A working motherboard inserted into the same machine produces a display (no sound, still have to recap that one).

Is the board irreversibly destroyed? If not, what is likely to be damaged and how can I repair it?

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    Start by checking the power supply tracks on the board with a multimeter. If they is zero, obviously nothing will work, but most likely some power supply components, or a fuse, have blown and that is repairable, Blowing the caps shouldn't have put reverse voltage or over-voltage on the rest of the board, so there is a reasonable chance it survived.
    – alephzero
    Commented Feb 3, 2019 at 16:29
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    Whithout knowing which 2 capacitors that were reversed it is hard to answer. I can onoy say that you can consider that you had a short at those two places. How hard it is to repair the board depends on what was short circuited.
    – UncleBod
    Commented Feb 3, 2019 at 16:32
  • @UncleBod C2 and C11.
    – wizzwizz4 ♦
    Commented Feb 3, 2019 at 16:43
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    So many suspicious traces... i.imgur.com/nZge3bA.jpg
    – snips-n-snails
    Commented Feb 11, 2019 at 21:51

1 Answer 1

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The schematics for the SE/30 can easily be found on internet, for example here

This is what I can see on the main circuit board.

A quick glance tells that C2 is connected between ground and +12 V through a ferrite (L11) and 5 Ohms (two 10 Ohm resistors in parallel, R14 and R15). That means that you tried to take up to 2.4 Amps from the 12V. I doubt it was designed to handle that much current. The question is if the resistors were burnt also.

C11 is connected directly between +5V and ground.

The schematics I got hold of might not be correct ones for the power supply. They contain 2x +12 V. Disk and Sweep. I will here follow the +12V Disk, since it seems the most probable.

Since the power supply is completely without protection in form of fuses or over current for the +12V, the transformer (TI51) might been burnt.

If the +12V (Disk) is missing, the transformer is most probably bad.

The +12V Sweep is generated in IC252, which seems like a normal voltage generator, so in the case of +12V Sweep being bad, I'd say that the problem is in IC252.

Edit 1 Seems the problem is solved: "Edit: Running the jumper wire from C25 to pin 11 of UB11 solved the problem. Thank you for your assistance. I don't think I would've been able to fix my SE/30 without it." (From reddit thread)

C25 is parallel with C2, and they are (normally) connected to UB10/UB11 pins 11 and 12. So, the re-cap did apparently damage the circuit board enough to make a open circuit there. (Well, it can happen the best. I've had to fix some boards myself with jumper wires...)

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  • Power supply transformers are pretty robust. If you start to fry a transformer, you can usually smell it before you do any serious damage. Smells are hard to describe, but it's very distinctive, rather like cooking with hot sugar. Small resistors (like 10 ohms) usually fail by catching fire, rather than invisibly going open or closed circuit. If there aren't any black soot deposits, they probably survived. Blowing some diodes in the power supply would be more serious, because AC power might have got onto the board instead of DC, and that could fry almost any of the chips.
    – alephzero
    Commented Feb 3, 2019 at 20:42
  • R14 and R15 look OK, and the power supply and analogue board work in another machine. What's the difference between +12V (Disk) and +12 Sweep, and how can they be checked?
    – wizzwizz4 ♦
    Commented Feb 5, 2019 at 17:40
  • @alephzero There was no smell.
    – wizzwizz4 ♦
    Commented Feb 5, 2019 at 17:40

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