Fostex X-55 blown PSU - fixable?

[Tom41]

My sister's Fostex X-55 mixer/tape recorder recently died. It suddenly popped and made a burning smell - she thinks she connected a power brick with the wrong polarity to it. I've had it opened up, and there's no visible damage to the main logic board or the mixer circuitry. However, on the power input PCB there is a diode that has popped open, and a small burn mark in the same place on the PCB base.

Would replacing the diodes be likely to revive this unit, or do you think something else has blown inside? The caps on the power input board have not bulged or vented, and there's no charring around any of the internal wires.

She's considering selling it for spares or repair so if it's not a simple fix, it's probably not worth attempting at all. Sorry there's no pictures, but she insisted I close it up again to keep it from getting any more damage.

Any ideas?

[999999999]

Re: Fostex X-55 blown PSU - fixable?

Not knowing the circuit, not having a schematic, it's difficult to say whether it is likely to have damaged anything else. A well designed PSU should have some polarity protection.

Since diodes are common and cheap, and it has high value, it's worthwhile replacing it to see if it works. If something else is damaged of course it may not but considering the minimal amount of time and expense it's worth a shot. You won't do more damage that matters considering what you wrote, that if it doesn't work at that point you'll probably scrap or part it out.

You might also see if there is somewhere right after the power input socket that you could break the circuit and add another diode to provide polarity protection in case she'd ever accidentally connect the wrong polarity supply again, odds are good it will still work with the typical 0.3 to 0.7V supply drop across a diode implemented in this way.

[PCBONEZ]

Re: Fostex X-55 blown PSU - fixable?

Are you sure that 'diode' wasn't just a PCB fuse?

[Toasty]

Re: Fostex X-55 blown PSU - fixable?

>>but she insisted I close it up again to keep it from getting any more damage<<

???? That's a new one. Sunspots perhaps?

>>...accidentally connect the wrong polarity supply again...<<

Always wondered why they don't use a bridge rect. to completely avoid the issue?

>>break the circuit and add another diode<<

Specifically - Inline with, not across the input.

[999999999]

Re: Fostex X-55 blown PSU - fixable?

If the circuit couldn't tolerate the forward drop across one series diode (which is doubtful, but I'll mention it anyway), a diode could be placed reversed across the PSU input, in doing so it would reduce the negative voltage to what is usually a safe level, that of the diode's forward drop. This would be done in conjunction with the input having a fuse or breaker before that point so it trips the fuse or breaker in a reverse polarity condition instead of blowing the PSU if it doesn't have it's own overcurrent shutdown circuit. Probably easier would be to just use a different type of keyed connector so the wrong PSU won't even plug in.