Can I nominate myself for an audio "Darwin" (Hafler repair)

[j beede]

How stupid can I be? I was making good progress on resurrecting a 30 year old Hafler DH-200 when I "blew it up... real good".

This "throw away" amplifier was rescued from a damp garage. The wiring and solder joints looked like they were 130 years old, not 30.

I spent 2-3 hours tearing it down, cleaning, reflowing and isolating channel 2. Power supply filter caps measure good. I powered the module up on the bench and it sings like a bird. Nice.

Now repeat the process for channel 1... all was well until I powered up this isolated module with the rails swapped--plus for minus, oops. The fuses flashed, but too late. I replaced the rail fuses and they blow instantly. I am hoping that the (rare) output MOSFETs go open circuit when they fail, not shorted. This happened late last night and I haven't yet begun debug.

Are the BJTs on the input board as hard to get as the outputs? If so I may have just snatched defeat form the jaws of victory.

[MagicBox]

You've been "punished" already, but I'd still like to advice you to make it a routine to doublecheck power rails wiring. Ommiting double-checks can be fatal if working with mains. You have only one life unless you're a cat. I know, it sounds extreme.. just trying to press the value of doublechecking. Electrolitics polarity is also a candiate for mandatory doublechecking.

[Frank Berry]

Some years ago at a television station where I was employed, the power supply in a digital video processing (Ampex ADO) system failed. One of the station Engineers replaced the 1kw supply and cross-connected the + and - rails.
When powered-up, the entire system went up in smoke and flames.
Hundreds of 7400-series ICs and ribbon cables were fried.
The old ADO went directly to the dumpster.
OUCH!

[sbrads]

The only time I've known FETs to go open circuit is after they've gone short circuit and blown themselves to bits usually internally and invisibly, sometimes with a hole in the case. Usually they just go short or very low resistance and blow a fuse or track or resistor, so they never get the chance to go open circuit.

[bear]

It's a nice amp. The repair shouldn't be too awful.

Try an ohm-eater across the output Mosfets.
One good thing, the amp will RUN with only one pair of outputs... of course you can't
run it hard into a load, but it will work.

Always power up unknown amps with a variac and an ammeter in series with the line,
that way if it's all messed up, you see it draw a ton of current FAST, you back off.

Worst case is a lightbulb in series with the AC mains... not as good.

You may have toasted almost all of the semiconductors, but you may have gotten lucky and let the magic smoke out of only a few choice ones...

_-_-bear