ampeg 350H got wet

[ilyaa]

okay as promised heres a SS amp -

ampeg 350H that was salvaged from a flooded basement. smells a bit musty in there and some of the chalk power resistors look like theyve absorbed a bit of water (the chalk looks darkish greenish kind of).

the good news is it powers up - my limiter tells me no shorts - and things actually look okay in there. the power resistors measure fine (should i be concerned about them) and im getting +/-60 and +/-16 on the main power rails.

edit: i had my scope set wrong. its giving me close to 20V p-p when cranked (into 8 ohms).

im not quite sure where to start - mosfet output stage but its hard to get to them because of an enormous heat sink they are glued to.

anyone give me some good advice for beginning to diagnose this kind of thing? (ive been in tubeland for a while and i have to be honest - my solid state theory needs some fleshing out)

i guess splitting the amp in half power/pre would be a good first step - ill do that and report back!

[Roly]

Transformers aside, most components don't really mind a bit of a wash.  It's not the water so much as the contaminants it brings with it, particularly in a roof cave-in, flood, or a dunk in salt water.  I've often given stuff a wash with warm water and a bit of dish washing liquid - BUT a good rinse with clean water followed by a wash with dry metho (which is hygroscopic) the a nice long dry in the sun.  Particularly with salt water it's more important to get the salt out as soon as possible rather than dry it out.  Wet trannies in particular need a good long warm bake to drive trapped moisture out. (but if you're running the amp they'll be getting that in situ)

Relax, FET's are more like valves than transistors.   :tu:

[ilyaa]

okay a little progress:

preamp looks fine - 50mVp-p in gives me 1.3Vp-p out

power amp not so good: everything is okay until test point 5 - where i should see 2.25V p-p im still only seeing about 1V p-p. at test point 6, where i should see about 5V p-p, im only seeing some weirdo looking waveform thats like 8Vp-p on one cycle and totally flat the rest of the time....

IC4 may be bad? all the DC voltages around there look fine....

one other note: the 180 ohm 10 W resistors in the power supply are getting SUPER hot while im testing it - they measure fine but they are really hot. ive disconnected the preamp via the long molexy jumper cable strand. is that a bad thing to do? its easier to access the board if i do that.....

also, because the amp turned on and did its thing more or less, i didnt take any measures to de-moisture it - should i?

[J M Fahey]

Now it´s too late, you already plugged it and turned it on.
You were VERY LUCKY it didn´t EXPLODE.

Don´t know where you live so neither what station you are in, but a couple days in warm summer sun will certainly help.
In winter put it 1 meter away from a stove. (The room heating type, not the cooking one).

After it´s reasonably dry we´ll retest it.

EDIT: by the way parts which are usually destroyed by immersion are pots, spray a good amount of WD40 into all them until you see oil foam leaving on all holes, move them back and forth a couple times and hope WD40 displaces humidity as promised on the can.

Same with jacks and switches which will become uselessly unreliable as soon as contacts rust even a little.

So basically the problem is on "mechanical" parts; resistors , capacitors and semiconductors couldn´t care less about a water bath or two.

[Roly]

Check that the Limiter function is actually off.

The DC conditions on the second IC4 are set by the feedback from the output, so check that TP11 is very close to ground as it should be.  If you have a dead OP device and standing voltage on TP11 this will be reflected back at IC4.

Considering the water, I'd give the area around IC4 a bit of a scrub with a toothbrush dipped in dry metho, then dry it out under a lamp o.n.o.

If you still seem to have okay DC conditions but odd AC conditions (and you are certain the supplies are actually getting to the IC) then just about any dual op-amp should substitute for a functional test, then get the low noise 5532 for the final repair.

But be a bit cautious here, IC4 is the start of the NFB loop and a fault anywhere in the loop will be reflected all the way around - what you are seeing may be a symptom not a cause.


As JM says, it's a bit late for drying out now it has been powered up.  Water as such is not the terminal nightmare some people think it is ("Hollywood" electricity) but it can cause some specific problems and gear that has been wet needs to be cleaned and dried before being powered up - flood waters are normally filthy and leave a lot of gruck behind in gear.