Thanks, Doc.  I'll try to get some closeups of the amp posted, haven't found anyone with a scope yet so this may turn into repair by component replacement.  I'm open to replacing parts, and testing whatever can be tested with my moderately cheap digital multimeter.  I'm reading through Teemu's book now, just to get a sense of how this thing is supposed to work.  I get the theory of it (at least in VERY broad strokes), and as Enzo said it's just not comparable to tube amps, so I'm learning as I go.  The mechanics of how it works are still a mystery, but I'm plugging along.

Thanks again,

Matt

Enzo--again, thank you, and I do understand why you're asking.  Just updating, I reflowed anything that vaguely resembled a cold or failed joint, no change. 

New data point, for anybody following along.  There are six 220 ohm resistors on the power board, in banks of two.  The resistors are shaped like square troughs, filled with a white ceramic material, with a metal lead from each end.  When I flipped the power board over to reflow the points, I noticed that the PCB was slightly scorched under one of the pairs, and a small amount of the white ceramic seems to have cooked off the top, as I found some residue nearby.  Both resistors came up to 220 ohm on my digital meter, so this may be a bit of a red herring, but I thought I'd post it.

Also, a correction to a previous post--the limiter only lights when the preamp is bridged to a separate power amp, and. On its own, the limiter is not lighting. 

The gain seems to go from 0 to 10 in the first 5% of the knob travel, and then from 10 to 50 from there.

Enzo--I completely understand where you're coming from, I'm a tech of a different variety and approach my problem solving exactly as you have requested.  However, to stretch your analogy, while a rattle in a car CAN come from almost anywhere, with certain makes and models there are frequent fail points that are often diagnosed faster through a repair, instead of an exact diagnosis. For example, a thumping rattle that moves with the tach in a late 80's BMW E32 is often the driveshaft center bearing.  Having a tech tear the whole car apart to confirm the issue is far more expensive than just replacing the part and checking operation.  Naively, I was hoping someone here could point me in a similar direction.  Please understand that I'm grateful for your feedback thus far, and hope that you'll continue trying to assist--it's very much appreciated!

Matt

Phil--

At least from the front panel, there is a limiter LED, and it is lighting with the distortion, so it's logical to assume there is a limiter circuit somewhere.  Willing to disconnect it, will try to figure out how. 

Thanks!

Time-consuming is not an issue, and pulling the boards is actually very simple in this unit--four screws on the back, four on the board, and the connectors.  I'll reflow the solder and report back.  If anyone has anything further, I'm all ears.  Thanks again for the efforts!

Matt

Thanks for the advice, Enzo.  The problem is I don't have a scope, and am unlikely to spend the cash to acquire one just for this project, given the relative value of the amp.  I can understand the need for data to pinpoint a cause, I guess I'm hoping there's a logical fail point (or a series of them) that can be component-swapped to a resolution.  The seller I acquired it from is willing to help with repairs, but given that I paid very little for it, there's only so much room for him to help.  The local pros are asking for some fairly hefty up-fronts, basically because the amp isn't worth much so they want to get their profit in ahead of the repair bill.  I'm just trying to figure out a solution.

Thanks,

Matt   

Further updates on the issue.  Preamp is cleared of all wrongdoing. I sent the preamp signal to a known-quantity combo and got a clean signal.  There does seem to be a bit of ground hum, a little more than I'm accustomed to but tolerable, and responsive to EQ/instrument controls.

However, I'm still experiencing the same heavy distortion at high volume, and the amp failed the whack test, no change upon applying a couple of good thwocks directly to the chassis with a rubber mallet.  One new data point, if I turn the volume up, and the gain to 0, I get a low-volume signal, but with a much cleaner tone.  As soon as I apply ANY gain above about a half (0.5 on the dial), the gain stage kicks in and we're off to stomp box metal-land.  Same happens (more or less) at max gain with 0 volume. 

If it was a tube amp, I'd be replacing valves.  I just don't understand what the equivalent components are in a solid-state design.  There are four round metal caps (Motorola imprints), secured by two screws each, along the back rail of the power section PCB.  They back up to a metal bracket that is apparently functioning as some kind of heatsink.  The bracket bends up from the PCB, and the upper section is flush to the chassis back wall.   Thermal paste was applied between the bracket and the chassis wall.  The round caps look kind of like where tubes would connect if there were any, at least in terms of size and proportion.  Are these openable/testable/replaceable?

Thanks for your assistance,

Matt

OK, finally got a chance to put probes to metal.  Spec calls for 15v pos and neg, I have 16.4v pos, 16.3v neg.  This is at the connector that passes voltage to the preamp.   

Might be both, or a layered high-freq hum/distortion.  Responsive to EQ and volume controls, but severe.

Thanks, everybody.  Really appreciate the schematic, I'll print that up and start checking later today if I have the time.

I understand Windex wasn't the best choice, but I've used it in a pinch in the past without too much difficulty, and some types of film deposits seem to respond better to it.  Would never clean a high-end piece with it, but for this it seemed OK.  To that point, the boards dried out this morning, and the pops went away.  I got some volume improvement in the signal but it's otherwise basically unchanged from the original post. 

Two new data points, though--the distortion seems worse at different frequencies.  I get an almost-clear note on an open G string, far worse on the open E.  The funny thing is that moving up on the G shows increasing distortion--almost as if the distortion was in phase with a G. 

Second, it would appear that at least part of the issue is a grounding problem somewhere--while powered up and connected I moved the chassis a bit, as soon as I touched it about half of the hiss vanished.  The distortion didn't change, though.

OK.  Thus far I've been unable to locate a schematic, and the amp is not labeled internally.  Other than identifying the big caps as 50v 4700mf's, and finding the wiring that's powering the prepamp off of the power board, I haven't been able to test anything.

That said, I did give EVERYTHING a thorough cleaning, and I went over every socket and plug to ensure that the wiring was tight.  All pots, jacks, and connectors got a good Windex paintbrushing.  Plugged it back in and am now getting a consistent pop-pop-pop (about 4 pops per second) that lights the limiter LED, with a speaker plugged in.  Without a speaker connected, it just powers up, I don't see the limiter engaging.  The good news is it's VERY loud, which at least speaks to the concept that the power amp can deliver volume, the bad news is that it's not responsive to the front panel controls--0 gain, 0 master, EQ at 0's across the board, and I still get that very loud popping.  The pop itself is similar to the sound of an amp at power up.

I'm hypothesizing that there's still something wet on the board, I gave it around 6 hours in a warm room, but I'm going to let it continue to dry overnight, hit it with a hairdryer, and try again in the AM. 

If anybody has a labelled schematic, I'd love to get my hands on that!

Thanks, all.  Will start with this and report back w/results.

Hi, all--

I purchased a Crate BX-160 amp chassis online (VERY inexpensively), was represented as a working unit, but well, it ain't.  Seller is certain that the unit worked perfectly prior to shipment, unit came pretty well packed, bubble wrap and peanuts, etc.  Box was unmarked, and there were no obvious signs of shipper abuse.  Installed it in a BX-160 cab that I already had.  Speaker cab works fine as an extension for my live rig, no issues there.

Unfortunately, the amp is seriously misbehaving--significant hum with or without cord plugged in, signal cuts out, severe distortion with instrument plugged in.  I tried three separate instruments, and several different cables.  All of the major controls work, and I can hear changes in the hum frequency when I move the EQ sliders.  The volume works, but the overall output is abnormally low and heavily distorted, with the exception of pops and crackles that come through strongly enough to activate the limiter LED at what sounds like a more "normal" volume.  The amp has an onboard chorus that works, but seems to self-engage at startup, and is buried beneath all of the distortion and hum.

I plugged a different amp's line out into the power in, same results, with the exception of the chorus--low power, lots of distortion, hum, intermittent total signal loss.  Tried a different outlet, no change.  Swapped all of the cables, no change.  Pulled the chassis back out, no obvious issues--caps all look good, no burn marks, no loose wires, no smell.  Didn't seem to run hot.  My best guess is that something is wrong with the power amp, but I don't know where to go from here.

Any thoughts on causes that I can check?  I have a basic working knowledge of electrical componentry but haven't really spent a ton of time in amps.  I'm likely to take this to a pro for repair, if it's repairable, but I'd like to be able to diagnose a bit beforehand so I know what to expect.

Thanks so much!