Crate BX-160 problem

[mtglick]

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!

[DrGonz78]

Well if the amp *was* working and, after it was shipped, now shows problems then something might be loose. To me the hum sounds like loose filter caps. Perhaps they were a bit loose and the time spent shipping the amp was the cause. That is assuming that is *was* working fine before the seller shipped the amp. Truly have you tried the Enzo Whack test? Hitting the amp with the ball of your fist might trigger a response to the amp. If pounding on the amp causes a change in the way the amp is behaving then chances are it is a loose intermittent component. In that case start at the mains filter caps and re-flow solder on the joints. That fact that it is happening through the power amp IN tells us it is something on the power amp side of the amp. Bust out the solder iron and start whacking.  :trouble

[J M Fahey]

Start by checking you have proper power amp rails (I'd expect between +/-40 and +/-50V DC, symmetrical within 1 V or 2) and +/- 15V feeding the preamp.

[SpareRibs]

Hello,
      I have a (Crate GX-40C+) that would not act right when I first tried to use it (same scenario used), try
cleaning all of the input jacks. It sounds strange but for some reason each jack is part of the ground circuit and if nothing is plugged into, and it does not fully close completing the circuit problems will occur.

[mtglick]

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

[mtglick]

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!

[DrGonz78]

Here is the power amp schematic...

[DrGonz78]

Oh and Windex is not really recommended either on the PCB boards. Denatured alcohol is best 99-100% and contact cleaner on the jacks. Of course others will chime in on their favorites, deoxit etc...

[Roly]

Agree.

Windex

4.0% isopropyl alcohol (a highly volatile solvent) 1% ethylene glycol monobutyl ether (a less volatile solvent), 0.1% sodium lauryl sulfate (a surfactant), 0.01% tetrasodium pyrophosphate (a water softener), 0.05% of 28% ammonia, 1% of a dye solution, and 0.01% perfume.

Not my first choice.  Some of this is good, some okay, some not so okay. 

A denatured alcohol such as metholated spirit is generally very available, and if fresh reasonably dry, and my normal first call for contact cleaning.

[mtglick]

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.

[J M Fahey]

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.

Hiss or hum?

Hiss is unaffected by grounding and shielding, hum is the exact opposite.

[mtglick]

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

[mtglick]

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.   

[mtglick]

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

[Enzo]

it is not a tube amp, any analogies stop pretty early on.

Those metal things are power transistors, and usually fail shorted, they don't wear out like tubes.

First look for ripple on your main power rails, and especially under load.   A scope will reveal power supply problems in a heartbeat.  Likewise scoping the output will show us the nature of your distortion.