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!
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
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.
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.
Thanks, all. Will start with this and report back w/results.
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!
Here is the power amp schematic...
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...
Agree.
Windex
Quote from: http://en.wikipedia.org/wiki/Windex4.0% 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.
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.
QuoteSecond, 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.
Might be both, or a layered high-freq hum/distortion. Responsive to EQ and volume controls, but severe.
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.
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
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.
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
Well there are the easy things that can be done too. Easy but time consuming... Like re-soldering the entire power board and especially the main filter caps. Perhaps just start there at the filter caps, but if you got the whole thing out you might as well hit a bunch of points. Look at the wire wound resistors and hit those solder points too. This approach might help or it might not, so my advice may be impractical. In fact, my advice might just be wrong/useless or it might help. So this is similar to shotgun repair without any *real* troubleshooting, except we are not pulling parts. So you might solder something better and fix the problem. Only problem is we won't know what was really wrong in the first place or if it will stay fixed. When I first started working on amps I had to practice soldering every amp including working ones. It really helped me improve my soldering skills, but now I realize it was overkill. Now when I re-flow a board it is because it really needs it bad. If you re-flow a joint and it turns cloudy then it is cold. You need to remove all that solder and put fresh solder.
Well I blabbed a bunch but just thought to share my novice thoughts here. Also, can't really hurt to change the filter caps on an older amp if you have it apart already. So that is my impractical advice that I used a few times in the past. Take it all with a grain of salt 8|
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
I guess one last thing I meant to add relates back to Enzo's post. When observing the voltage at the filter caps we look for the DC voltage. However, not having a scope to see AC ripple is a problem. Try reading AC ripple with your DMM set to AC voltage. That may help test and sometimes it harder to see observe with a simple DMM. If there is excessive AC ripple it will help us test that hypothesis.
This power amp looks like it has a limiter setup and if that is faulty it would likely give early distortion.
Look at OC1 at input, they join up to points a/b/c at output (R19-20 / D11-D14)
I figure you could disconnect the Opto to see if it is causing the problem.
Phil.
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!
Your description of what the gain pot is doing sounds kind of odd. Is it possible it is defective and going straight from zero to full?
The problem here is one of information. You report distortion, but that is like reporting a rattle in your car. It could come from anywhere.
I know I am asking for a scope where none sits, maybe you know some electronics hobbyist with one who might assist. But distortion simply means the output does not match the input in shape. but it doesn;t specify ripple or failing filter, nor does it tell us if it is clipping, and if so symmetrical or non, or one sided, and if the waveform is missing on one side. Another thing is that certain types of failure only occur under load, but without a load you cannot hear any changes, though a scope can see. Until we have a better idea of the nature of the distortion all we can do is guess from the list of a thousand things what it might be.
SO we are back to the rattling car. Is it a loose steering thing? A flapping body panel? a failing motor mount? Something loose in the trunk?
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
Then you understand. Your car tech has the added information that the noise is associated with engine speed and has a thumping nature. When he sits in the car he uses sensing equipment - his ears - to detect the type of rattle sound, and the direction it comes from, plus the association with RPM. In your amp we use a scope as our ears, and the waveform of the distortion points us in the direction.
If I knew that the output had half the waveform missing it tells me where to direct you. Makes me look smarter than I am when I do that. Then if I also know that the waveform looks intact unloaded, but the distortion is associated with load - not unlike higher RPM - that points me in a different specific direction. If you have hum and distortion along with a high amplitude RF waveform, I will point you at a certain resistor - in your zobel network. You cannot hear RF oscillation, but it has a certain hard to describe sound, with hum and weak distorted sound. Your auto tech might instantly realize the parking brake is stuck on by a sluggish response. I know I know it every time I do it in my truck. "Gee, this thing is a pig today and... oh, right..." Well, I usually recognize the sound of RF oscillating amps as it affects their audio performance.
I have a squeal coming from under the hood. COuld be a bad water pump, could be a failing crankshaft bearing, could be a slipping belt, or a bunch of other stuff. Overall, maybe one is more likely than another, but one would really want to listen to localize the issue. And that is what our test equipment does, it "listens" closely.
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.
Well those resistors you speak of are called cement wire wound resistors. The two in question are probably R36 and R37 as they are dropping voltage to the preamp. Since the wire wounds drop a significant amount voltage they will heat up and that is perfectly normal. The PCB board will always start to look cooked in those areas as with age and use.
Quote from: phatt on January 01, 2014, 09:05:52 AM
This power amp looks like it has a limiter setup and if that is faulty it would likely give early distortion.
Look at OC1 at input, they join up to points a/b/c at output (R19-20 / D11-D14)
I figure you could disconnect the Opto to see if it is causing the problem.
Phil.
Perhaps there is a way to remove that Opto OC1 as Phatt had suggested. I am not really sure about doing that but someone else here might be able to help. I guess if you send some close up pictures that can only help too. 8|
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