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Crate BX-160 problem

Started by mtglick, December 20, 2013, 02:09:24 AM

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mtglick

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   

DrGonz78

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|   
"A person who never made a mistake never tried anything new." -Albert Einstein

mtglick

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

DrGonz78

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.
"A person who never made a mistake never tried anything new." -Albert Einstein

phatt

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.

mtglick

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!

g1

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?

Enzo

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?

mtglick

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




Enzo

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.

mtglick

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. 

DrGonz78

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|

"A person who never made a mistake never tried anything new." -Albert Einstein

mtglick

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