Got a problem with an amp

[SolidxSnake]

Several years back, I got a brand new Crate GFX120.  It's one of the older ones, not the ones with the red/black scheme, but blue/black color scheme.  It shot sparks from a short last year.  It is a 1x12 120w combo.  I recently decided to fix it up. Replaced the fuse (3A 250V Slow Blow), and the thing powers up and plays fine. However, after maybe 10 minutes of playing, the sound starts to become extremely inconsistent. For instance, I will play a chord on the guitar, and the sound fades to a very low and quiet level. It will keep fading in and out according to the output volume of my guitar (I assume). If I cut the volume on my guitar (turn down the knob all the way), then the hum from the distortion channel from the amp slowly comes back and stays consistent. This leads me to believe my guitar is "overloading" something in the amp. A visual inspection leads me to believe nothing is wrong with the amp (no blown caps or ICs etc.).  It is quite hard to explain.

Any clues? I can take pics of the amp, videos, sound clips etc.

I've confirmed it's not the speaker of the amp.  My gut tells me it's in the poweramp stage.

[teemuk]

Email me if you need the schematic. teemukyttala[removespam]@gmail.com

[jimmy peters]

you have a cap. or a resistor breaking down when the amp gets hot. be careful of slow blow fuses, they are designed strictly for motors.
ger your amp to a good repair shop fast.

good luck
jimmy peters

[joecool85]

you have a cap. or a resistor breaking down when the amp gets hot. be careful of slow blow fuses, they are designed strictly for motors.
ger your amp to a good repair shop fast.

good luck
jimmy peters

Slow blow fuses are used in amps all the time, in fact, Brian (chipamp.com) recommends using a slow blow fuse for any of his gainclone amps.

[SolidxSnake]

The fuse that came with the amp stock was a slow-blow fuse.  My tube amp uses fast-blow fuses.

Is there any way to pinpoint what specific part is breaking down from heat?  I'd assume it's after the preamp stage as it occurs for the clean and the gain channels.  I can try rigging a fan above the amp to see if it stops/occurs later.

[teemuk]

Slow blow fuses are common in amplifiers that have high inrush current.

"Is there any way to pinpoint what specific part is breaking down from heat?"

Yes. Cold spray, for example.

Anyway, plugging signal source into the "insert" jack should tell you whether the problem is in the preamp or in the power amp. (Be careful, there is no volume control for it in the amp.) You said the amp "shot sparks" and that doesn't sound too good. Sparks generally mean high current and high current melts things like solder joints etc. Was the short over speaker leads or something else? I second the opinion about thermal issues but you also have to narrow down the search area to locate the faulty part.

Just to make sure, I would first check that the quiescent current is right to find out if bias servo is still working correctly or if there's trouble with the power transistors. You should read 7mV across R72 with no signal and no load. Also, check the DC offset at the positive speaker terminal (use the negative terminal as reference instead of ground) - this has likely nothing to do with your problem but you want to be "safe" anyway. If it's anything higher than few millivolts then you should likely worry.

If it's the poweramp... Well, the concerned circuit in that amp is pretty basic so I would attack the section around the drivers/power transistors first to see that all those parts carrying high (or moderately high) current are OK and that there is no problem in their solder joints etc. Do you have the schematic for the amp?

I hope you do use a current limiter (i.e. light bulb) inline with mains since at current state it is reasonable to assume that the amplifier might just suddenly develop a major fault. Just because it seems to "work" now doesn't neccessarily mean it will do that for long. Do not measure the bias with current limiter inline though.

[SolidxSnake]

The short was under the amp, it wasn't the amp's fault, human fault for the shorts (some metal rod got stuck under the amp between the chassis and the board).    The amp was doing the exact same thing before the short, so I'm just about positive the short had no effect on the amp besides blowing the fuse.

I do need the schematic.  Email me at MetallicAman91 [at] gmail [dot] com

[SolidxSnake]

R72 was not reading 7mV, but after adjusting AP1 a bit, it is now 7mV exact according to my cheap DMM.  This was without any input and with the speaker disconnected.  That was how I was supposed to measure it, correct?  With no signal/inputs and no load (speakers)?

I turned the amp on for all of a minute or two with the speaker connected but no input, and noticed the glue around the resistors towards the top-right of the PCB (R61, R88, R89, R90 and one other which I can't make out the name) was getting very warm.  Maybe there's a problem there?

For the record, all of the solder joints on the board are fine from eye inspection.  The input jack is missing.  My Ext. Speaker jack broke, thus I replaced the Ext. Speaker jack with the Input jack.  I made sure to solder the two points that weren't going to be connected with the jack gone (The jack was a switched jack, as in when a 1/4" cable was plugged in the switch was open, otherwise the switch is closed).  I still have yet to test to see where the problem is, the poweramp or the preamp.

I don't have the greatest electrical knowledge, I know the basics.  I don't have that many tools on hand either, I have a soldering iron and a cheap DMM...

[teemuk]

Yes. According to schematic that is the way to measure the quiescent current. If it wasn't way off it's normal. What you really want to know as well is will the reading fluctuate once the amp has been on for some time. It should stay pretty steady once the amp has warmed up. BTW, do not adjust it "cold" (wait few minutes after turning the amp on so operating points have time to settle). Anyway, it seems like there is no problem with the bias.

According to schematic R88, R89 and R91 are series resistors for regulators. They are supposed to heat a little (thus the 5W rating) but if you think there might be a fault then check the voltages. There should be about 15-16V after R89 and R90 (+V and -V rails & zener diode regulation). Naturally, 5V after the DSP regulator - which is IC.

R61 is an emitter resistor. Basically, R72, R74 and R64 should heat as much as it does. If there is excessive heating in only one emitter resistor it is very likely an indication of a problem with the concerned transistor. At idle (no load, no signal) there should be little more than 1W wasted in the emitter resistors (again they have 5W rating) so they will heat a little.

However... Did the glue start getting warm after the adjustment? If so, recheck the adjustment. I don't know how Crate designs stuff but usually nothing should melt when the amp is on idle. Basically, less bias will do no harm. It will decrease the heat losses but in turn will also increase crossover distortion.

The schematic shows tables for voltage readings at certain test points (marked TPxx). You can check if they match. Naturally, they don't need to be exact, just reasonably close. I trust that you also know the basic transistor/diode measurement techniques. Your greatest problem will likely be trying to get the amp to exhibit the fault condition while you do the measuring (this is why intermittent problems and alike are really tricky). I already mentioned how to find out if the problem is in the power amp (the insert jack method).

[SolidxSnake]

The glue was getting warm before adjustment (the voltage across R72 was 2mV then).  I haven't checked the resistors in suspicion since adjustment.

For voltages of the regulator resistors, I should not be measuring across the resistors, instead using the chassis or a ground as a common point, right?

The testpoints look as if they include the use of an oscilloscope, correct?

I can identify transistors and diodes, and know how to test diodes to see if they are still working.  However, I don't know exactly what you mean by basic measurement techniques for each.

[teemuk]

The glue was getting warm before adjustment (the voltage across R72 was 2mV then).  I haven't checked the resistors in suspicion since adjustment.

2 mV seems a bit low but now it should be up to the specs. The amp is likely running a bit hotter, though.

For voltages of the regulator resistors, I should not be measuring across the resistors, instead using the chassis or a ground as a common point, right?

Right. You can see the zeners shunting the rails to ground after the 5W resistors. At those points you should read about 15-16V + or - according to the rail you measure. The 5V for the DSP circuitry is regulated with IC. I trust it should be pretty OK but you can measure that the circuit has an output of 5V. However, I don't think it helps you in the trouble you are having with the amp. If the voltage for digital circuitry would be way off you'd be having some other problems. I'm also pretty sure that the rail voltages are correct as well.

The testpoints look as if they include the use of an oscilloscope, correct?

Well... sure. But you see that one of the tables quotes also the DC. This you can measure with an ordinary DMM.

I can identify transistors and diodes, and know how to test diodes to see if they are still working.  However, I don't know exactly what you mean by basic measurement techniques for each.

Well, I meant just that. You are familiar with concept that you should measure about 0,65V across base-emitter junction, likely more than 2V across collector-emitter. With fets about 1-4V gate-source and more than 2V drain-source. Basic silicon diodes should have voltage drop of about 0,65V etc. Since you know some voltage references you can test if semiconductors are OK.

However, I suspect that there is a failing part, intermittency introduced by heat or some other problem that is very difficult to locate by taking simple measurements. While the amp is functioning normally it will (very likely) also give readings that seem to indicate that everything is fine. I can't help you much further. All I can say is that...

1. Isolate the problem to certain parts in circuity as much as you can.
2. Think about the symptoms and what kind of problem in the concerned circuit might cause them.
3. Measure, inspect etc. to see if you are correct. Here is the major problem since you have to get the amplifier to exhibit the fault while you measure. If the fault is triggered thermally (at least it seems like it is) then a hair dryer and a can of cold spray will be quite useful tools. Naturally, they are a lot more effective if you can locate the fault to a certain part in the circuit. Heating or cooling the circuit randomly usually doesn't reveal anything concrete.
4. If the first educated guess was wrong think about other alternatives. Repeat 3 and 4 until you locate the problem.

[SolidxSnake]

Thanks for your help so far.

I brought the schematic to school and tried to explain to my electronics teacher what was going on, and she suggested it might be something from the power source.  She said to try checking the diodes, and the test points around the filter capacitors (TP14 and 15).

For the test points, an oscilloscope is helpful for the AC readings.  For the DC ones, a signal generator is required.  Thankfully, my electronics teacher lent me a signal generator.  The schematic says a 1KHz signal to the insert tip for the poweramp testpoints.

I'll post back with results as to what is happening if I can't figure anything out.

Once again, thank you for your time.

[teemuk]

If you get it fixed, please inform us. I'm quite curious to find out what was going on.

[SolidxSnake]

Strangest thing.

Today, I was gonna work on my amp, measure test points and such.  I first wanted to create a soundclip of what was going on to show my electronics teacher (as well as people on this forum) to see if that would help narrow down what was going on.  I unattached everything from the chassis besides the transformer (all of the PCBs, the fuse holder and the power switch are not attached to anything), and then started playing to reproduce the problem.  After 10, 20, 30 minutes of playing, I didn't get one single problem.  I tried the caveman's testing for temperature by touching various components, and some resistors and transistors were BURNING.  Resistors in question are R61, R64, R88, R89, R90.  Transistors are Q12, Q13, Q15, Q16.  The transistors I mentioned are attached to a metal plate which is normally attached to the chassis with some screws (some TIM in between it) for the chassis to act as a heatsink.  However, even after the resistors and transistors were extremely hot, I had no problem.

This is very strange.  The only difference is that the board is not enclosed or screwed into the chassis at all, and I have no problems.  I can take a picture of my setup at the moment if you don't understand what I am saying.

[joecool85]

Sounds like when it's in the chassis you are experiencing a short.