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Fender M-80 Bass Amplifier making distorted output

buck1108

buck1108

New Member
Hi,
I'm pretty new to diagnosing electronics, although I've built kits for several years.
I've been working on a Fender M-80 bass amplifier (circa 1993) which has a distorted output.
The signal is clean (in scope wave reading and audibly) through the pre-amp section, at least to the "effects send" jack, which is also clean.
In the push/pull section, in the test points (from the schematic), there is a clipped wave. Alternately, the entire positive half of the wave 'disappears.' This happens seemingly on a random basis, during testing.

In tracing with the scope, I've gone back from the speaker jack, through the various paired NPN/PNP's, and I can't seem to isolate the origin of the clipping. A somewhat clipped wave appears even as far back as transistor Q8 (2SC2362K). I've replaced all 2SC2362K's and 2SA1016K's in this region, and the readings remain the same. Additionally, I replaced the transistors on the heat sink (Q22 - 2SC3201 and Q24 - 2SA1302), with the readings remaining the same afterwards. All electrolytics have been replaced, as well.

What might be a more informed method for troubleshooting, and what might be causing the wave to change appearance in this manner?

Thanks,
Buck
 

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You seem to have an intermittent circuit or connection in the positive half of the PP amplifier ("Alternately, the entire positive half of the wave 'disappears.' This happens seemingly on a random basis, during testing")

Try poking around with an insulated stick at all the wiring or connections on the PCB while watching the scope.

Mark T. :music:
 
Also, I see that Q8 receives the signal from U4, pin 1, that TL072... Those small OP amps can fail sometimes. Check the signal getting in that IC (pin 2), the output (pin 1), and the supply voltage (+ - 16V at pins 8 and 4). If the signal arrives OK, and get out distorted, and the supply voltage is steady, the IC is probably bad. Also, this IC is between the point you have good signal, and Q8 where the distortion appears... I'd suspect of it..

You have U5 and U6 also, between the OK point (effects send) and Q8.

You are on the right way, narrowing the source of the distortion. I don't know other method apart of tracing the signal.

Also, did you do that MOD suggested by the manufacturer, replacing 4 resistors to lower the bias?

I have an M-80 right now here, but finished the work on it, and closed and mounted on a large cabinet, not easy to disassemble to take measurements.
 
markn2wae said:
You seem to have an intermittent circuit or connection in the positive half of the PP amplifier ("Alternately, the entire positive half of the wave 'disappears.' This happens seemingly on a random basis, during testing")

Try poking around with an insulated stick at all the wiring or connections on the PCB while watching the scope.

Mark T. :music:
Click to expand...
Thanks, I will try to do that. I've often wondered where to get a good set of insulated sticks? Are they that fancy? :)
Also, it looks like there is a solder pad that is 'half missing,' although the half that is there, is the part connected to the trace. Continuity tests have indicated that the solder joint is good there.
Would you recommend a solder pad replacement, or the wire wrap technique, etc? Thanks
 
elnaldo said:
Also, I see that Q8 receives the signal from U4, pin 1, that TL072... Those small OP amps can fail sometimes. Check the signal getting in that IC (pin 2), the output (pin 1), and the supply voltage (+ - 16V at pins 8 and 4). If the signal arrives OK, and get out distorted, and the supply voltage is steady, the IC is probably bad. Also, this IC is between the point you have good signal, and Q8 where the distortion appears... I'd suspect of it..

You have U5 and U6 also, between the OK point (effects send) and Q8.

You are on the right way, narrowing the source of the distortion. I don't know other method apart of tracing the signal.

Also, did you do that MOD suggested by the manufacturer, replacing 4 resistors to lower the bias?

I have an M-80 right now here, but finished the work on it, and closed and mounted on a large cabinet, not easy to disassemble to take measurements.
Click to expand...

Thanks very much. I will have a look at these points.
I'm a little ahead of myself here, but if a chip is bad, would there be a method to look for possible causes of damage?

I noted, soon after I started on this repair, that several components had been replaced, per a Fender tech bulletin.
Where was the MOD to which you refer (did it have a bulletin number, etc)? I will double check the ones replaced on mine.
 
Bias modification: bias_modificacion.jpg

The OP amps can be damaged by some little overvoltage, or any electrical event. If the voltages are OK, you usually replace them. If they fail again, trace the problem.

But look at the signal, you'll find he fail for sure.

About JFet, I usually look at the signal at the input and output, and the supply voltage.
 
elnaldo said:
Bias modification: View attachment 651041

The OP amps can be damaged by some little overvoltage, or any electrical event. If the voltages are OK, you usually replace them. If they fail again, trace the problem.

But look at the signal, you'll find he fail for sure.

About JFet, I usually look at the signal at the input and output, and the supply voltage.
Click to expand...

Many thanks - will look at that soon.
I was wondering if this technical bulletin post was for the M80 guitar amp or the bass amp?
 
Thanks.
I've been testing the circuit over the past few days, and it seems that the non-clean wave is coming into the collectors of Q8 and Q9. My testing continues.
Also, as it's been very dry over the past couple of days, I've been wearing my grounding strap (clipped to the chassis of the amp). Yesterday, however, the tip of my scope probe was sparking on a few occasions, when I was testing the Q8 and Q9 transistors.
I was wondering, short of using a humidifier, what you might recommend?
 
Well don't clip your ground strap to the chassis.. You need a ground mat with the strap you connect to that or just a good metal bench. I guess perhaps in some cases the scope may be in error? some people seem to draw more electrostatic charges or just charges than other people.

Just a hint; in my last abode an apartment, to do serious work I had to walk barefoot as carpet was a static generator, even my dmm would read stupid.. however those ac circuits were noisy even on a good T.V. on cable.
 
buck1108 said:
Thanks.
I've been testing the circuit over the past few days, and it seems that the non-clean wave is coming into the collectors of Q8 and Q9. My testing continues.
Also, as it's been very dry over the past couple of days, I've been wearing my grounding strap (clipped to the chassis of the amp). Yesterday, however, the tip of my scope probe was sparking on a few occasions, when I was testing the Q8 and Q9 transistors.
I was wondering, short of using a humidifier, what you might recommend?
Click to expand...

The schematic shows the test points to follow the signal path, and the "test conditions" (5mV AC 1KHz sinewave at the HI input).
I'd post here your findings, a schematic with your notes, where the signal is OK, and where the signal is distorted.

Q8 and Q9 collectors are the main V+ and V- rails. Check you have + and - 48V at the collectors and you have +25.5 and -25.5V at TP 28 and 29.

Also check signal at U12 output, pin 6 (power amp input), and narrow from there to the output. Don't discard the simplest: a bad output transistor. If in doubt , replace the pair.

To avoid any spark, or transistor damage, I'd suggest to attach the scope probes before powering the unit, with hook type probes, and then power ON and read the measurements. It's VERY easy to slip a probe and short a transistor...
 
Thanks very much. I apologize for not posting sooner, but we've had much heavy winter ice storms, with damage, in the area since the last post.
I am currently getting my testing area back into shape and hope to post soon.
Thanks again for your input with this. :)
 
Well, I've resumed testing the bass amp, and I've begun unsoldering one leg of all the diodes, to try testing that route.
As I'm still fairly new to using the scope, and as the suggested test points in the schematic yielded no clues, I opted to go with a DMM and analog voltmeter.
I came across a small zener (1N5245B), which tested with full continuity 'both directions.' It showed '0' v on the DMM, 'both directions,' as well. On the schematic, the "B" might be an "8," but I wasn't sure if there was such a thing as the "8" variation?

If this is a sign of failure for the zener, would the possible cause be unstable voltage from the -16v rail? Or, could it possibly be a more 'random' or defect-oriented/lifespan failure? How could I ascertain the more likely cause? Thanks.

Attached is the zener in its place in the schematic (lower left corner of the full version of the schematic (linked in first post)).

I have a few more diodes to test, and I was wondering if there are advantages of using a scope to test the diodes out of circuit?

Thanks for your input in this. :)
 

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To properly test diode CR10 you must disconnect one end and measure it.

If the diode looks good, then check Q10, it may have a base to emitter short (leave CR10 disconnected for this).

Without disconnecting these parts, you would not discover the one with a short circuit as they are connected together.

Mark T. :music:
 
Actually Mark, you are usually right, but in this case the diode is shorted. Looking at the print, one side goes to the negative rail. The other side goes to the base of a transistor. That transistor only has so many paths to that negative rail, the emitter goes through a resistor and would not read zero. And resistors do not short. You follow everything that comes off the collector of that transistor and the best it can do is about 1 K ohm. Not enough of a short to result in a zero reading.

Bottom line, the Zener is shorted. Sure, lift one end and make sure, and when you do that DISABLE THE AMP because if it gets powered up with that disconnected it could to be overbiased or whatever and could blow something. That means take the fuse out, unplug the output boards or whatever it takes so it does not go up in smoke when someone "forgets".

The diode comes back as a run of the mill 15 volt Zener. Simple math says 50 mW with those circuit values (15K ohms 50 volts). A half watt will be fine. Don't go higher because higher wattage Zeners take more current to push them.

One nice thing about this particular repair is that this failure probably didn't blow any other parts. If that turns out to be the case consider yourself lucky, and it is quite possible.
 
markn2wae said:
To properly test diode CR10 you must disconnect one end and measure it.

If the diode looks good, then check Q10, it may have a base to emitter short (leave CR10 disconnected for this).

Without disconnecting these parts, you would not discover the one with a short circuit as they are connected together.

Mark T. :music:
Click to expand...

Thanks Mark! Yes, I had de-soldered one lead of CR10 before testing (with the shorted results), and Q10 tests ok, based upon your test recommendations (thanks for the recommendation/explanation).
I appreciate it! Buck
 
JURB said:
Actually Mark, you are usually right, but in this case the diode is shorted. Looking at the print, one side goes to the negative rail. The other side goes to the base of a transistor. That transistor only has so many paths to that negative rail, the emitter goes through a resistor and would not read zero. And resistors do not short. You follow everything that comes off the collector of that transistor and the best it can do is about 1 K ohm. Not enough of a short to result in a zero reading.

Bottom line, the Zener is shorted. Sure, lift one end and make sure, and when you do that DISABLE THE AMP because if it gets powered up with that disconnected it could to be overbiased or whatever and could blow something. That means take the fuse out, unplug the output boards or whatever it takes so it does not go up in smoke when someone "forgets".

The diode comes back as a run of the mill 15 volt Zener. Simple math says 50 mW with those circuit values (15K ohms 50 volts). A half watt will be fine. Don't go higher because higher wattage Zeners take more current to push them.

One nice thing about this particular repair is that this failure probably didn't blow any other parts. If that turns out to be the case consider yourself lucky, and it is quite possible.
Click to expand...

Thanks! Would it be possible that the negative rail may have caused the failure, or is that too likely? I tested the four rectifier diodes (unsoldered one lead of each prior to testing), and their collective measurements ranged from .487V to .545V. Would this range be in acceptable range?
 
The only thing other than the Zener diode simply failing would be the transistor shorting and I think you would have noticed that by now.

It is unlikely the rail fail caused it, but it could happen. It would have to be running and that trail go from its normal to ground in a hurry, enough to cause enough forward conduction in the Zener to damage it. they are designed to operate beyond their Prv all the time, and to breakdown at a certain voltage. but they are still able to be a rectifier. And they do not like forward current - OH NO. Not at all. No matter what happened you need a Zener diode.

I think it simply failed.
 
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