Fender M80 : distortion

[garyk]

Hi

I have a Fender M80 combo (1 x 12") and is has some pretty horrible
distortion...sounds a bit like a bad speaker, but it is the amp (tried speakers.)

I think it might be a cross-over distortion, worse at low frequencies and reasonably low output. Crackly breakup. Power output itself seems okay.

You play a few notes, most fine, then suddenly bad, loud, crackly distortion.
Intermittent.


I am okay with tube amps, pretty lost on solid state.


Can't see anything obvious, R71 gets pretty hot, and the far right transistor seems a little warmer....it maybe a cap?? http://music-electronics-forum.com/images/smilies/confused.gif

(Anyone recommending electrolytic swaps in a 1989 amp?)

I have a schematic copy.

All assistance gratefully received.

Gary

[Enzo]

I imagine R69 gets hot too. There are voltage test popints for those resistors on the schematic, do they compare well?

While the board is out, closely inpect to solder under all the filter caps.

Are both HV rails present, up to voltage and relatively free of AC?

How about the LV rails for the preamp? Got both +/-15VDC? Is it clean? CR25,26 are the zeners for that.

God gave you a power amp in jack and a preamp out jack, so use them. Plug the guitar into the power amp in jack. Is it strong and clean or crackly and distorted? Run a cord from preamp out to the input of another amp to listen to the peramp signal. How does it sound? Clean and proper or does it sound like your symptom. This divides the amp into preamp and power amp so we can determine which half has the problem

[garyk]

ok ... generally the DC test points are close: although TP4/5 are 42v rather than 48V and the rails are 17.6v rather than 16v.

All other points pretty close.

Haven't rigged for the AC volts tests. Generally checked all diodes.

I did pull the amp out and check for any bad joints, and replaced the bias resistors as per the Fender update.

Will try driving the input/output to see if problem can be isolated, but trying a guitar into the output stage probably isn't driving the o/p stage hard enough to show up the problem.

I took care to keep the amp upside down so the + & - rails were reversed for down under readings......:D

[Enzo]

Oh just plug the guitar in there, it takes two seconds. It will be loud enough to test things.

[garyk]

What I mean to say is that I did try that, but didn't seem to cause the same problem...but I think it needs to be driven harder and the problem does appear to happen whether you use the normal or distortion channel...

its late at night and I can't turn it up,
but at lower volumes I'm still hearing distortion in the M80 and not the slave
amp.

Any thoughts on electros..'cause the problem seems worse on loud transients?

[garyk]

One of the power electros is somewhat warm...it could be sucking in a bit of local heat, but it is noticeably warmer than its brother ...and given that the cracking up seems to happen on certain notes + transients ...could be a power supply problem????

[Enzo]

Power supply looks fine, your voltages are up there and even. Power supply won't pick out one transistor to overheat. Or is it not transistors heating?

Electros? Electrolytic caps? If a cap is getting warm, change it. SCope the output and look for ripple in the waveform. Scope the HV rails and see if they are excessivly rippled.

Whack the chassis with a mallet, does the amp react? SHouldn't. If the preamp out is clean and hte power amp is not, that l;ocalizes the problem to the power amp. Grasp each main filter cap and wiggle it, does the amp react? SHouldn't.

A main filter could have a broken leg, even though the solder is intact.

[garyk]

Not finding anything dramatic ..one transistor might be marginally warmer, doubt it is overheating, though.

Yes, one electrolytic seems too warm.

No mechanical shock seems to affect amp.

Scope would be great, sadly don't have one.

Okay ...I'm changing tack here.

Might be speaker problem(s).

You say, "what an idiot, that should be the first thing"...but...the original speaker isn't poling or rubbing, then I have tried another two speakers
and the problem was still there.

Just plugged the amp into a heavy duty external box and I don't seem to be getting the same effect...so either I am going mad (probable) or I have lucked out 2 - 3 bad speakers????

Hmmmmm.

[Enzo]

Regardless, I don't like that hot cap.

And if those other speakers were mounted in the combo, they all would vibrate the chassis. Have you operated the chasis while removed from the combo cab? Use extension wires.

[garyk]

Yes, all speakers were tested via extension wires...and I have made plenty of physical bangs on the amp ...when it was out of the case, I touched up any vaguely suspect joints (very few looked even suspicious) plus replaced the bias resistors, and tightened up heatsinks...etc...

the original, and two other speakers clapped out when trying this way. :confused:

Put it all back together and rewired the speaker...(replaced original)..via insulated socket...all "seems" well right now..thanks for input, still dunno if something needs attention, though, but so far....so good.... :D

one slight possibility is that the speaker wires were crimped hard in the retaining clamp...any possible capacitive problem??

[Enzo]

I'd doubt it.