Oscillating Bipolar Guitar Amp

Hello
I'm still searching for a problem in the Attached Circuit of the Fender Ultimate Chorus Guitar Amplifier. This one channel is Oscillating, the other, not. all of the components were still original. I had previously replaced Electrolytic Capacitors as advised by Members in a previous post, but the problem still exists, so I hav started a new post in the hope that someone can help!
Kindest Regards; Telnet100
 

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well that's confusing it seems what your describing is a preamp channel problem but your attached schematic is of the output section...are you certain that the low voltage supply filters are ok? perhaps that's the filers you where supposed to look at under previous advice.


this should be in the instrument and amps section.
 
Most of the HF oscillations I repaired were caused by a fault in the power amp/section not a preamp where low voltages/low current are found but this circuit is a bit unusual as--if I read it right distortion is intentionally introduced as an "effect ".

The full circuit is available at manuals lib.com.
 
Hello, Everyone - Thanks for the help --
To Help prove that it is one of the Power Amps which is oscillating, I plugged an Earthed 1/4" plug into the Mono Effects Return Socket, and the problem still exists in the Right Hand Power Amp Circuit. It is taking off at a Very High Frequency, but so far, I have not connected it to the Frequency Counter to find out how high. Is it possible that the Driver I/C, the MC1435, could be faulty? I have Attached the full circuit to try and help --
 

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I just re-read the replies; The Main Power Supply Electrolytics were replaced before my first post, so these are new. They, or at least their connections into the PC Board, have been a known failure Point in this model of Fender. The other downstream filter electros have all been checked for ESR, and all look/test Okay.
Thanks for the help --
 
The little bitty electrolytics are far more likely to be bad than the big supply caps. The seals are not as good, and if there is little or no DC bias they can degrade more rapidly than if they have voltage. They may still “work” but have increased ESR, and get worse at high frequency. Those small electro’s in the feedback loop of the op amp driving the darlingtons would be what I would suspect. This is one thing that can change over time, leading to oscillations that suddenly appear one day.

An op amp directly driving emitter followers is inherently stable, as long as the op amp is compensated for the required closed loop gain. If the closed loop gain defined by the feedback drops too much, you get oscillation. If the impedance of the caps in the feedback loop goes too high so that the high frequency (above audio band) gain drops too much it will oscillate. If the capacitance drops too much, you will lose bass response. I’ve seen this happen too. Not all op amps are unity gain stable, and additional phase shift in the darlingtons will degrade the inherent phase margin requiring more than unity gain for stability in some cases. This may or may not be the problem, but easy to check. Temporarily bypass C65, 66, and 67 with a 0.1 uF ceramic, and if it stabilizes you’ve found it.

Power supply decoupling caps can go bad over time too, also leading to mystery oscillations. But since they have a DC bias, they tend to stay better longer than coupling caps with no DC.
 
Hello
Thanks for all the info -- Checked the .47 Ohm Speaker Return resistor -- OK.
Tried doing as suggested, bypassing those Original Caps. No apparent change, so I then pulled the Main Board and replaced all of the Caps (which tested OK, out of circuit) in Both Power Amplifiers. The Problem still exists. Why does this particular Power Amp have no Zobel Network? I was under the impression that the Zobel was to prevent exactly this sort of thing from happening -- Is it possible that somehow the MC1345 Opamp could be faulty? (it is the original) I have since found that the other Channel is also Oscillating, but not to the same degree??? This is happening with the inputs to both Both Power Amplifiers disconnected from the preceding preamps, so this is not something that is going on in the preamps and being amplified. -- Where to from here?
 
The Zobel network is there for the interaction of what the amplifier sees when outputting into a moving coil loudspeaker network to stop re-entry interaction causing HF oscillation its not there to tune the amplifier design that's what a compensation capacitor is for.

I would check the small value comp. capacitors and change for polystyrene.
 
Why does this particular Power Amp have no Zobel Network? I was under the impression that the Zobel was to prevent exactly this sort of thing from happening -- Is it possible that somehow the MC1345 Opamp could be faulty?
By all means add Zobels.
Amps *seem* to work without them but then sometimes oscillations appear, under certain loads or simply improper cables , why risk it?
 
Hello, and thanks for that --
I did as suggested, and swapped out both Output Devices for ones which I bought from Mouser here in New Zealand, who always supply a Certificate to say that the parts are from original manufacturers. This is what I try to do to avoid Clever Chinese Replicas, so these parts SHOULD be Genuine. However, the same problem Still Exists. I do not know why this particular Power Amp Circuit does not use a Zobel; if all else fails I might Add One using 10 Ohms 1 Watt, and .1 uF 100v in series, does this sound like what might be advised? Regards, Telnet100
 
well the low voltage supply is derived from the 40 v supply which is the what the output's run off of and with only a zener acting as a regulator there's not much isolation so why wouldn't a problem there not crop up/effect the output stage?
i've serviced many of these which where oscillating so badly they where producing audible squealing and all those turned out to be bad preamp supply filters or bad 16 V zener's.


as to your theory about the zobel not being there (and i freely admit i may be wrong) isn't the speaker return (- line) tied to the inverting input and sufficiently compensated? Enzo pointed you to the most likely point of failure that would cause output stage instability.
 
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Hello
Thanks again for the help --
I will check those Zeners and the Power Supply Filtering Electros. Previously, I had checked the ESRs of those Caps, which were found to be the same as new ones, but I will now be looking further into that. The Main Power Supply 4700uF capacitors are new.
Yes, You are correct regarding the Speaker return .47 Ohm Resistor, and had checked them, both OK. Do you think that the MC1345 Opamp might be at fault? At his point, I'm running out of ideas --
Thanks, and Kindest Regards - Telnet100
 
the other caps i'd be taking a good look at are the 47 mfd's that are bypassing the op amp rails (C68,69)


as crazy as this may sound have you cleaned the jacks for the effects loop? i've had countless problems with those creating all manner of signal problems as many go unused and over time become intermittent/corroded.
 
Hello, Thanks for the help.
Those Capacitors, (C68 and C69) were the very first ones I changed on advice.
And, No, had not looked at Effects Loop Jacks, as the Output Power Amps are still suffering from Oscillation with the Inputs to them having been disconnected from the Preamp Section of the system, so I naturally was concentrating efforts on trying to find the cause of this.
I will report on findings --
Kindest Regards, Telnet100