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Oscillating Bipolar Guitar Amp
Oscillating Bipolar Guitar Amp
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Old 22nd July 2021, 05:27 AM   #11
Telnet100 is offline Telnet100  New Zealand
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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?
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Old 22nd July 2021, 09:32 AM   #12
duncan2 is offline duncan2  United Kingdom
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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.
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Old 22nd July 2021, 02:02 PM   #13
wg_ski is online now wg_ski  United States
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If lack of a zobel is causing oscillation, check the amp with a 2 watt carbon comp or metal oxide resistor as a dummy load (instead of a speaker or wire wound dummy load). If that makes the oscillation go away, then add the zobel using this type of resistor.
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Old 22nd July 2021, 08:09 PM   #14
Bone is offline Bone  United Kingdom
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It is not unknown for the TIP 142/147 transistors to cause oscillation. Try changing them. They are cheap enough.
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Old Yesterday, 07:48 PM   #15
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Telnet100 View Post
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?
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Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
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Old Today, 12:08 AM   #16
Telnet100 is offline Telnet100  New Zealand
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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
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Old Today, 02:47 AM   #17
turk 182 is offline turk 182  Canada
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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.

Last edited by turk 182; Today at 02:52 AM.
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Old Today, 08:39 AM   #18
Telnet100 is offline Telnet100  New Zealand
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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
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Old Today, 12:53 PM   #19
turk 182 is offline turk 182  Canada
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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.
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