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Oscillating Bipolar Guitar Amp
Oscillating Bipolar Guitar Amp
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Old 26th July 2021, 06:02 AM   #21
Inch-o is offline Inch-o  Slovenia
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Join Date: Aug 2008
Hi,

Just an intermezzo question... Where are you attaching your oscilloscope GND clip? I hope not to the "-" output terminal.

Also when you insert a jack into the effects rtn, do please short the "hot" and gnd. In this way you will be sure no strange "things" are being picked up by the amp.

Good luck!
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Old 26th July 2021, 08:45 AM   #22
turk 182 is offline turk 182  Canada
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Join Date: Oct 2012
well that's even more confusing ....so what components did you remove, or how did you isolate the amp section? was it left open ended ?
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Old 26th July 2021, 11:02 AM   #23
Telnet100 is offline Telnet100  New Zealand
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Join Date: Jun 2007
Location: Whakatane, North Island, New Zealand
Hello
First Answer: The Scope was grounded to the System Ground on the Main PC Board, adjacent to where the Reverb Tank is grounded. I hear what You are saying re the Hot and ground, I will try shorting these two absolutely.
Second Answer: I had Replaced, in this order:1; The Main 4700uF Filter Caps; 2; the Bypass Electros on the Opamp supply leads; 3; All of the other Series Electros in the feedback loops, and, 4; lastly, the two Output Devices, which I have been assured have come from Original Manufacturers. The Amp Outputs were connected to my 2 x 8 Ohm Workshop Test Speakers.
Thanks for all the help -- I will keep you informed on progress.
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Old 26th July 2021, 04:14 PM   #24
Inch-o is offline Inch-o  Slovenia
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Join Date: Aug 2008
Hi again,

Check the components(resistors & caps) in the three NFB(negative feedback loop)
1. NFB directly around the opamp(680k & 3.9k + 2.2u to gnd)
2. DC offset(servo) - done from the output through two 27k in series + 2.2u cap to
gnd(now that I think about it, the 2.2u should represent a dead short to gnd to the hf oscillations signal)
3. NFB from the .47ohm resistor back to the opamp(3.9k + 4.7u to gnd)

Check the resistors by desoldering at least one pin, so not to measure the complex circuit resistances.

Also check the two 47u caps tied to the opamp supply pins and the 220ohm resistor going to the output. This circuit is a boot-strap circuit to the opamp.

As somebody else here suggested, be sure to eliminate the possibility of the oscillations entering the power amp from the preamp/preamp psu...

Check for any interrupted gnd tracks(just as a tip: thin tracks running between IC pins and alike)
Good luck!

Last edited by Inch-o; 26th July 2021 at 04:26 PM.
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