Screaming Bright Switch Cap??? - 1974 Fender SF Twin Reverb Master Volume Push Pull Switch

[TelRay]

try lifting the FB circuit

well, I lifted the 820R in the NFB.



I played my guitar, the nasty high frequency distortion is still there. In any case, I expected a less mid-scooped sound, more aggressive. I investigated a little more.
The wire linking the 820R with the EXT SPEAKER jack showed continuity with the chassis, but according to the schematic it shouldn't



then i checked the jack... this appears to different comparing it to the SPEAKER JACK



I do not have a spare of this thing at hand but... what do you think?

[Enzo]

I think you are not looking at the schematic carefully.

If no speaker is plugged into the main jack, then the shorting contact grounds the NFB line. Even with a speaker plugged in, the DC resistance of the OT secondary is under an ohm and so appears to be grounded. All perfectly normal. So the wire to the NFB resistor will always measure continuity to ground.

In your photo, the two jack tips are wired together with the yellow wire - as per the schematic. The jack on the right must be the main as it has the cutout wired to sleeve, which is grounded.

[TelRay]

I think you are not looking at the schematic carefully

i am most likely not! hahahha...
thx Enzo, I have started looking for things that could be wrong based on the fact that lifting the 820R had no effect.
If it’s not that it has to be something else

[Chuck H]

The things that are standing out for me are:

1) The screech is reverberated. Therefor the actual screech signal is prior to the reverb input circuit.

2) The mid knob, reverb knob (which is past the reverb input) and the trem intensity knob (which is also past the reverb input) all affect the occurrence.

So it's clear that later circuits are affecting earlier circuits. There must be a positive feedback loop. This is usually due to failed or incorrect filtering allowing interaction within the power supply that does not get "decoupled". Or sometimes a grounding fault. This is such an extreme case in an apparently stock amplifier circuit that I really thought the culprit would show itself by now. I've looked at the pics. And though I am able to trace some stuff, I can't trace everything clearly. I'm currently at a loss. Though I am confident in my diagnosis of the fundamental cause of the problem I cannot find it and I'm not sure what else to try just yet. Except...

Have you pulled V3 yet? I believe I mentioned it once earlier. I still wonder if some remnant of the stupid boost circuit may not be causing this thing. Pulling V3 would null the reverb, of course, but if there is signal there causing positive feedback it may also null the problem. This wouldn't be my solution. Just a way to narrow the search for a solution.

[The Dude]

I noticed in the initial video (post #1), it looks like there's something plugged into the amp's input. Does this amp "screech" with NO input, or could we be chasing some input feedback loop? I ask because I believe you said the amp doesn't squeal if V1 is removed.

[Zouto]

Did you check/exchange the 250pf treble cap? I recently had a series of three brandnew faulty 250pf SilverMicas, which were leaking, causing lots of trouble as screeching, high DC Voltage on vol.pot etc...

[Chuck H]

Did you check/exchange the 250pf treble cap? I recently had a series of three brandnew faulty 250pf SilverMicas, which were leaking, causing lots of trouble as screeching, high DC Voltage on vol.pot etc...

I had thought of that too, but TelRay reported no significant voltage on pin 7 V2. Of course I don't know that he had the volume control up at the time of measurement. So this may still be something to check.

EDIT: Except that I believe from the pics that TelRay has used ceramic caps rather than mica.

[TelRay]

The things that are standing out for me are:
1) The screech is reverberated. Therefor the actual screech signal is prior to the reverb input circuit.
2) The mid knob, reverb knob (which is past the reverb input) and the trem intensity knob (which is also past the reverb input) all affect the occurrence.

and...
3) SCREECH can ONLY be generated with V1 pulled out of the circuit

Have you pulled V3 yet?

sorry, I forgot to give you feedback on this. Yes I have tried with V3 out
1) with V1 in the circuit (and V3 out) the SCREECH cannot be reproduced. All normal, no strange noises.
2) with V1 out (and V3 out), there is a severe HUM(but cannot get the amp to SCREECH, see video)

I am able to trace some stuff, I can't trace everything clearly

I really appreciate the effort. The following layout describes what is in the amp.

if some remnant of the stupid boost circuit may not be causing this thing

I am almost certain I removed every bit of wire of the PULL BOOST feature as this was always my main candidate to be responsible for this failure mode (additionally even changed the MASTER VOLUME pot).





I am now also intrigued about the fact that lifting the 820R NFB has no effect in the guitar sound... apparently this is a mod that should change the amp's sound dramatically.

[pdf64]

I suspect that the nasty distortion also stops (ie as well as the screech) with no tube in V3?

[TelRay]

I noticed in the initial video (post #1), it looks like there's something plugged into the amp's input. Does this amp "screech" with NO input, or could we be chasing some input feedback loop? I ask because I believe you said the amp doesn't squeal if V1 is removed.

thx for que question, it's a good detail to keep in mind.
the answer is YES, the SCREECH is present both with something plugged or not into the CHANNEL INPUT

[TelRay]

I suspect that the nasty distortion also stops (ie as well as the screech) with no tube in V3?

yes, the guitar distortion (the nasty one) is notably reduced when V3 is not in the circuit.
good point!

[pdf64]

Great, you might wish to reconsider the grid stopper mod for V3 suggested previously.

[Enzo]

What am I missing here?

SCREECH can ONLY be generated with V1 pulled out of the circuit

Are we really trying to solve a condition that ONLY exists when tubes are removed? In other words in a fully tubes amp, the problem does not exist?


My mom's car likes to drive in a circle, but ONLY when one wheel is removed. Like that?

[TelRay]

My mom's car likes to drive in a circle, but ONLY when one wheel is removed. Like that?

hahahahaha... I liked that.

Thank you Enzo for your contribution so far, nevertheless in this case the fact that your remark is funny does not make it applicable to this case.

The amp has a problem, with all tubes in and with the VIBRATO CHANNEL VOLUME at 7+.
The problem is there is a high frequency distortion present when I play my guitar.
The sound of this distortion is not nice and that is preventing me to use anything but a super clean tone in this amp.

As a way to understand what was wrong I have started looking for a way to push this failure mode to the limit and found a combination of removing tubes, moving knobs and pulling switches that created that nasty SCREECH.
My approach might be not the right one (I am here to learn and hopefully to contribute in the future in the same ways you guys are doing now) and I am more than open to try something else.

Clearly, the target is to eliminate the distortion in the guitar sound. I can leave with using the CHANNEL VOLUME at 9.5, the BRIGHT SWITCH OFF and the TREBLE below 9

Again, thank you.

[TelRay]

Great, you might wish to reconsider the grid stopper mod for V3 suggested previously.

hey man, I am willing to give it a try to see what's the effect. Can you help me out understanding how is it done?

Ultimately I want the amp sounding good as intended in the original schematic without adding too many "fixes in the mix"

thx!