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Post subject: Re: Reverb Knob Creating White Noise 67 Super Reverb
Posted: Fri Feb 20, 2015 7:57 pm
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Joined: Sun Oct 07, 2012 6:48 pm
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I kinda knew the gtr analogy would lead to 'don't stick your finger in there' :wink:

Absolutely, the safety part is quickly the first thing everyone learns as all the experienced techs here mention it several times.

I am very very safe when working on amps. I routinely power up the amp (both switches in the play position) then turn the On switch to off to drain as much current as possible from the caps. Then I unplug the amp. Then I test the power tubes sockets with a DVM. Then (usually as an extra precaution) I open up the dog house, test there, and then alligator clip in a ceramic resistor. Once there is less than 0.01 VDC..I move on.

Safety is not the issue in my case, the issue is knowing whats what. One of the hardest things for the novice, is that in the Chassis, everything seems connected, to everything else (or several things at least) With that in mind, the goal is to change as little as possible to get the problem zone.

So when the answer is something like 'check the caps, or the resistors" (which is totally the 'right' answer) the newb looks into the chassis, and goes "holy F**, which one"...which is usually met with "read the schematic"

..in any case It took me most of the day, but I think I have started to locate the components that Arjay was talking about. I believe he was referring to the pair of 100k resistors that connect to pins 6 and 1 on the schematic. (V1, V2, and V4 all have them...I think V4 are the ones to look at)

He also told me to check out the caps. The filter caps are in great shape and are the big blue Spragues, clearly from the last 10-15 years. The coupling caps are blue Mallory's, those 'round 'chocolate drops' and a cpl more modern tiny black spragues. They all look good. No visible leakage (I've seen this on other amps)

Arjay told me once that the best way to test a resistor is to lift one leg. I can and will do that, but Im hoping to get a better picture of what components relate to the reverb circuit, and which ones are unrelated...as I said everything is connected, so lifting and testing the wrong components is just going to make things worse.

I read somewhere today that the grounding of the reverb pot is an issue for many people....wondered if anyone and any insight/experience with that.

....If I knew how, I would build a tube guitar amp in a straight line, like a long piece of plywood with all the components one after the other from input jack to speaker jack. The visual aid would be super helpful. In the real world its more practical to have it all mashed together in the chassis, but not great for understanding and visualizing the signal path.

Thanks for the help
Dave


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