1973 super NO reverb

[clock10]

Hi, Ihave a silverface SR that I have blackfaced and replaced all the caps on. Somehow the reverb has gone. I did have high voltage on V3 pins 2 and 7 which I have now tracked down to a faulty 500pf cap. But I still have about 13v. Ive been through the circuit dozens of times but cant find the fault. The reverb pan is OK as I checked it with another amp. Tried swapping valves still nothing.
Any ideas anyone?
Cheers

[Billy R aka DynaFreak]

Hmmm. Sound like maybe you dont have the grid resistor on V3 properly connected/grounded? With power off and all voltage drained, check the resistance from pins 2 and 7 to chassis. Should be about 1M. There are other possibilities, but I would start there. And I would make sure you did not replace the bad 500p cap with another bad one.

[clock10]

Hmmm. Sound like maybe you dont have the grid resistor on V3 properly connected/grounded? With power off and all voltage drained, check the resistance from pins 2 and 7 to chassis. Should be about 1M. There are other possibilities, but I would start there. And I would make sure you did not replace the bad 500p cap with another bad one.

Hi, yeah resistance is about 1M. Ive tried taking the 500p cap out and still finding 7v on pins 2 and7. Also did the same with the .003 cap.

[52 Bill]

Try a different tube. Pull the tube and see if the voltage remains on 2&7, bad tube socket.

[Enzo]

OK< that cap is on the part board isn't it? And you have 7v or whatever on the tube grid sode of that cap? Put your meter probe on the part board itself beside the eyelet, not touching the eyelet. We want to measure if there is any voltage on the actual insulating material of the board. If you get ANY voltage that way, you have a conductive eyelet board. DO a search for conductive eyelet board. I know we have discussed it. Assuming that is your case.

I'd be willing to bet you have that, and also that maybe that first cap wasn;t really bad, but heating the eyelet while soldering in a replacement cooked some of the conductive moisture out of the area.

[clock10]

Hi Enzo. I replaced that cap back and it works fine. I still have about 7.3mv on pins 2 and 7 of V3 and no reverb. I took the tube out and its the same. I couldnt get a fixed reading off the board, but there are millivolts present, so not sure if that is sufficient to be a problem. But sounds like you have a good point. If that is a the problem whats the best fix? Is 7mv on those pins enough to stop the reverb working or do I have another problem.
Thanks for the help.

[pdf64]

If it's gone down from 7V to 7mV, then you're good.
Is the V3 cathode resistor (and bypass cap if fitted) good?
Have you tried a known good 12AT7 in V3 yet?
What's the V3 pin1,2,3 Vdc?

[clock10]

I have tried a good 12at7 still no reverb. If I turn the reverb up to 8 it starts rumbling?farting. Readings are;
V3
pin1 433v
pin2 9mv
pin3 7.43v
pinn6 433v
pin7 9mv
pin 8 7.43v
I have new caps fitted. I have swapped another new cap in but still the same. I have the original 2200 resistor fitted. Ill look at that, seem to remember finding it hard to source a cc to replace it.

[Enzo]

You have done repeated soldering around the same area now, is that right? And after each operation the unwanted voltage gets smaller. Do this, pick some area a few inches away that you have not soldered near, and probe the fiber board right next to one of the 100k plate resistors for some tube. Do you get any stray voltage on the board there? We may have cooked the problem out of the reverb drive tube part, but if one part of the board is contaminated, then the whole thing is, and other problems will crop up. You need to know for sure.

That aside, why are we concentrating on the reverb drive tube? That drives the springs in the reverb pan, but it doesn't send sound back to the amp. The OTHER end of the reverb pan does that. Pull the cable from the OUTPUT jack on the pan. Turn the amp on and the reverb control up midway. Now touch the tip of the plug on that reverb cable. DO you get a good hum from the speaker? If not, then either the recovery circuit is not working, or you have the wrong cable plugged into the reverb pan. Just to be sure, pull the other cable out and touch it. If THAT one hums, then your cables were reversed.

A simpler test is to turn the reverb up part way and then rock the amp to crash the springs. Hear the crash? then the recovery stage works. No crash? then the stage is not working or the pan is not sending anything or is miswired.

[clock10]

I couldnt get any firm reading from the board elsewhere as suggested, just constant change of readings.
I get the crash from the reverb. I have pugged the reverb pan using its leads into another amp and I get reverb.
Still nothing on the super though. Seems strange to get the rumbling/farting when the reverb is turned above 8.
I didnt get any hum when touching either plug tip though.

[clock10]

I think youre right, Ive been concentrating on V3. Looking at V4 now, swapped a tube and the rumbling after8 has gone, still no reverb though.
Ill keep investigating, but any advice would be welcome.
Cheers all.