Fender Super Reverb Shorting

[admin]

Greg: My 1971 Fender Super Reverb seems to be shorting out briefly in the second channel (V2). The shorting is pulsating in nature.This happens only when the volume is turned up from zero to about 2. From 3 until 11 ( ) it is fine if I keep the volume there. I had this pot replaced but the problem persists. Do you have any idea what might be going on? I have systemically replaced all tubes without any change.

[soundmasterg]

Greg: My 1971 Fender Super Reverb seems to be shorting out briefly in the second channel (V2). The shorting is pulsating in nature.This happens only when the volume is turned up from zero to about 2. From 3 until 11 ( ) it is fine if I keep the volume there. I had this pot replaced but the problem persists. Do you have any idea what might be going on? I have systemically replaced all tubes without any change.

Hi Peter,

Sorry to not get back to you on this until now. I've gone back to school and had finals this last week and a half so have been extremely busy.

I think the problem with your amp is that the ghost of Leo Fender is haunting the non-blackface Fenders.

All joking aside, I need a little more info about the nature of the problem. Is it a popping sound that sounds like the signal is cutting in and out? The trem roach could be having issues, but usually that manifests itself differently. It could be an intermittant ground somewhere, though with that it would usually occur more as the amp gets turned up...especially on 11! It definitly sounds like a Gremlin though.....Have you been transporting the amps in any AMC products lately? (Sorry, couldn't resist another joke)

Its hard to say exactly what is going on with it without being able to hear it and poke at it. What I would normally do would be to get the amp on the bench and get it to repeat the problem, and then poke the parts with a chopstick to see if I can make it stop and start. This helps to localize the stage where the problem is manifesting itself, and once you narrow the scope, then eventually you can solve the problem. I've fixed problems this way on amps before....a friend's Sunn Spectrum II would sound fine all all notes except the low G, F#, and F, where it would get all fuzzy and distorted. It ended up being a loose solder connection on the reverb and trem board inside the amp, but it took him banging away at loud volumes while I poked away at it, while it was sitting on top of the speaker cabinet to induce the vibration. Not a very safe thing to do if you're not careful, and hard to hear over the racket, but its things like those that are hard to pinpoint sometimes.

Try pulling the trem oscillator tube (12AX7) and see if the problem still occurs. The fact that you said it is pulsating in nature leads me towards something in the trem, since that oscillator pulsates even when the trem isn't engaged.

Greg

[admin]

Greg: Thanks for your comments. I realize that you are very busy these days. I got to the bottom of the problem and perhaps my description of the difficulty was inadequate.

At the end of it all I sprayed the volume pot with the slightest amount of Deoxit D5 and the problem disappeared as quickly as it appeared.

[johnallg]

Greg: Thanks for your comments. I realize that you are very busy these days. I got to the bottom of the problem and perhaps my description of the difficulty was inadequate.

At the end of it all I sprayed the volume pot with the slightest amount of Deoxit D5 and the problem disappeared as quickly as it appeared.

Well that was simple and easy.

[soundmasterg]

Well Peter, thats great that you fixed it! If it is a newish pot and still doing that, then there may be a cap that is leaking DC onto the pot somewhere in the circuit.....though usually that will cause scratchiness rather than a pulsating type of sound. If a cap was leaking DC, it would have to be the 250pf treble cap, the .1uf bass cap, or the .047uf mid cap. To check those, you unsolder the end that is connected towards the volume pot electrically, and turn the amp on with your meter attached to the free lead. (The cap should be hanging in the air.) You let te amp come up to operating level with standby not engaged, and you measure from the meter to ground with the black lead, and from the meter to the free end of the cap with the red lead, on DC voltage. Better make sure the meter is set to handle at least 450v just to be safe, though at that point in the circuit, 350v is probably enough. The voltage will spike and then reduce down and will somewhat stabilize at a lower level. Whatever it stabilizes at is the leakage DC volts. Any leakage will cause the bias of the following stage to get messed up, and will also cause the pot to be scratchy or possibly other things. Caps usually have some leakage, even modern ones, but anything more than about .3v is a problem and the cap should be replaced. If the amp works and sounds fine otherwise and the tone controls all work, then it is probably ok, but the next time you're in there, you may want to check it just to be sure.....or if the problem reoccurs. If it is hard to remember where each part is supposed to connect, then only undo them one at a time, and be sure when you shut the amp off to leave the standby on so it will drain the caps, AND check the plate pins on the power tubes and cap positive leads to make sure there isn't a voltage on them before you put your hands in there. If there is, then you need a 10k 2 watt resistor hooked up to some insulated leads to drain the voltage out slowly. I have a couple of these made up and use them when I work on amps. The only key is to remember to disconnect them when you are done or you'll get smoke...haha.

Greg

[admin]

Greg: Thanks for this great tutorial. It will certainly come in handy should the symptoms reoccur.

[soundmasterg]

You're welcome Peter.

I just want to make it a little clearer though as I don't think I was clear enough above. When you are checking the caps for leakage, the amp should be in PLAY mode.

Also, as I'm sure you've heard before, safety is #1 so do not be sticking you hands inside the amp unless you know what you're doing please.

Greg