Popping sound troubleshooting

[bluesguitar]

I just finished a cap job on a Deluxe Reverb. I replaced filter caps as well as cathode electrolytics in the chassis. I checked plate voltage and set the bias with a dummy load, so without a speaker I couldn't hear any problems. All seemed fine. When I tried the first play test next morning I heard a pop about 10 seconds after taking it off standby. I turned the volume up to 3 and didn't hear a single note, but it started popping like a string of firecrackers being set off. I have done a visual of the chassis and nothing looks fried. I turned it on in the dark to see if I could see any arching, but nothing. Any suggestions on where to begin my troubleshooting? Or what the main cause could be?

[Firestorm]

Did it work before you recapped it? First thing I'd do is check the voltages at each power supply node and at all the plates. Also confirm that all grounds are intact.

[bluesguitar]

Did it work before you recapped it? First thing I'd do is check the voltages at each power supply node and at all the plates. Also confirm that all grounds are intact.

It worked fine before. This is the fourth cap job I have done on my different fender amps. First time I've had a problem. I'm meticulous on making sure I put it back together the same way I take it apart. All grounds seem visually fine. I don't know what you mean by each power supply node. As for the plates I take it you mean pin 3 from the power tubes to the output transformer.

[Firestorm]

You should measure each ground point with your meter, especially the power supply grounds, since that's what you worked on. I meant to measure voltage at the preamp and PI plates. The power supply nodes are the positive end of each cap.

[selloutrr]

The 100ohm carbon comp resistors in the fender preamp section are prone to pops. With new caps giving full voltage it maybe shit just flows done stream. If your installation of the caps is good look at the resistors. IMHO swap them for metal film very little to no audible difference. Swapping all carbon comp to metal film is know as the studio mod it helps quiet the old fenders a lot.

[bluesguitar]

I located the problem. As I was getting ready to turn the amp on and do some tests I noticed the "ground" toggle switch on the back of the amp. I remember it being toggled to the left when I pulled the chassis to begin the cap job. I had read somewhere that it was supposed to be in the center, which is like a neutral position if I remember right. So, I put it in the center, and forgot that I had changed it. Well, I toggled it back to the left before turning the amp on and you wouldn't know that I ever had a problem. The amp worked fine. Anyone know why the position of this toggle switch would make or break the operation of the amp?

[martin manning]

Is there still a cap connected from the ground switch to ground? If so it should be removed. The popping may be an indication that it is leaky.

[bluesguitar]

Is there still a cap connected from the ground switch to ground? If so it should be removed. The popping may be an indication that it is leaky.

Yes there is a cap there. It looks like it is an original. Do I just take it out and thats it?

[Reeltarded]

That's called "the death cap". The ground thing should be totally bypassed with a 3-prong power cable and proper ground so nobody gets hurt.

[martin manning]

Yes there is a cap there. It looks like it is an original. Do I just take it out and thats it?

Yes, just clip the cap out so that the switch does nothing. If you haven't done so, installing a three-wire grounded power cord would be a good idea, as Reeltard suggests.

[Reeltarded]

Leo Fender tried to kill me.

[bluesguitar]

Yes there is a cap there. It looks like it is an original. Do I just take it out and thats it?

Yes, just clip the cap out so that the switch does nothing. If you haven't done so, installing a three-wire grounded power cord would be a good idea, as Reeltard suggests.

I can do that. However I'm a bit confused. This is a '72 Deluxe. It has a three prong cord, which I don't think is original, because the plug is black instead of red. Yet the female outlet on the amp chassis is three prong. If they were putting grounded plugs and outlets on amps at this time, why the problem with this cap?

[martin manning]

The ground switch connects the cap to one side or the other of the AC line, so if the switch is positioned such that the cap is connected from the hot side to chassis ground the cap has line voltage across it. Now, if the chassis is ungrounded and the cap fails shorted then you have line voltage on the chassis, and on your guitar strings. If the power cord is polarized and the chassis is grounded (modern practice), then the shock hazzard is removed (as long as the ground connection is good, and there is no guarantee of that), and the mains fuse in the amp would blow. In your case the cap is evidently leaking enough to put some noise voltage on the chassis but not enough to blow the fuse. It might well trip a GFCI, though.

[bluesguitar]

The ground switch connects the cap to one side or the other of the AC line, so if the switch is positioned such that the cap is connected from the hot side to chassis ground the cap has line voltage across it. Now, if the chassis is ungrounded and the cap fails shorted then you have line voltage on the chassis, and on your guitar strings. If the power cord is polarized and the chassis is grounded (modern practice), then the shock hazzard is removed (as long as the ground connection is good, and there is no guarantee of that), and the mains fuse in the amp would blow. In your case the cap is evidently leaking enough to put some noise voltage on the chassis but not enough to blow the fuse. It might well trip a GFCI, though.

Thanks. Good explanation of the potential for danger. Would you agree that just clipping the cap is sufficient with no rewiring of the current three prong plug?

[martin manning]

As long as the 3-wire cord is wired properly, clipping the cap out is all that's needed. Look for the power switch and line fuse to be on the hot side of the AC line (black wire), the safety ground (green wire) going directly to the chassis on a dedicated bolt, and for the correct polarity at the courtesy outlet.