DC on input jacks

[drewl]

Dang had this problem on a clone before but this is a real '57 5E3 Fender tweed deluxe.
The worst jack is about .3 volts unloaded, maybe 10mv with a guitar plugged in causing scratch noise when guitar pot is turned.
And no, it isn't the guitar.

Gotta' be leaky 60 yr old board material, right?

Does it without preamp tubes installed, 1M resistors are good, 68k resistors are good.

This guy always gives me old stuff that's a pia!

I don't think it's worth it to replace all the front end stuff on an original amp.

Anything else I could possibly check?

[The Dude]

I would try grounding the grids of the first stage and see if the volume is still scratchy. I'm more apt to believe the problem is on the output side of V1 rather than the input side since the volume control is there. Simple enough to check. A couple of clip leads will prove me wrong.

[Chuck H]

Try another preamp tube first!

[g1]

He's getting the problems without preamp tubes installed.

[The Dude]

He says, "Does it without preamp tubes installed", which I take to mean there is still a little DC on the input jacks. I guess we don't know for sure what "Does it" means. I doubt the volume control is noisy as there's no preamp tubes installed. So, it wouldn't hurt to try a new tube as Chuck said. I think the DC on the jacks isn't the cause of the problem, but I could be wrong which is why I suggested grounding the grids. I would also check/tighten all grounds. Sometimes things get loose, especially given the age of the thing. That would include snugging up the nuts on the jacks themselves. Also, if there is a brass grounding plate, the ground wires soldered to those come undone every now and again. If the grid leak resistors aren't getting a solid ground that may cause some residual DC to show up at the jacks.

[Chuck H]

Ah. Skimming bad.

I can't see any reason to have DC on the input and volume pot with no tube in the socket other than voltage on the chassis. That's a bad ground. Have you measured DC from the chassis to the AC mains ground?

[Enzo]

Or if the input jack lead goes to a leaking eyelet board, rather than a straight wire to the socket grid pin.

[drewl]

I tried cleaning all the jack connections, ground connections and jacks.

tightening the tube socket screws lowered the dc voltage but guitar pots still make scratch noise.

Owner doesn't want the 2 prong cord replaced.

[Chuck H]

Owner doesn't want the 2 prong cord replaced.

Then it must be assumed that the owner has no plans to actually use the amp and it's only a collection piece. So scratchy pots don't matter.

Just being a smartass (or a dumbass, depending). I'd still measure voltage on the chassis relative to an AC mains ground. If there's significant voltage on the chassis it could be coming from someplace other than the input or preamp and that would be helpful to know.

I haven't worked on an amp where the owner didn't want the safety ground installed. But I don't work on many amps. I've considered what I might do if a person didn't want the safety ground installed and I'm still on the fence as to whether I'd turn them away or not. Turning them away would be on the premise that I wouldn't want to be the last guy to work on an old amp that electrocuted someone. That is, only amps "I" believe are safe should leave my bench.

[Leo_Gnardo]

Owner doesn't want the 2 prong cord replaced.

Introduce him to your friend the life insurance salesman. It's only right he should be prepared for the inevitable.

[Chuck H]

Does it without preamp tubes installed,..

Ok... I'm rethinking this and I now believe that there MUST be voltage on the chassis. There are only two preamp tubes in a 5e3 and they are the entire preamp and phase inverter. If the amp still makes scratchy noises without those tubes in (you DID say "tubes" and not tube) there is no way for shifting DC to be coupling through anything but the power supply ground. I've never heard of anyone REPLACING a death cap, but... And I don't suppose if your guy doesn't want the two prong cord replaced that he'd be up for changing the original power supply caps.?. Does he expect you to effect a repair without replacing bad and obsolete parts?

[Enzo]

Voltage on the chassis with respect to what? He gets voltage on the jack, presumably he means on the tip with respect to the chassis. The guitar pots go scratchy from this small voltage. That means a voltage from chassis to tip, as the guitar connects to nothing else. I am not sure how a guitar can have scratchy pots with the tubes not in the amp. I am assuming he means the voltage is there without tubes. I can see how an AC hot chassis can cause hum in a working amp, but not how it can put voltage across the input jack. Just my thinking.

[drewl]

Yes, there is DC voltage on the input jack tip to ground.
Voltage is present without preamp tubes installed.

It is around 300mv unloaded (open 6" patch cord plugged in) around 10mv with guitar plugged in.

ground the junction of the 68K input resistors and the voltage at jack is zero.
Same result on either channel.

owner is selling the amp, says buyers prefer the original 2 prong cord- whatever

[Enzo]

So follow the wire back from the 68k resistors on the jacks, does it go to the board eyelet, or direct to the tube grid? If the wire goes to an eyelet, turn the amp on, ground your DC meter to chassis, and poke the probe at the eyelet, still see that 300mv? Probably do, now move the probe tip to the black eyelet board material right next to the eyelet, but not touching the eyelet. Is there voltage? If so, your board is conductive.

Maybe a better test is to find the pairs of 100k plate resistors on the board, and poke your probe NEXT TO one of their ends, do you read voltage on the board? That might be easier to detect, and if it is there, it is likely over by the input wiring as well.

We have discussed conductive eyelet boards here in the past. I have had success baking the contamination off the boards with a heat gun. The boards get a waxy buildup that adsorbs water from the air.

[Leo_Gnardo]

Yes, there is DC voltage on the input jack tip to ground.
Voltage is present without preamp tubes installed.

It is around 300mv unloaded (open 6" patch cord plugged in) around 10mv with guitar plugged in.

ground the junction of the 68K input resistors and the voltage at jack is zero.
Same result on either channel.

owner is selling the amp, says buyers prefer the original 2 prong cord- whatever

Yes it's looking like a conductive board all right. I run wires right from the input jacks to 68K resistors soldered to the preamp tube socket. "Yes but buyers prefer . . . " Yeh, I know. So you can try Enzo's heat gun solution. It only has to work long enough for the cash to change hands, ka-CHING! If later on the buyer finds his guitar volume control is crackly due to leakage on the circus board, "that's authentic, Charlie" and get over it.

FWIW when I build new 5E3, always use the direct wire solution, and I order the board made without eyelets for the 68K resistors.