Amp noise question.

[PedroC.]

Hi. I replaced the filter caps and the power resistors in an old Fender Deluxe 90 after some trouble with them. The thing is that now the amp makes an awful noise when nothing is plugged in, but the noise goes away when I plug the guitar -there's not even hum, the sound is crystal clear.

Why is this happening? Will I risk ruining the amp if I use it that way (since there is no noise with the guitar plugged)?

Thank you.

[TD_Madden]

Maybe input jacks not grounding themselves with nothing plugged-in...? Check to see it they are bent.

[PedroC.]

I checked the input jacks. All in nice shape, but I found out that what makes the noise go away is lifting the middle contact in any of the two input jacks or the "Pwr In" one (those are the three jacks that have three contacts, tip, ring and sleeve, all normally closed).

Any thoughts?

Thank you.

[Enzo]

Check resistance to chassis from the ground connections at the input jacks. I'd bet it was open. That would explain why the "ground" is injecting noise into the circuit. I think that ground has no continuity to real ground.

[PedroC.]

Sorry for taking so long to reply, but the past weeks were hell at work.

Concerning the amp:

I've checked and the input jacks are well conected to ground.

The schematic is here.

By opening the sleeve switches in J1, J2 or J4, the "MUTE" area cannot go to ground, and the noise stops. The thing is that "MUTE" going to ground is suppossed to mute the amp, not to add noise... so I guess there is a problem in that area (bottom center in the schem, around Q6 and Q7).

Any ideas will be greatly appreciated, since I'm reaching the limits of my small electronics knowledge. I've already checked for bad solder joints, but I'll do it again, though it seems that something else is causing this issue.

Thanks.