Weird Problem I can't figure out...help

[Contech]

Hello Techs,

I am in the middle of repairing and upgrading a 67’ Fender Twin Reverb Amp. This poor amp was tortured with multiple mods over the years. I’ve gotten back to most of its original specification and I replaced all the filter caps, installed new tubes, re-biased, and replaced few bypass and coupling caps. I’ve now got it working pretty well. However, something just happened and I have the strangest problem that just popped up and I simple can’t figure it out. The problem occurred when I tried to change the input jack on the Vibrato channel.

Essentially, if I plug into the Normal channel, signal still leaks into the circuit even if the volume is 100% down. You can hear muffled signal (almost microphonic) ring through the amp. What’s even more odd is that when you turn up the Vibrator Channel volume, it is active on the Normal channel. It’s almost like there is a grounding problem.

Any ideas?

[bnwitt]

...you tried to change the input jack....did you, didn't you, why were you doing so?  Photos?

[shooter]

All I have handy is a '65 schematic, probably close, If you turn all the normal TS knobs and gain to 0 does it still bleed through?
One "odd" thing to me, at least on the '65, V1b shares a cathode R with V2b, the vibrato channel.

[Contech]

I was replacing the input jack because it was not making a good connect when plugged in. It was mechanically unstable and really noisy when touching it. I think it's a grounding problem. I'm checking the connection of the brass plate to the chassis. Any other thoughts?

[Contech]

I found the issue. It was a faulty ground between the brass plate under the pots and the chassis. All of the grounds for each bypass cap included a buss wire to the brass grounding plate. They all were solid connections. However, there was NOT a good solid ground connection from the brass plate to the chassis. I just added a buss wire from the brass plate to the chassis and viola! It worked! I made sure this the brass plate was solidly ground to the chassis down near the input jacks. It's only grounded a one end. It really cleaned this up nicely. Thanks again. The volume pots now drain the audio signal to 0. No guitar bleeding through the circuit...cool!

I'm not that experienced with working on tube amps but I'm pretty familiar with tube amp electronics. Troubleshooting is an art form and experience certainly helps.