1965 Fender Reverb unit noise / hiss problem

[poligow]

Hi everyone

Recently put my hands on an old '65 Fender reverb unit.
The unit was in it original shape, except for the 3 prong power cord.
It has some background noise, and crackling / hiss noises.

I replaced the 3 electrolytic caps and the resistor for the power supply, and the 3 electrolytic caps on the circuit board.
I fixed the background noise, but the hiss / crackling noise are still here.

Even if the mixer knob is set to 0, the noises are still here, so i assume they are not relative to the reverb circuit.
If i remove the input jack, the noises disappear.
I tired to replace the 7025 tube > same problem.
The tubes sockets, jacks and pots were cleaned (with some KF F2 contact cleaner).

I've attached a sample to let you hear the noises ... i turn the volume louder at 10 seconds.
If i sweep the volume knob on the guitar, the noises disappear when it's at 10 ... and if i put a Boss TU2 between the guitar and the reverb it's ok

Do you know which could be the faulty component ?
I was thinking about the 2,2M resistor feeding the pin 7 of the 7025 ....
Thanks for your help !

[52 Bill]

If i remove the input jack, the noises disappear.

Please clarify this statement. Do you mean that the noises stop when you remove the instrument cord from the input jack?

[poligow]

yes, i don't have any noise if i run the unit without a jack connected
if i plug a jack, even without a guitar at the other end, i have the noises.

[trobbins]

If you pull out the 12AT7, then you should also get no change with the noises, and that (or using the reverb pedal switch) would confirm it is the direct path that is the problem circuit.

If you disconnect one end of the 0.05uF coupling cap from input to 7025 then do you get the noises? That cap is likely to be leaky - as that would explain the difference between guitar input and a pedal that may have its own output coupling cap.

That unit could do with some heater elevation as a mod, and a bridge rectifier.

[poligow]

Thanks for your help.

Yes i tried to switch off the effect with a footswitch, same problem.

For the cap, if i unsolder a leg, if i'm right, it will have nothing to bring my guitar signal to the tube ?
Is there an other way to test it for leakage ?

[trobbins]

One way to test is to replace cap with a new part.

Another way to test is to unsolder a leg - that does provide good diagnostic information - if the amp shows reduced levels of noise/hiss then you have confirmed the position of the problem.

[poligow]

ok thanks for the feedback.
i'll buy a replacement cap to try to replace it.

[mac dillard]

Check the two 100K plate resistors.

[poligow]

Check the two 100K plate resistors.

thanks Mac for your help.
yes, i ordered a few 100k resistors today

i'm also suspicious about the 2,2M resistor, as the unit is quiet when nothing is plugged in (so hot tip of the input goes to the ground if i'm right) but starts to be noisy as soon as i plug a jack in :

that's also the only point on the circuit where my voltage is way off (around 102V instead of the 120V expected if i remember well)

[g1]

Have you tried other 12AT7's ?

[poligow]

Have you tried other 12AT7's ?

No.
Maybe i'm wrong, but i still have the noises even if the mixer knob is set to 0 (full dry signal) or if i swith the effect off with a footswitch, so i think my problem doesn't come from the reverb path where the 12AT7 is used ...
I can try to source a 12AT7 just to be sure ...

[g1]

Even though the signal does not run through the AT7 with effect switched off, a bad tube there could still put DC on the input jack (which it sounds like your problem may be).
I believe this is why trobbins suggested removing the AT7 in post #4. You can try that if you don't have one.

[poligow]

Even though the signal does not run through the AT7 with effect switched off, a bad tube there could still put DC on the input jack (which it sounds like your problem may be).
I believe this is why trobbins suggested removing the AT7 in post #4. You can try that if you don't have one.

I didn't think the 12AT7 could be a problem. I'll follow your advices and remove the 12AT7 for a test and keep you informed

[poligow]

just tested without the 12AT7, same noises ...

[g1]

If you disconnect one end of the 0.05uF coupling cap from input to 7025 then do you get the noises?

Did you try this? (listen with reverb on, not bypassed)