Fender '63 Reverb standalone

[xtian]

I'm getting very little reverb out of this unit. All DC voltages are good, all tube gain stages appear to be working. Look at the attachment--I inject 40mV 1KHz sine wave at reverb return, it gets amplified properly to 800mV by V3A, and then most of the signal disappears before the output. I cannot probe any test points between the plate of V3A and the output without disassembling the thing, which I will do, but wondering if anyone had an idea where the signal is being attenuated.

63reverb.png

[xtian]

Forgot to mention: it's an intermittent problem. While probing around the first time, reverb started to work properly. I play tested, then turned it off and put the back panel back on. Then moved it, plugged it back in, and the reverb didn't work again. But I can't chopstick the thing back to life.

[pompeiisneaks]

Forgot to mention: it's an intermittent problem. While probing around the first time, reverb started to work properly. I play tested, then turned it off and put the back panel back on. Then moved it, plugged it back in, and the reverb didn't work again. But I can't chopstick the thing back to life.

Could that pot be going bad? Maybe the wiper makes some pretty poor contact at times so most of the signal is going to ground instead?

~Phil

[martin manning]

Bad solder joint? Inspect all connections carefully and reflow as needed.

[xtian]

This has been quite a journey. I addressed and re-addressed all mechanical possibilities--retouched all solder joints, swapped tubes, checked ribbon and other connections--all good.

I simplified to the basic problem by removing the AT7 and 6V6, leaving only the AX7 in place, and plugging a signal into the reverb return. When in fault state, the signal is strong before and after C8, and vanishes after. But it's not the tone or mixer pots.

I had a revelation when I probed R16 for plate voltage, and my signal popped back into existance at the output. Again, I made sure there is not a mechanical intermittence. It's some latching thing...maybe grid blocking. So I checked R11/12/13 the grid reference compenents, and all seem to be OK.

What could cause this latching grid blocking symptom, where just probing the anode resistor on V3A bring it back to life?

Here is a simplified schematic showing the section of the circuit I'm troubleshooting.

simplified.png

[xtian]

Confirmed. Totally repeatable. Now I have all tubes back in, and set up for normal operation with guitar and amplifier. I turn unit on, no reverb. I probe the junction of R16 and pin 1 of V3A and the reverb starts working. Works fine until I turn the power switch off, and I hear a POP which indicates that the fault state has returned.

[pompeiisneaks]

I don't know the 'answer' but in my head at least that means 'providing a very high resistance to ground gets things going' your DMM has like a 10M resistor inline that allows a very small current through. What is that doing that 'starts things going'? Maybe it's creating some kind of grid leak to ground for the next stage?

Not sure where to look, just talking out loud on what that may mean.

~Phil

[martin manning]

Failing resistor or poor tube pin contact with the socket?

[jjman]

Could be some sort of parasitic oscillation that gets removed via probing. I would add a grid stopper to pin 2 as the 1st attempt. Make sure it's wired as a stopper and not a dropper.

[xtian]

Appreciate that suggestion, jjman.

I added 100pF in parallel with the V3a anode resistor, and I replaced the last filter cap in the chain, 47uF. The unit has been stable for the past day and a half, surviving multiple power cycles. Just sent it home with the local owner, so fingers crossed!

[didit]

Could be some sort of parasitic oscillation [...].

This. Makes most sense given the story so far.

Best .. Ian

[tubeswell]

Assuming the pin clamp for the plate was tight and that there were no bad traces, pads, or jumpers or cold solder joints, it could've been a bad plate resistor.

[Jerry Pbury]

I had problems similar to this. It was all caused by the 1/4” connectors being loose. Tightened up all connections and all was good.