Fender Blues Deville - No Reverb

[Wildcard]

PROBLEM SOLVED!

Hi all,

It's been quite a long time since I had to bother you guys but I've run out of ideas with a problem I have. I have Fender 4x10 Blues Deville with no reverb. The amp had some drinks splashed onto it at a gig resulting in a shorted 6L6 and roasted R58. That side of things has been put right but on testing the reverb has disappeared (it previously worked fine before the accident apparently).

Here's what I've done so far:
Tested the pan - there's resistance at the in and outputs.
There's a healthy buzz on black output plug when touched.
When I bang the pan I can hear a faint reverb splash through the speakers.
The pan's internal wiring checks out fine.
I've even swapped in another known working pan from another deville and that didn't help.
There's continuity from the red and black cables back to the board so the cables aren't broken.
I've taken readings from the ic (U2), they're fine and very close to the schematic readings.
Out of paranoia i've even changed U2 for a new one, the results are the same.
I've been through all the resistors in the reverb circuit, they all test as per spec.
I've tested the reverb pot and cleaned it...no change.
I'm getting very close readings from all the test points in the reverb circuit.

Any other ideas? I hope you can help.

[52 Bill]

There's a healthy buzz on black output plug when touched.
When I bang the pan I can hear a faint reverb splash through the speakers.

If there is a healthy buzz on the return cable, you should have more than a faint splash from the tank. Have you got the tank plugged in correctly?

Can you feed the send signal that is going to the tank to another amp?

[Wildcard]

If there is a healthy buzz on the return cable, you should have more than a faint splash from the tank. Have you got the tank plugged in correctly?

Can you feed the send signal that is going to the tank to another amp?

Yep, black to output / red to input.

Just to the input of another amp? Yeah sure, what should I hear?

[Wildcard]

OK Bill, I connected the send signal to another amp and there was a strong signal - just touching my thumb on a jack that was plugged into the deville made a loud buzz through the other amps speakers.

Presuming this is good, where should I look next?

[52 Bill]

You proved that the return circuit worked and now you've proven that the drive circuit works as well, so therefore you don't have a problem.

Based on what you've said, you've checked everything that there is to check. Is the tank the correct one for the amp?

[g1]

Like 52 Bill stated, healthy buzz from touching return cable but faint crashing sound from banging on tank does not compute. You have verified it is not the tank by swapping in a known good one of correct type. What you are calling healthy buzz is not very concrete. Can you get access to the amp you borrowed the tank from and try the same test with the same settings? (touching return plug)
You said all the measurements agreed with the schematic, does this include the AC voltage readings with signal injected? If so, I'm leaning toward something between the reverb pot and where it mixes in with the main signal.
If indeed all the AC readings in the reverb circuit are good, I would first look at the reverb muting FET, Q2. For a quick and dirty test, disconnect the leg that connects to the R39/R40 junction (or just clip it out).

[Wildcard]

You proved that the return circuit worked and now you've proven that the drive circuit works as well, so therefore you don't have a problem.

Based on what you've said, you've checked everything that there is to check. Is the tank the correct one for the amp?

Hi Bill, yes, the tank is the original.

[Wildcard]

Like 52 Bill stated, healthy buzz from touching return cable but faint crashing sound from banging on tank does not compute. You have verified it is not the tank by swapping in a known good one of correct type. What you are calling healthy buzz is not very concrete. Can you get access to the amp you borrowed the tank from and try the same test with the same settings? (touching return plug)
You said all the measurements agreed with the schematic, does this include the AC voltage readings with signal injected? If so, I'm leaning toward something between the reverb pot and where it mixes in with the main signal.
If indeed all the AC readings in the reverb circuit are good, I would first look at the reverb muting FET, Q2. For a quick and dirty test, disconnect the leg that connects to the R39/R40 junction (or just clip it out).

Thanks for your input, much appreciated. I disconnected the leg of Q2 but that has had no affect. After that I had an idea and after much digging around found the footswitch that came with my blues deville. Plugged in and with reverb selected on the pedal (red light on) there's no reverb, however, when de-selected, while there's still no reverb there is a nasty buzzing hum. This disappears when I unplug the footswitch.....ideas???

[booj]

Is your "power amp in" jack J4 good? The output from K2A goes to J4 pin 4 and comes out of pin 5 to the reverb circuit.

[Wildcard]

Is your "power amp in" jack J4 good? The output from K2A goes to J4 pin 4 and comes out of pin 5 to the reverb circuit.

Thanks Booj,
I ran a signal from the line out of another amp into the power amp in (j4) socket and along with the signal I'm getting that same nasty buzz / hum that I got when the footswitch was plugged in...could that be the actual socket (j4) causing that???

[Wildcard]

OK - good news, i've found the problem, a dry joint on the P6 ribbon cable that connects to the tube board, i think this feeds R29 and C14 (though i might be wrong there), which acts as a mixer for the dry / wet reverb signal?. It must have broken the connection when i removed the tube board to fix the previous problem.

Thanks to all that contributed, it is very much appreciated!!!