Blues Deville no Verb

[LarBal]

I had an old Blues Deville come in with a crackly volume pot, and visibly leaky filter caps. WHen I pulled the board the Vol pot was rocking back and forth, with lifted pads.
I replaced that, did the cap job, put it all back together, amp sounded great. No issues....
except that the Reverb worked when it came in, and now it doesn't. Here's what I know:

https://www.thetubestore.com/lib/the...-Schematic.pdf

-Tank works fine on another amp
-cables show continuity
-Footswitch changes channels OK but not Reverb
-I can hear the relay for the channel switch physically clicking when changing channels, but not when I turn the verb on and off via FS
-I'm getting a splash when I shake the take, so the return is fine
-When I plug in the footswitch cable when the splash is ringing out, it kills it immediately
-At TP 34, for " +15 Reverb off, -15 Reverb on", it just stays on -10 or +13, but this is effected by the channel switch?!?!? The reverb switch does nothing here.

Any ideas?

[Enzo]

You had to extract the board to get at the controls. You fixed a volume control, but whatever caused it to be loose could have affected reverb control. A key to working on amps and such: any time you do some work and now a NEW problem has been added, it is 99% sure that it is the result of your work. Doesn't mean you did anything wrong, but in getting at the volume control we may have bent the reverb control one time too many. Or a tiny bead of solder may be grounding out a reverb circuit.

[LarBal]

Ain't that the truth. Well I replaced the Reverb pot, poked, prodded, and chopsticked everything, no luck
I've only got 13 and -13VDC for my lo volt supply. Could that be an issue? I do have 4VDC at the pan, TP17

And am I correct in that if I'm getting signal when I tap on the return RCA end, nthat the red are is the only area I'm concerned with? Or do I need to backtrack to the footswitch jack as well. That's what I'm confused about with this circuit, actually. Even if something was busted in the footswitch circuit, the reverb would still be "on" right?

[LarBal]

Across R32( from pin 1 to 2 on the op amp U2A driving the tank), I'm reading 26ohms. Is this my problem? So basically, 26ohms between the red lead and shield to the pan

[nickb]

R32 is in parallel with the tank so 26 ohms is just the DC resistance of the input coil of the tank. I would expect nearer 60 ohms for a 4EB3C1B tank that the DeVille uses so you should check that at the tank with the cable unplugged. A much bigger problem is 4VDC on the tank, it should be close to zero. Check the DCV on pin 3 it should also be zero.

BTW, TP17 calls out 4.63VAC under the specified conditions.

[glebert]

And am I correct in that if I'm getting signal when I tap on the return RCA end, nthat the red are is the only area I'm concerned with? Or do I need to backtrack to the footswitch jack as well. That's what I'm confused about with this circuit, actually. Even if something was busted in the footswitch circuit, the reverb would still be "on" right?

If you get a response when you tap the return cable then the footswitch enable/disable shouldn't be the problem, since the footswitch circuit shorts the reverb return to ground.. Do you get a clang if you tap the tank when the cables are plugged into it?

[LarBal]

R32 is in parallel with the tank so 26 ohms is just the DC resistance of the input coil of the tank. I would expect nearer 60 ohms for a 4EB3C1B tank that the DeVille uses so you should check that at the tank with the cable unplugged. A much bigger problem is 4VDC on the tank, it should be close to zero. Check the DCV on pin 3 it should also be zero.

BTW, TP17 calls out 4.63VAC under the specified conditions.

Oh yes, VAC, my bad.
Well, I definitely have VDC on pin 1 of TL072, with reference to ground, with no tank connected. I f I measure from the tip of the red RCA input cable to it's shield(instead of ground), I have zero VDC, so I gather that is what the input of the tank is seeing? 0V, since the shield isn't actually at ground potential?
But here's the thing, and what I noticed this morning with a clear head, when I touch the tip of the red input RCA in the the chassis, and short it to ground, the ~4VDC that is at pin 1 goes down to zero, but at the same time, my +/-16VDC supply(that has been at 13VDC), slowly climbs back to 16VDC! As soon as I pull the tip from the chassis, the pin1 VDC slowly climbs back up(takes a few minutes) to 4VDC and the supply goes down to 13VDC .

This tank is marked 4BB3C1B, and I looked it up and indeed these were the original Blues Deville tanks

If you get a response when you tap the return cable then the footswitch enable/disable shouldn't be the problem, since the footswitch circuit shorts the reverb return to ground.. Do you get a clang if you tap the tank when the cables are plugged into it?

Yes! I do, and the clang can be adjusted with the reverb pot. However in my haste to clean up my mess this morning from working all day on this thing yesterday, I knocked the tank off my bench. I plugged it into my Blues jr(as it worked in there yesterday), and nothing...I've got another on the way

[nevetslab]

Your +/- 16VDC supplies shouldn't be varying in level the way you're reporting them. You may have 'cooked' solder joints at the two 16V Zener diodes CR22, CR23 & at the dropping resistors R85 & R86. Those resistors run very hot, and tend to bake the PCB below it, as well as cook the solder joints to/from them, as well as on the Zeners. Common problem on the Hot Rod Series and the Blues Deville amps. Oxidation also sets in. I'd carefully remove the four components, scrape away the oxidation on the leads, then carefully re-install them (or replace them). If replacing the resistors, you could put kinks in the leads to elevate the resistors above the PCB. If the solder pads are damaged from the long-term heating, you may need to lay in buss wires to restore the connections.

[LarBal]

Your +/- 16VDC supplies shouldn't be varying in level the way you're reporting them. You may have 'cooked' solder joints at the two 16V Zener diodes CR22, CR23 & at the dropping resistors R85 & R86. Those resistors run very hot, and tend to bake the PCB below it, as well as cook the solder joints to/from them, as well as on the Zeners. Common problem on the Hot Rod Series and the Blues Deville amps. Oxidation also sets in. I'd carefully remove the four components, scrape away the oxidation on the leads, then carefully re-install them (or replace them). If replacing the resistors, you could put kinks in the leads to elevate the resistors above the PCB. If the solder pads are damaged from the long-term heating, you may need to lay in buss wires to restore the connections.

Alright, thanks, I just made a Mouser order for a bunch of things on this amp's part list, and will rebuild the 16V supply .

[LarBal]

I replaced the entire +/-16VDC supply with no change, so that was the two 330r, the 100uF and 1000uF caps, and the 2 zeners. I Still have ~13VDC out , unless I ground the red verb cable(pin 3 of TL072), then it pops back up to 16VDC.
To restate, the verb worked on this amp before I repaired what it came in for, a busted Volume pot and cap job. I still have no Verb, and I even bought another tank to be sure.

So really, all that's left is everything after the power amp in jack (starting at C15 to the tank input) Oh I also replaced CR2, CR3, C16, as well as the Q1 and the Master volume pot, just to be sure none of those were leaking signal to ground.

Any thoughts on the best place to start?

[Enzo]

Don't assume the schematic will help, it might, but might not. As I mentioned, a errant drop of solder can connect two points on the board together that have zero to do with each other on the schematic.

[glebert]

This tank is marked 4BB3C1B, and I looked it up and indeed these were the original Blues Deville tanks

Ok, this is perhaps my own bit of delusion, but the original Blues Devilles had a 4BB3C1B, but IIRC at some point Fender changed them to 4EBxxxx for the Blues reissues and the Hot Rod amps with no change in the circuitry. IMHO the amps sound much better with 4BB tanks, I find the reverb to be quite clangy with 4EB tanks. I alsways wondered why Fender would use a worse sounding tank. The only reason I can come up with is that a 4EB tank (being higher input impedance) would put less stress on the TL072 than the 4BB. Might be worth a try to replace U2.

[LarBal]

I should also say that I still have 4VDC at the tank input/TL073 pin 1 and ground, but 0VDC between the same point, and the shield

[nickb]

Oh yes, VAC, my bad.
Well, I definitely have VDC on pin 1 of TL072, with reference to ground, with no tank connected. I f I measure from the tip of the red RCA input cable to it's shield(instead of ground), I have zero VDC, so I gather that is what the input of the tank is seeing? 0V, since the shield isn't actually at ground potential?
But here's the thing, and what I noticed this morning with a clear head, when I touch the tip of the red input RCA in the the chassis, and short it to ground, the ~4VDC that is at pin 1 goes down to zero, but at the same time, my +/-16VDC supply(that has been at 13VDC), slowly climbs back to 16VDC! As soon as I pull the tip from the chassis, the pin1 VDC slowly climbs back up(takes a few minutes) to 4VDC and the supply goes down to 13VDC .

This tank is marked 4BB3C1B, and I looked it up and indeed these were the original Blues Deville tanks

In the above you state that without the tank connected you have both 0VDC and then later on 4VDC. Which is it? This whole slowly changing power is very odd. If the source voltage on the input to the 330R resistors is steady then you have to be drawing too much current. That suggests a short. Does U2 get hot when the supply goes to 13V?

Uh, yup, I see it is a 4BB3C1B. I guess I forgot.

[nickb]

I should also say that I still have 4VDC at the tank input/TL073 pin 1 and ground, but 0VDC between the same point, and the shield

You should have 0VDC on pin1, 2 and 3 of U3. If not then something around it is open, shorted or U3 is bad.