Fender Twin Amp Has No Reverb

[PfeifferElectronics]

My Twin Amp fell out the back of my rig onto the concrete floor. when I played it, everything seemed to be ok. Then I started hearing a crackling noise. So I opened it up and found some bad solder joints and reflowed them. Everything is working great now except I don't have any reverb, not even a little. I checked the tank and the cables to/from the tank and that is not the problem. I check the 12ax7 and the 12at7 that runs the reverb and they check out ok. I don't know what else to try. Please help.

Thanks

[trem]

Just curious what you mean when you say you "checked" the reverb tubes.
Did you replace them.?
The reverb tank is a real fragile piece, with tiny wires and springs that can break.
Best

[Tom Phillips]

First step is to determine if the problem is in the reverb drive section or the reverb recovery section.
1) If you turn up the reverb control and shake the tank do you hear the spring crashing sound?
2) Swap the reverb tank leads at the chassis connectors and shake the tank again. Do you hear the spring crashing sound now?
Please do #2 no mater what the answer is to #1
Report back and we will proceed from there.
What moderl of "Fender Twin" do you have?

...I checked the tank and the cables to/from the tank and that is not the problem. ...

How did you check?

Regards,
Tom

[PfeifferElectronics]

First step is to determine if the problem is in the reverb drive section or the reverb recovery section.
1) If you turn up the reverb control and shake the tank do you hear the spring crashing sound?
2) Swap the reverb tank leads at the chassis connectors and shake the tank again. Do you hear the spring crashing sound now?
Please do #2 no mater what the answer is to #1
Report back and we will proceed from there.
What moderl of "Fender Twin" do you have?

How did you check?

Regards,
Tom

I don't hear springs crashing either way.
I have a 94 Twin Amp.
I checked the tubes with a tube checker.
I checked the tank by connecting the amp's reverb cables to my brother's identical amp. Still no reverb.


Thanks!

[Chuck H]

I checked the tank by connecting the amp's reverb cables to my brother's identical amp. Still no reverb.

That statement is a unclear to me. Do you mean to say that you connected your tank to your brothers amp? Or do you mean to say that you connected reverb cables from your AMP to your brothers AMP? Or do you mean to say that you connected reverb cables from your amp to your brothers tank?

[Tom Phillips]

Thanks for the info.
It appears that your reverb recovery circuit is not working.
If the amp was in my shop the next thing I'd be doing is visual inspection of the return circuit components and voltage measurements at the reverb recovery triode pins. I'd also make sure that the FET that kills the reverb return is not permanently ON. (It shorts the reverb return signal to ground)
Since the amp was dropped there may be a cracked solder joint in part of the return circuit. It could even be in the tube's heater supply. Can you see that both heaters are glowing in the return tube?

Are you capable of troubleshooting to the circuit level inside the amp? Have test equipment?

Tom

[Tom Phillips]

...I checked the tank by connecting the amp's reverb cables to my brother's identical amp. Still no reverb.

After reading Chuck's reply I need clarification too.
If there was "still no reverb" with your tank connected to your brother's amp then there could be a bad cable OR the wires to both of the transducers in your tank could be broken OR the springs could have broken off the transducers in the fall.
Have you looked inside your reverb tank?

[PfeifferElectronics]

To clarify, I connected my brother's tank to my amp using new cables. His reverb works fine so it is not the tank or the cables.

[Chuck H]

OK. That's true enough. But remember that you didn't test your tank. You may want to since they're very sensitive. You'll have a confusing time finding other problems if your reverb tank doesn't work.

[52 Bill]

If you have an ohm meter, try reading the resistance across the RCA plugs that come from the tank. One will read around 1-2 ohms (input) and the other should read around 200 ohms (output).

If either one or both wires in the tank are broken or not making good contact, it will cause the symptoms that you have described.

[Tom Phillips]

To clarify, I connected my brother's tank to my amp using new cables. His reverb works fine so it is not the tank or the cables.

Understood. You have done some good troubleshooting. If you want to be absolutly sure that your tank and cables are still OK you could hook them to your Brother's amp and verify that they work corretly by producing reverb. It's double checking but would be nice to know. However, now I think we are back to my post #6 above. How do you feel about those questions?

[J M Fahey]

Fine, so far.
Please let me add to:

I checked the tank and the cables to/from the tank and that is not the problem. I check the 12ax7 and the 12at7 that runs the reverb and they check out ok. I don't know what else to try.

that the reverb circuit is made of not only those but also about 20/30 extra parts, such as resistors, capacitors, wires, sockets, pots, connectors, etc.
To boot, those parts are interconnected in a certain way, forming a system, which does a specific task (give you reverb sound).
They are mounted in a board or PCB, they are grounded and fed power, etc.
So you still have *a lot* to check.
Please google its schematic, or ask it from Fender (great guys) and post or link it here, so we can suggest further tests.
Otherwise, we are as blind as a bat.

[PfeifferElectronics]

Thanks for the info.
It appears that your reverb recovery circuit is not working.
If the amp was in my shop the next thing I'd be doing is visual inspection of the return circuit components and voltage measurements at the reverb recovery triode pins. I'd also make sure that the FET that kills the reverb return is not permanently ON. (It shorts the reverb return signal to ground)
Since the amp was dropped there may be a cracked solder joint in part of the return circuit. It could even be in the tube's heater supply. Can you see that both heaters are glowing in the return tube?

Are you capable of troubleshooting to the circuit level inside the amp? Have test equipment?

Tom

Thanks Tom.

I have test equipment and know how to short the filter caps to prevent electric shock. I tested the rca leads to/from the tank and I get 200 ohms from output and 1.3 from input.

I took voltage readings.

TP 26 = 15.65 AC, 257 DC
TP 27 = 4.47 DC
Pin 2 of V6A = -.3mV
TP 23 = -.2mV **** this doesn't seem correct.

I did a visual inspection of all the solder joints and components surrounding TP 23 and everything looks good.

Schematic here http://www.pfeifferelectronics.com/9..._Schematic.pdf

[Tom Phillips]

What's the DC voltage at Pin 1 and at Pin 3 of V6A?

[PfeifferElectronics]

Pin 1 = 200V
Pin 3 = 1.57V