Hot Rod Deville with no Reverb

[flythair1]

MEF Folks,

I found a HRD 2x12 that is about 10-12 years old. It was with the original owner and looks pretty much untouched. He said it needed tubes and one of the original power tubes was discolored somewhat. I put in a good set of 6L6GC's and the amp works excellent & sounds great except for the reverb not working. I did swap out each of the pre-amp tubes and the originals seem ok. Nothing on the board looks burnt. If you tap the tank with the reverb knob on 0, you hear nothing. I you tap it with the reverb knob turned up, you can hear the faint splash of the springs. I checked the archives for threads related to this but found nothing on "no reverb".

Any ideas as to how to troubleshoot this problem would be appreciated. I did find the service manual download in one of the other threads so I have the schematic & layout available.

Scott

[Jazz P Bass]

So if banging on the tank gets a response, that means the return is working.
Now you must check the send circuit.
Measure the resistance of the input coil on the tank.
It should measure pretty low.
Make sure you have continuity of the send wires from the amp down to the pan.
If taht is all good, then you have to dig into the send circuit itself.
U2A: measure the Vdc on pin 1.
It should read zero volts.
Insert a test signal into the amps input & check that the Vac voltages agree with the schematic.
Test point 19 & 20.
Post back what you find.

[bob p]

RG's tube amp debugging page has a good link on reverb repair.

Basically, you need to check the driving stage, the tank, and the recovery stage.

It's pretty easy for people to bugger up reverbs when moving heavy amps around. I happen to have a '78 Fender on the bench right now for a similar set of symptoms. It looks like the amp got body slammed one time too many. The reverb tank had the small gauge wires ripped out of the transducer when somebody manhandled the amp. To test the integrity of the tank, measure the impedance across the input terminals, then across the output terminals. Input terminals are typically low Z, outputs are typically in the k-ohm range.

edit: Jazz P Bass beat me to the punch. I also overlooked the fact that you got noise when banging on the tank. This suggests that if the tank is the problem, it would be at the input side. Should measure low ohms if it's OK. My Fender tank had a 2 ohm input resistance. Otherwise look at the driver circuit.

[bob p]

The driver circuit is a low powered Class-A amp. If the driver circuit is working, you should be able to hook the reverb driver up to an 8-ohm speaker and have audible output. Just plug an RCA cord into the reverb send jack, and hook it up a small 8-ohm speaker. If you get sound, the driver is working.

[g1]

bob p, this is a solid state reverb circuit, not the typical tube type so I would be leery of connecting a speaker to the op amp driving the tank.
flyhair1, you said you could hear the springs "faintly". With the reverb control up tapping on the tank (don't be shy) should produce more than a faint response.

[bob p]

oops. nevermind. i'm showing my age...

[flythair1]

Thanks guys for your replies.

I was finally able to get back to this amp and I pulled it apart and first checked the tank & wires and all checked out ok. I pulled the board and looked hard for a problem. I resoldered one joint but cleaned up some spots that could have been a solder bridge. There were places around the reverb wires that could have been if-y. Put it back together and darned if it dosen't work. I did not check it after just unplugging and replugging the tank leads, but after reading another thread about a 410 Deville reverb, I have to wonder. The RCA plugs and jacks look clean and nice but could they have just had a little corrosion and stopped the signal?

Anyhow Thanks for your comments.

Scott