Fender Pro twin reverb not working

[floridamike]

Im pulling my hair out, Before I pull the chassis or sell the amp, I want to pick your brains.The model is an early 2000s Fender Pro Tube Twin Amp (not the evil twin). The Reverb used to work great but for a while now, nothing.

Ive tested the tank with a multimeter though the RCA cables and got some resistance

I've also bought a new tank and used 3 sets of cables. All to no avail
.
I also got a new tube for V3 12AX7WA but didn't make a difference

This amp has a tremolo that works fine. When I crank the reverb dial up I can hear some light white noise and that exists whether I engage the reverb through the foot switch or not

Switching through tons of cables and banging the tank trying to get some reverb noise on the output side at least, I get nothing.

Im now thinking it could be the reverb transformer, or maybe a wire break at the RCA jacks at the chassis. Could there be any other explanation. I thought about the footswitch but it engages the Trem/channel just fine and the light comes on the foot switch for reverb when engaged.

Any troubleshooting I can try?

Thanks, Mike

[g1]

Pro Series Twin schematic attached.

Disconnect the cable from the 'out' connector of the tank and touch the tip with your thumb. With reverb control turned up, do you hear a hum from touching the tip?

[Enzo]

Yes, first stop guessing, that just wastes your time. Any reverb circuit has three main parts, the pan itself, the drive, and the return.

Turn the amp on and the reverb up midway. Plug cables into the chassis reverb jacks but not into the pan jacks.. Now touch the tip of each cable free end with your finger. ONE of them should make a hum out the speaker when you touch. That one is the return. If neither one hums, your return is not working. If one does hum, plug it into the OUTPUT jack on the reverb pan. If the pan is working, you should get loud spring noises if you bounce it around.

The pan you test with an ohm meter. For the Fender you have a low impedance model like 4AB2C1B or similar. Across the input jack, you get a pretty low resistance, on the output jack you get I forget, maybe 200 ohms.

That leaves the drive. Fender uses the small transformer to drive the pan. And all that is really is a tiny little power amp. So instead of connecting the drive cable to the pan, just connect it to some speaker. Can you hear the guitar through that? Note the transformer is before the pan, so if shaking the pan makes no sound, we have a return problem, and the transformer has nothing to do with that.

There can be a ton of things that cause lack of reverb, that is why we use a systematic approach to isolate the problem.

[floridamike]

Okay, trying with with two set of cables amp turned up and trying reverb between 5 and 10, no hum at all with the test you mentioned.

[g1]

Do you have approx. 1.4VDC at V5 pin3 ?
If so, I would suspect some problem in the reverb mute circuit, Q2 & Q3.

[floridamike]

just to confirm multimeter onto DC, pull V5 put the red lead of the multimeter into pin 3 and the black onto the chassis, then amp on and standby on?

[g1]

No, this measurement would need to be done with the tube installed. It is to check if that tube and it's circuit are working.
If you are not familiar with all the safety precautions, you should not proceed.

[Enzo]

Okay, trying with with two set of cables amp turned up and trying reverb between 5 and 10, no hum at all with the test you mentioned.

OK, so your return is not working. I suspect the drive side is OK< but did you try the little speaker test, just for fun?


One test you can do to check those transistors: Pull V5, disconnect the reverb pan,and with POWER ON and the amp wanting to run, measure RESISTANCE to ground from pin 2 of that tube socket. In a good amp you should see maybe 220k or more. If you see something like 2000 ohms or less, then one or both those transistors are either bad or are being turned on.

At the very bottom center of your schematic, see TP68? (I think, hard to read) for control signal REV_MUTE. Nearby it explains the voltages there for reverb on and off, check them.