Reverb on Twin Reverb Reissue stopped working while playing

[belf]

Hello everybody


The reverb on my Twin Reverb Reissue stopped working yesterday right while I was playing. Things I've already checked :

Green and black cables in reverb pan are properly soldered. Springs are ok. Reverb RCA cable is ok.
Tubes are ok.
If I bang on the amp, I can hear reverb, so I suppose V4 is working.

Voltages on V3:

Plate = 454 VDC
Cathode = 8.3 VDC

TP7 (R24 1M ohms, connected to Grid) = 0.08 VDC

TP10 (Reverb Transformer Green and Black) = 0.1 mvdc
TP11 (R26, connected to grid on V4) = 0.1 mvdc

Someone suggested testing the reverb transformer with an 8 Ohm speaker, but I don't have such a speaker lying around. It's also worth mentioning that when reverb and vibrato are on at the same time the amp makes a noise resembling the one of a helicopter. When the reverb is off, tremolo works perfectly.

Any ideas ? Do the voltages seem ok ? Thank u.

[HotBluePlates]

The reverb on my Twin Reverb Reissue stopped working yesterday right while I was playing.
... when reverb and vibrato are on at the same time the amp makes a noise resembling the one of a helicopter. ...

Are you the original owner? Is the amp still under warranty? If so, I'd take it to the music store and have it fixed on Fender's dime.

If I bang on the amp, I can hear reverb, so I suppose V4 is working.

If that happens, everything from the reverb tank springs to V4 output is fine.

There might be a break between C13 (500pF) and the reverb tank springs, or there could be another issue. For reference, the schematic and layout of the Twin Reverb reissue are here.

Do the voltages seem ok ?

All voltages are fine. The 0.1mV are probably really 0vdc; eithe way, they are close enough we'll call those 0v.

I notice CP28-31 are connectors from the reverb send circuit to parts off the p.c. board., including connections to the reverb transformer and the reverb send jack. If any of those are loose or making intermittent contact, you'll have problems with your reverb. Have you checked to make sue these are in place and tight?

... It's also worth mentioning that when reverb and vibrato are on at the same time the amp makes a noise resembling the one of a helicopter. When the reverb is off, tremolo works perfectly. ...

Helicopter sound not made by the trem? 

Just kidding. Both of these circuits share a filter cap, so they could influence each other. But that cap is also used by the output tube screens, so it seems unlikely it's bad. At the power end of the board ae connectors for the "Z" filter cap and its ground (CP24, CP25, CP18). Each of those should be on and tight.

But really, you ought to have a Fender service center be responsible for Fender fixing it if there's any chance it's still under warranty. It's possible there is a tricky fault you're not prepared to troubleshoot (meaning a test signal and an o'scope or speaker would tell you if everything is good up to the output of the reverb transformer).

[belf]

Hi, thanks for helping. I bought the amp 2nd hand, so no warranty. I changed some caps and resistors and i was really digging the sound. Everything was working ok until yesterday when I was playing Ticket to Ride with my father-in-law. Reverb all of sudden stopped working. I turned the amp off and on again. Reverb shortly came back for 5 -10 seconds, then it stopped again, this time for real. I guess I won't be playing the Beatles anymore 

I will re check CP28 - CP31 and the circuit starting from C13.

About the voltages: how do you know they are fine ? are there tables on the internet to check that ? I've noticed that the voltages on the schematics are not really the ones I'm measuring. Do I need an oscilloscope to get these values ? By the way, I'm in Europe (230 V AC) and i'm operating the Amp with 1 speaker and only 2 of the 4 output tubes, so i guess that might also change things. I've recently bought a kit amp, so an 8 ohm speaker is on the way...

I'd rather buy an oscilloscope than pay for a tech here....i'd probably pay the same price, believe it or not. Besides i'm interested in learning.

[sluckey]

Someone suggested testing the reverb transformer with an 8 Ohm speaker, but I don't have such a speaker lying around.

Sure you do. There should be two 8Ω speakers in the TR cab. Use one of those.

Just unplug the reverb send cable and connect the speaker to the send jack (You'll have to temporarily adapt). This essentially makes a low power single ended amp using the 12AT7 reverb driver. Your volume and tone controls will work with this temporary setup.  If you get a reasonable bedroom sound level, the V3 driver circuit is OK.

I'd pull all output tubes during this test or connect a dummy load to the speaker jack.

[HotBluePlates]

... I bought the amp 2nd hand, so no warranty. I changed some caps and resistors and i was really digging the sound. Everything was working ok until yesterday ...

I hate to say it, but there's a 99.99% chance any issue is related to changes you made to the amp. So what did you change?

About the voltages: how do you know they are fine ? ...

Well, there's the schematic and layout I linked earlier.

... For reference, the schematic and layout of the Twin Reverb reissue are here.

You can also look back at the original AB763 Twin Reverb schematic/layout.

... I've noticed that the voltages on the schematics are not really the ones I'm measuring. Do I need an oscilloscope to get these values ? ...

The voltages on the schematic I linked come in 2 varieties: d.c. voltages in rectangles, a.c. voltages in ovals.

You get none of the a.c. voltages unless you have a signal generator applying a test signal to the input jacks of the size indicated on the schematic, and with controls set according to the schematic notes.

Now if you're worried about the little bit of variance between the schematic d.c. voltages and your numbers, that's "nothing" in tube electronics. A problem would be likely if the voltages were 50-100% different than the schematic values.

[belf]

Hi, thank you all for the really insightful feedback. Just to give you an update: reverb is working again. While I was poking around in the amp a cable from the reverb jack came off. I resoldered it and reverb works again. It's all very weird because I thought I had checked it earlier and there was continuity between the components....Another weird thing: there's a C46 cap on the dual reverb jack which isn't listed on the schematics....how awkward is that ? 

Here is the list of components I have updated (same values, simply better quality) :



@sluckey: so the reverb send cable is the one going into the INPUT of my reverb pan ? do I leave the other cable connected ? I'm going to try this method anyway tomorrow just to be sure.

Cheers

[sluckey]

@sluckey: so the reverb send cable is the one going into the INPUT of my reverb pan ?

yes

do I leave the other cable connected ?

it doesn't matter.