Fender Twin Amp Has No Reverb

[Tom Phillips]

OK. Those voltages are OK and tell me believe that the first triode in the reverb recovery stage is alive.
Now...I'm just thinking out loud about what I'd do next and assuming that you know how to safely probe around inside this amp with the power on.
1. When you touch a test lead to pin 2 of V6A do you hear hum/buzz though the speaker. (Reverb control must be turned up)
2) Measure the DCV at TP49. The schematic lists what you should measure for both Reverb ON & OFF conditions

[PfeifferElectronics]

Tom,

I do hear hum/buzz when I touch a lead to pin 2.
TP49 = On/-15V, OFF/15.9V

[Tom Phillips]

That leads me to believe that the reverb return circuitry from V6A pin 2 to the output of the amp AND the reverb foot switch control circuitry is working properly. I suspect a problem between the reverb tank return jack and V6A pin 2. Not a lot of stuff.

There are two different versions of the circuit shown in the schematic.
Rev A uses only FET Q1.
Rev D uses two FETs, Q1 & Q2. Do you know which version you have?

[Tom Phillips]

...I do hear hum/buzz when I touch a lead to pin 2.

Note that if everything was working properly then you should hear the same hum/buzz in the output when you touch the test lead to the reverb tank return jack center contact. Try that too.

[J M Fahey]

I´d add that I want to know for sure that you hear a hum/buzz touching V6A(2) when the Fet is open (TP49=-15V) but it disappears when the Fet is closed/shorting (TP49=+15V).
Or it makes no difference?
Thanks.

[PfeifferElectronics]

Ok, it seems that I'm chasing a moving target here because now I do not hear a hum on pin 2 of V6A. TP49 is still working properly. I have the revision with two FETs revision D.

[J M Fahey]

Dear Pfeiffer, you give me two unrelated answers when I´m trying to find a relation between two fenomenae.
That "TP49 works properly" (I *think* you mean voltage changes there) tells me *nothing* whether change affects hum ... or not.
Please answer

I´d add that I want to know for sure that you hear a hum/buzz touching V6A(2) when the Fet is open (TP49=-15V) but it disappears when the Fet is closed/shorting (TP49=+15V).
Or it makes no difference?

[PfeifferElectronics]

Fahey,

I get no hum regardless of the proper voltage changes at TP49.

[g1]

Are your DC voltages on V6 pin 1 & 3 still good? If so, you could try removing both Q1 and Q2 or at least disconnect source or drain on both. If the reverb then works, recommend replacing them both, rather than just the bad one. Aside from those items, resoldering all connections in the reverb recovery side may be in order. A bad solder connection could have been the result of the original impact.

[J M Fahey]

Fully agree, and add: do these tests first, V6 might be bad or not receiving proper voltages (after all the presumed failure mode here is cracked tracks or solder) or its sound path might be cut somewhere.
Touch the plate with the end of a screwdriver (be careful, don´t touch the metal part) or even better with a neon lamp screwdriver, which is "noisy" and listen.
Follow the output path until you hear something.
If along a track, touch both end pads, one might be dead, the next one live, evn if *visually* the track shows no problems, you might have a hairline crack, hidden under the epoxy solder mask.
Move until you hear *something*, then backtrack.

Edit: also although TP49 might be "working properly", you have no guarantee that that voltage reaches the FET gates ... or the series diodes just before them.
The way they are connected, *no* voltage means killing sound; they need a negative voltage to open and let it pass.

[Daniel Toledo]

Try measuring the transformer secondary reverb, green and black wires.

[oc disorder]

This is the amp with the two RCA sockets mounted on the top of the chassis (well bottom when the amp is in the box).
I am suggesting that it might be something stupid like physical damage to the actual plastic mounting pair of RCA's in there.
They are the typical Red and White rca's with a mounting screw in the plastic like the type found on mixers for tape in and out etc.
I serviced an amp like that once and matched up the colors of the lead to the amp socket when putting it back together.
A day later the guy rang up to say the reverb wasn't working and had to drive for an hour and a half to bring it back.
(I did tell him over the phone to swap them but was not mechanically minded).
So red to white and black to red and all was well (lead - red and black, sockets - red and white !).
That was my experience!
Now I always check the reverb before it leaves !!!

[PfeifferElectronics]

Update... It appears that the reverb switching voltages are not making it from pin 7 of U3B to the FET gates. The voltages go from -15 to -2 when reverb is on, and +16 to + 12 on the other side of R128. I put a jumper across R128 and heard loud crashing reverb springs.... I'm tempted to just bypass R128 but there must be a reason for it. Any ideas?

Thanks guys... You have been great! By the way, I replaced the two J111.

[Enzo]

Look at the schematic, the left end of R128 is TP49 at U3-7. What does the drawing say about TP49? It says +16 for reverb off and -15 for reverb ON.

The voltages go from -15 to -2 when reverb is on, and +16 to + 12 on the other side of R128.

I don't know which end of R128 is which in this statement, but neither set of voltages would be correct. If TP49 is not correct, then nothing downstream of it, in this case the reverb JFETs, will work either. Bypassing R128 is not fixing the problem with U3.

ANd there are TP47 and TP48. TP47 should be a steady voltage as a reference. Then note TP48 doesn;t specify voltages, just greater than and less than that reference voltage. WHat are they doing in your amp?

And I see the reverb has an assign switch. Is it set in the center for "BOTH"? I hope we are not using channel 1 with the reverb selected for channel 2.

[PfeifferElectronics]

Good news! Reverb is working properly now. However, I'm not sure what I did to fix it. I have a theory that i would like to get some opinions.

The reverb started working after I took a pointed tool and scraped off some of the dried yellow flux on the ribbon cable connection to the reverb assign switch and around the reverb assign switch itself. My question is this, does that flux residue conduct electricity? It seems that this isn't the first time I've had this happen. Does anyone else experience this phenomonem? The only other thing I did was move the assign switch back and forth a few times but I did this previously without getting reverb back. If I move the switch through the different positions it works as expected with no problems whatsoever.