Gretsch Fury Reverb issue

[johnk_10]

i just finished replacing all of the filter caps, quite few coupling caps in my buudy's Gretsch Fury and the amp it's sounding great, but for the life of me, i can't figure out why the reverb isn't working.
i've repaired reverb circuits on 100's of fenders, and it was always either the tank (broken leads to the tiny xformers in it, or an open one), the RCA cables, the reverb drive transformer or reverb drive tube, or the recovery circuit or the tube.
on this one, when i turn the reverb level up, the (unreverberated) signal level drops in level thru the speakers quite a bit. when i shake the amp, i get the spring crash thru the return, so it's definitely in the send/drive circuit. it uses a 6973 (RCA) to drive the tank and i tested it on my triplett 3444 and it tested at new spec, i also checked every resistor, cap, and even subbed a fender drive tranny in it, and also ohmed out the reverb tank's input and output xformers (all good), but still the same results. i'm getting absolutely no signal on the scope on the drive signal to the tank. does anyone have idea of what i'm missing here?

here's the schematic of the circuit:


my voltage readings on the 6973's (V4-A) tube socket:
pin 3: getting plenty of signal with it's level controlled from the amps input volume pot
pin 7 : .035V (IMO, this is the problem!)
pin 8: 314V
pin 9: 384.5V


any help would be greatly appreciated as i'd like to get this one off of my bench.

[johnk_10]

i'm stumped and since i don't have another 6973 on hand, i'll have to wait to try another one in it until a couple of them that i ordered arrive on thursday.

[johnk_10]

crickets chirping..............................

[johnk_10]

fixed it today.

[dc74]

Hi there, I have the exact same issue on my Fury piggyback. Can you tell me how you fixed it, I am totally stumped.
Thank you - Dc

[Enzo]

No reverb is such a broad complaint, it could be a ton of things.

Unplug the cables from the reverb unit, then measure resistance across each jack. Is either end open?

Now you have a drive circuit, and a return circuit. Play a guitar or other ssignal through the amp. Plug some speaker into the Reverb footswitch jack. DO you hear the guitar through that? That tests the drive.

You have the two cables for the pan, leaving then unconnected still, one should have a shielded wire, the other may have just one wire. With volume up and reverb control up, touch the tip of the one with the shielded wire. DO you hear a loud hum? That tests the return.

These three tests will tell you where the problem lies, the pan, the drive, or the return.

[dc74]

Firstly, thank you for the response!

Neither of the RCA cables is open, tested that already.

I do get a hum when I touch the top of the shielded line.

I have tried three pans, all of wyI believe to be functional. I can hear the spring when I crash the pan.

I have not tried the speaker trick you mentioned, but based on your suggestions, I’m led to think it may be the drive circuit.

I do think there may have been some work done previously on the amp. The footswitch jack is completely different than all the others (it’s a square black plastic box as opposed to the standard switccraft style with the PCB board material sandwich). Also, the RCA lead and the signal lead were both soldered to the tip. The wiring diagram I have (looks to be the same as yours) shows signal to ring and RCA to tip. I resoldered them accordingly, but still have the same symptoms. I get signal, no reverb effect, and I get a volume/tone loss as I turn the pot up.

Thanks so much. - Dc

[dc74]

In your situation, do you recall what the fix was?

[52 Bill]

I do think there may have been some work done previously on the amp. The footswitch jack is completely different than all the others (it’s a square black plastic box as opposed to the standard switccraft style with the PCB board material sandwich). Also, the RCA lead and the signal lead were both soldered to the tip. The wiring diagram I have (looks to be the same as yours) shows signal to ring and RCA to tip. I resoldered them accordingly, but still have the same symptoms. I get signal, no reverb effect, and I get a volume/tone loss as I turn the pot up.

The jack may have been replaced, but that shouldn't matter.

You have read the schematic wrong, both the output from the drive transformer and the hot of the reverb tank drive cable must be connected to the same tip connection of the foot switch jack. When the foot switch is used, the signal going to the tank is grounded, turning off the reverb.

The reverb control is like a balance control between the reverb signal and the straight signal. As you turn up the reverb the straight signal is reduced, so what you are experiencing is normal.

Have you checked the driver tube? Do you have a voltmeter? Do you have the skills to read high voltages in the amp? If you do, read the voltages on the driver tube.

[dc74]

Thanks, I’ll switch the lines back to the tip when I replace the jack.

I replaced the driver tube, and it should be good. I will check the voltages, from the tube as well as the drive transformer to see if that’s where it lies.

[dc74]

I get just about 4 volts dc off of the tube socket pins that the input leads (? red & blue) for the driver transformer connect to, and there is no voltage on the black leads (which connect to the lead that runs to the footswitch jack).

So I am assuming with my rudimentary knowledge that the transformer is dead. I cannot identify the type of transformer it is however, other than the schematic I have indicates "24:1", the transformer itself has no real manufacturer markings. Just reads: T - 109 (model?) 524 67 07 (date code?

[g1]

If the tank is the same input impedance as the standard Fender type, I'd just put the standard Fender transformer there.

[Enzo]

Why are you assuming the transformer bad because there is no voltage on it? I see two 500 ohm resistors in series from the B+ supply. Are either one open? Is there B+ at both ends of each?

[52 Bill]

Or are you reading the voltage across the transformer primary, from red to blue?

[dc74]

Yes, there is voltage across both that reads at 388v, which is high (according to the schematic, should be 340v feeding to pin 9 & the transformer correct?)...I suppose that means they are bad. One does not look very good, the ceramic is cracked, hadn’t noticed it because it was hidden under a wire. There is also a wire that has melted onto them that feeds from the 450v caps.