Gibson Super Medalist Reverb issue

[cjlectronics]

I am working on a Gibson Super Medalist and the reverb is not working properly. When enabled, I get a loud hum, the amp still works and I can hear the guitar, but there is no reverb. If I tap one of the caps in the reverb circuit, I can hear the spring reverb. However, the hum is over bearing and there is no reverb. So far this is what I've done:

1. Replaced all the capacitors in the reverb circuit.
2. swapped tubes between the NORMAL channel and the reverb circuit. I learned that the schematic has a typo... V4 should be 6EU7 and NOT 6AU7. This has me chasing my tail for a while.
3. On the attached schematic, I noted voltages in the reverb circuit.
4. Also, I found the transformer in the reverb circuit wasn't grounded on the secondary. I grounded the secondary and confirmed the reverb tank is at ground potential with the chassis. The secondary of the transformer measures 71 ohms.
5. When the reverb is enabled, the reverb light does illuminate.

All these things and I just can't get rid of the loud hum and no reverb. When the reverb is not enabled, the amp is quiet and sounds great!! Any suggestions are welcomed and encouraged.

Thanks in advance,
CJ

[Jon Snell]

When you touch pin 8 of V4, do you get hum? if you don't the fault is with that section of the valve, if you do work backwards.
Do you get spring noise from thumping the tanl with your fist? if you do
have you got drive on pin 7 V3?
Have you got signal on pin 2 of V5?

Doing that simple fault check will show you directly where the fault is.

[The Dude]

Also, check input and output resistance of the tank itself. You could have an open transducer or broken wire in the tank.

[Jazz P Bass]

Sounds like the Return is working.
Check the input drive circuit.
Do you have input at V3 pin 7?
Output at pin 6?
Output at the secondary?
You may have a bad transformer.

[cjlectronics]

Thanks for the input. I found the issue. The hum was caused by the footswitch cable. It didn't have the cable shield grounded. Once I tied the cable shield to chassis, the hum magically disappeared. HOORAY!!

The reverb was fixed by a poor ground going to the tank itself... Good call "Dude". So, grounding was an issue on this old amp. Now, the Tremelo isn't working and I'll start with checking all the grounds. Thanks again!!

[drewl]

assuming V4 is good and the trem circuit has the proper voltages, the .01 and .022uf caps are usually the cause of the tremolo oscillator not working.

But check your footswitch first to make sure it's not grounding or turning off the oscillator, you already had a footswitch problem.