Fender Custom Shop dual professional hum

[Opry Audio]

Hey everyone,

I have a fender dual professional on my bench that has just been fully recapped and worked on by another tech. The work looks good, but he’s not local so it was brought to me. I am still in the process of moving my shop so I don’t have access to all of my tools like my scope and a comfortable workspace at the moment. I took it in for a friend so apologies if I do not provide very specific info about the fault conditions.

The amp is relatively quiet but there is hum coming from the reverb circuit. When tapping what is C302 and C301 on the reverb tone control pot. Those 2 capacitors are extremely microphonic. The conditions are the same with the reverb tank attached and detached. The hum is 60hz. Is my best bet to clip in other caps and see if the condition changes? Are there other aspects to this circuit that I may not be considering? When the “mix” knob for the reverb is completely off the hum goes away. Thanks so much in advance for all of your help!

[72Blazer]

Why don't you post the schematic so the Pros on here can help you/don't have to look it up.
 
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[Dave]

Reverb recovery stage cathode ground loop would be my guess. I always suspect that connection on any Fender with reverb that has such symptoms. I suggest lifting the resistor and cathode bypass cap at the ground connection and try grounding them someplace else. Alligator clips come in handy here.


Dave

[Opry Audio]

Sorry guys! I meant to post the schematic here but forgot. Here it is https://schematicheaven.net/fenderamps/dual_professional.pdf

[Latole]

Sorry guys! I meant to post the schematic here but forgot. Here it is https://schematicheaven.net/fenderamps/dual_professional.pdf

You must wait to have your scope

[Dave]

Any luck?


Dave

[shooter]

The hum is 60hz

indicates coupling to sensitive signals, check for solid grounds at the jacks, input circuit for the RB, shielding...

[AlNewman]

Maybe just a good cleaning and work all the switches and relays will help?

[Opry Audio]

Any luck?


Dave

No luck as of yet. I gave the pots and relays a good cleaning but no change. There is a significant increase in the level of hum when switching channel B from FAT mode to normal mode. And an additional increase in hum when turning up the “mix” knob for the reverb. I see that the balanced cables are grounded to the chassis via the pots and not on the bus bar. There’s a chance that it’s a ground loop that’s being sent into the reverb circuit and the rest of the amp from the input stage. Suggestions for alternate grounding? Im a bit confused because I have to assume it wasn’t always like this. The ground scheme wasn’t changed but this hum came up after it was recapped by another tech.

[Opry Audio]

Thanks again guys and sorry it took so long to reply to the thread