rebel 30 mk1 head effects loop debugging

F

flakeygeek2000

New member
My Rebel 30 MKI Head had to go to the shop after one of my cats hacked a hairball into it.
The amp got substantially fixed except for the effects loop. I did some experiments and
took some measurements.

Before the cat incident, the fx loop worked well with my Strymon Bigsky, sounded just glassy and wonderful. Now there is added hum.

I did some experiments:

fx send cabled to fx return = good

fx send to Strymon Bigsky in, Bigsky out to FX return (normal effect in loop configuration) = hum added with desired sound

fx send to Strymon Bigsky in, Bigsky out to humbuster to FX return = loud hum

Cable from fx send to input of Strymon Bigsky (nothing plugged into fx return) = no added noise

1/4" jack that shorts signal to ground plugged into fx return = medium level hum

1/4" jack that shorts signal to ground plugged into fx return and shorted to ground of fx send = silence

I took some measurements with a DMM:

fx send gnd is 10 ohms to board gnd

fx return gnd = can't find it tied to anything

I really don't want to take that whole board out to see if the fx return jack is broken or
somehow no longer has a ground connection. Any hints or suggestions that make sense of this
new reality would be appreciated.

Alan
 
I'm not expert. That's an understatement. Safety issues included. That said:

The fact that a direct patch from send to return gives a clean signal suggests to me either an earth loop with externally connected gear or an earth problem in the loop itself. It seems you have indeed found an earth problem in the loop return.
So:
What if the return jack signal earth is independently connected to chassis earth or signal earth (say by a jump lead with croc clip terminators)?
If that works, then you either go in and fix the return jack earth or keep kludging a jump lead (say between send and return earths, such as metal jack bodies).
It no solution in the above:
Is the 'in loop' equipment mains or battery? Sticking to battery removes a variable.
What results with an external line-level signal straight into FX return (say an mp3 player or externally pre-amped guitar)? Start at minus infinity gain on the external device, I suspect the return is post-master volume.
 
The send is raised 10 ohms above ground to reduce loops. The return should be ground. You could probably go in and check if you can see what is broken. Reconnecting the jack to the nearby ground without removing the board may be feasible?
 
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