Diagnosing a Fender bass amp problem

[Den.]

I'm looking for ideas. My friend's Fender Rumble 100 was working one day and the next day it wasn't. I told him that I would open up the amp and look for something obvious but I didn't see anything suspicious. On the normal channel, with the Gain and Master full up, it barely passes the sound of the guitar. On the Overdrive channel it is extremely noisy; so much so that it can't be used. And, as one starts to push that lower "vintage" tone button you can hear that loud beep tone. Plugging into the Effects Return jack yields a guitar sound that's fixed; a signal that isn't affected by adjusting the Gain, Volume or tone controls. I'll try to post links to the scheme and a short video.
Video
Schematic

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[Tassieviking]

When you plugged into the effects return with the guitar did it sound nice and clean ?
When you plug straight into the return jack you are bypassing the whole preamp and all the controls associated with the preamp.
A preamp boosts the guitar signal up a lot for the power amplifier, that's why a guitar straight in to the return is not very loud.
Do you have another amp you can use ?
Plug the send of another amp into the return  on the Fender and you will be using the preamp of the other amp and the power amp of the Fender.(guitar into other amp)
If that works then the Fender power amp is fine.
Try an overdrive pedal straight into the return and you are bypassing the preamp again.
You have to try to determine if the problem is the preamp or power amp.
Preamp is guitar input on front to the send jack on back.
Power amp is return jack to speaker out.

[Den.]

When you plugged into the effects return with the guitar did it sound nice and clean ?
Yes, it did.
Do you have another amp you can use?
No. None of my three amps have an effects loop, but I'll ask around for a loaner.
Try an overdrive pedal straight into the return and you are bypassing the preamp again.
My friend will loan me his Boss Dist. pedal.
You have to try to determine if the problem is the preamp or power amp.
Preamp is guitar input on front to the send jack on back.
Power amp is return jack to speaker out.
Thanks, I'll report back.

[Den.]

Today I hooked up the borrowed Boss distortion pedal between a guitar and the RETURN jack on the amp and the distorted sound of the guitar was present. Of course all of the amps front panel controls were inoperative. I also tried plugging it into the front guitar input  jack again with the same results as reported in my first message. Does this indicate the problem is in the preamp section?

[Jazz P Bass]

It appears to be a power amp issue.

[g1]

I think there may be some confusion now about the 'distorted sound'.
You are getting the proper sound of the Boss pedal using the FX return?

[Den.]

I think there may be some confusion now about the 'distorted sound'.
You are getting the proper sound of the Boss pedal using the FX return?

Yes, I am getting the proper DS-1 distorted sound as expected from that pedal.
Also, I neglected to mention that I had also tried a patch cord between the Send and Return jacks with no improvement.

[Tassieviking]

I would say the problem is with the Pre-amp, the reason I asked you to try the pedal is to see if you got the sound you expected.
I think the best way to go forward would be to inject a clean signal and check the test-points with an oscilloscope.
Being a SMD build it gets a bit harder to check and replace components.

[Den.]

I think the best way to go forward would be to inject a clean signal and check the test-points with an oscilloscope.
Being a SMD build it gets a bit harder to check and replace components.

Thanks!

[joecool85]

I recommend making sure that the input jack is clean and connected well to both line and ground.  If not, it could be only letting some signal through causing these issues.

[Den.]

I recommend making sure that the input jack is clean and connected well to both line and ground.  If not, it could be only letting some signal through causing these issues.

I will, thanks.