B-3 vibrato/chorus low volume and distorted

[tabtech]

My old B-3 is feeling its age. I just noticed that having the chorus/vibrato switches on drops the overall volume considerably and is distorted compared to when it is off. I have a service manual but am not an electronics expert of any scope. After reading through the manual, I tried replacing the vacuum tubes associated with the chorus/vibrato on the pre-amp (V2, V4, and V5) with some spares I had but no improvement. Anyone have ideas what to try next?

[enor]

Most likely a failure of the plate or grid resistor on V2.

[tabtech]

So I replaced the tubes on V2 (6AU6), V4 (12AX7), and V5 (6C4) with other spares but the issue persists. The drop in volume is considerably more than what the Normal/Soft rocker switch does. It is on either manual; not so much distortion as an attenuation of maybe 20 - 30 db. Any resistors or caps might need replacing?

[enor]

Did you read my answer in post #2?

[tabtech]

enor, yes I did read your answer and thank you. As a musician and not an electronics guy, I am trying tube replacement first as I have plenty of those. Identifying the plate and grid resistors on the schematic and matching that to what's under the hood must be the next step.

[kzinkmusic]

A couple things to try, first try swapping the V1 and V2, that is the non-vib and vib tubes,to see if the problem moves to the non-vibrato side. Also,V3 serves to drive the vibrato too ,replace it and go from there.Karl

[enor]

I disagree, don't replace tubes blindly! It's a very expensive and unnecessary excersise, when the problem is almost always the resistors. A multimeter tells the story in a couple of seconds, and at no cost.

[kzinkmusic]

Different skill levels require different troubleshooting methods,if you are good with a meter yes ,check the R6 and R7 on V2 as they do fail. A good 12bh7A will cost 20$ bucks or so.karl

[KC9UDX]

But the old one goes in the trash. That's the expensive part. Resistors are cheap (for now).

[tabtech]

So I found R20 on pin 6 and the stripes matched the 2.2 M ohms on the schematic. Trying to un-solder one end (so I could test it) resulted in the wire being pulled out of the resistor itself; and the other side as well. Once I get that replaced, I'll post my results. My eyes and hands are not what they used to be.

[KC9UDX]

Hands and eyes may not be a problem here. Either that resistor was already falling apart because it had deteriorated inside (very likely!), or, your soldering iron isn't delivering enough heat (a common problem).