Blackface Bandmaster no bass control on channel 2

[Spillrw]

I have been refurbing my 65 Bandmaster and there is no change in the sound when turning the bass control knob using the vibrato channel. It works fine on the normal channel. Any ideas.

[BobbyZ]

Check all the grounds and solder joints. You may get lucky.

[Spillrw]

I checked connections and one leg of a .01 disc cap had melted. I replaced the cap and the bass control worked for about a minute. Any ideas what may have burnt the original cap?

[muchxs]

Disc cap is part of the trem circuit. The "bass" cap would be a .1 film cap. Has the amp been modded? You might have one channel modded to a single tone control or you might have a broken amp.

Pics are mandatory with stuff like this. I can't begin to tell you how many times I've assumed everything is kosher when there's something obviously wrong and readily apparent in a pic. Otherwise,

"Take it to a tech."

[Spillrw]

I thought the .01 was for the tremolo so it didn't make sense the the bass control worked for a short period. Modded.... No unless you count that cigarette burn from 1982. I have owned the amp since 1973 and no one has been inside it until about a month ago. It has performed great for many years except for the tremolo. I never used tremolo so I didn't worry about fixing it. It has lost volume over the years but was still very usable. When the bass control stopped working I decided it was time to investigate. New electrolytics and a good cleaning brought both channels up to traditional volume but the bass control on the tremolo channel and tremolo still do not work. I will post some pictures this evening.

[BobbyZ]

You've had it long enough to know the trem won't work with out the pedal or a shorting jack in the pedel plug.
Worth repeating because a lot of guys new to Fenders don't know that.
(the trem on a Vibro Champ will work with no pedal)

The pics will be helpful.

[Glen W]

Originally Posted by Spillrw

I have been refurbing my 65 Bandmaster and there is no change in the sound when turning the bass control knob using the vibrato channel. It works fine on the normal channel. Any ideas.

No change in the sound... Does the amp sound thin, and you can't add bass? Or are you trying to lower the amount of bass, and can't?

[Spillrw]

I short the tremolo RCA and the volume decreases as I turn up the intensity so I think the circuit is working but the fluctuation is not. I should fix it while I am in there shouldn't I?
I start with all knobs at 0. I let the amp warm up on standby for about 10 minutes, turn it on and gradually add volume, treble and bass. Volume and treble both increase normally but the bass does not increase from 0 so yes, thin is a good description. The normal channel works fine. Great actually. I have recently modded a 2- 12 cabinet to look similar to an original. It has Pile Drivers and the bass knob adds some amazing sounds.

Here are some photos...

[Spillrw]

I did some more testing last night and found something strange to me. I compared VOM reading from the 2 different channels. I was hoping readings should be the same for each bass circuit and if not I would pinpoint the problem. When measuring the resistor soldered to the pot case, Ohms were steady on the normal channel but would fluctuate as I turned the bass knob on the 2 nd channel. Ohms were good when checking the resistor isolated.

[BobbyZ]

The caps with the Bs on them should be changed as well as the big paper one by the power tranny. And the filter caps under the cover.
But that may not fix this bass pot thing. Hows that bass pot look (with the amp off and caps drained) when you check it with the ohms meter? (compared to the other one)
There is a chance that one of the tone or coupling caps (the blue ones) is leaking but thay almost never go bad.
Amp looks great inside by the way !

[Spillrw]

That B label was identifying cap replacement. Meaning a bag of 20ufs in bag B to replace caps labelled B. All caps you mentioned have now been changed and my wife can no longer hear her TV shows.

I did not write down the pot values but I didn't notice any obvious differences. I also performed a quick capacitance check on the tone caps as compared to each other and did not notice anything either. I also seems when I power up the amp, the bass tone works for almost a minute or so then fades away. Is it possible to get a good cap or resistor reading at idle but it fails under power?

Thanks for the compliment. I have tried to take care of it.

[BobbyZ]

I'd bet on a resister.
Takes a bit of time but check all the resisters cold then play it till it acts up and check them again.
(amp off and drained)
Save a little time and check the 100K resisters first then the trem channle ones.

[Spillrw]

I am getting eratic readings from the pot so I think I'll order one and try it.

[Spillrw]

New bass pot installed and tightening up some grounds seemed to do the trick. I only tested for a few minutes but it sounds pretty good. Tremolo hasn't worked for 35 years but it does now. Only works on slow speed (1to5), but it works. Thanks for the help everyone!