Volume fluctuating on 68 Bandmaster

[scotto]

Guys,
Plays great for a while and then huge bump in volume with distortion. Been recapped a while back. Only seeing it on normal channel. Changed v1 and plate resistors. Still there. Any ideas?
Thanks!

[pompeiisneaks]

A sudden increase in volume to me seems to hint at potentially something bypassing hte tone stack there. I.e. the 100k slope resistor maybe heating up and failing open until it cools down again? Does the volume pot impact it? (I mean make it stop doing that?) Maybe the volume pot is also a potential area if there's no ground reference it may suddenly dump all signal across the bright cap (if engaged?, which is also easy to figure out, if it's in that state try disabling the normal channel bright switch and if you lose all volume or it's really weak, then the volume pot may be having intermittent issues with the wiper contact.

Just a couple areas to check... have you tried swapping the actual tube and/or cleaning the tube socket etc?

~Phil

[Stevem]

Sounds like its temperature related so the first thing to do is to reflow all the solder connections in relation to the function of that 2nd channel.
Trying to find a bad connection due to a thermal issue with the chassis out of the cabinet can be impossible at times, (but adding heat from a hair dryer can help) so just bite the bullet and reflow all the connections, it takes all of 10 minutes if that!

Also go half way down the board to where the 2 channel output wires that rap around each other, go to the two 220K ohm mixer resistors and feed the phase inverter stage, as a bad solder joint just there can make for that issue.

After these things it's time to hook up a volt meter and see if you have one out of the two 100k ohm plate load resistors failing in the normal channel.

In thinking this over again I think your issue may just be a bad solder connection on the Cathode bypass cap ( 25 uf@25 volts) or the Cathode resistor ( 1500 ohm ) that's off of pin 3 on that first preamp tube on the right as you view then from the rear of the chassis.

[pdf64]

More detail needed, eg do all controls still work as normal when it’s in the fault mode?
Reflowing all solder joints may be beneficial but 10 mins seems somewhat optimistic? Especially as a big Bertha may well be needed for those to the brass plate.

[Stevem]

In the 100s of Fender amps I have worked on that have the brass grounding plate under the pots I have never found I Bad solder connection going to them, but I have found pots that where not bolted down tight to them which made for a bad ground!

Other then the pots there are only 15 components that make up the normal channel, so that 30 points to be reflowed , certainly that can be done in 10 minutes with a iron of 35 watts or better!

[scotto]

It turned out to be a loose tube socket. Tightened up and no more problem.
thanks all!

[Stevem]

Even though the China Sino tube factory burnt down a few months ago you still need to whatch out for these preamp tubes as many times I have found that they have undersized pins and if your tube sockets are on the edge of being too loose with other make tubes, the China made tube will be the straw that snaps the Camel's back so to speak.

Good to hear it was a easy fix!