AB763 Bandmaster Troubleshooting - Fizzy and Ratty

[Jake152436]

Starting to troubleshoot my 1968 Fender Bandmaster. The amp seems to function fine at lower volumes, but when hit around 5 on the volume knob, I start to get this god awful fizzy, ratty, almost vibration like distortion. It almost seems that is independent of the core note/notes I am playing.

The amp has been recapped, the pots have been cleaned, etc and this is still happening. My guess is the issue lies after the preamps as this is happening on both the channels, albeit a little worst on the Vibrato channel.

have attached a gut shot of the amp, voltages as well a sound clip.

So far I have:

-Pulled the entire board to make sure nothing was shorted underneath
-Checked every resistor value
-Double checked and reflowed every solder connection, ground etc.
-Chopsticked all connections - nothing obvious here
-Performed Lead dress

Worth noting:

-The amp has an aftermarket Allen TM40 OT installed
-Mid Pots added to both channels

Going to address a couple of the lead dress issues when I get home later, but the amp is still behaving improperly.
Not sure where to go from here!

Audio link: https://soundcloud.com/user-47032081...sharing

[The Dude]

Since the OT has been replaced, Can you post specs or links to specs for it?

[g1]

when hit around 5 on the volume knob, I start to get this god awful fizzy, ratty, almost vibration like distortion. It almost seems that is independent of the core note/notes I am playing.
...
-The amp has an aftermarket Allen TM40 OT installed

Incorrect OT phasing can cause oscillation that will sometimes result in the symptoms you are hearing.