Blues Junior question

[Doc Stitches]

Fat switch is not working. I'll pull the chassis later today.

Any ideas?

Thanks
Doc

[Jazz P Bass]

Fender Blues Junior Fat Switch

Here is the schematic.
First thing I would check is TP 2 voltage.
Then Q1.

[Enzo]

Without even opening the amp, I'd stick a common footswitch into the FAT SWITCH footswitch jack and see if that works. The cutout contacts in that jack are in series with the switch. If the footswitch works it, then either the FS jack is faulty or the panel switch is, or the wiring between them.

Test point 2? Plug a cord into that same jack and measure voltage at the tip of the other end plug. Got about -14v? Grounding that tip is all the FS does anyway.

My vote is jack, switch, wiring, and then electronic parts.

Of course this was an election year, so votes don;t always work out like you want them to.

[Twin]

Enzo - We bought a footswitch for it(Fender, mono, no LED) & the FAT(clean boost) does engage using the FS, however if the panel switch is actuated & the FS is engaged the amp squeals at an alarming level! This is intensified with an increase of the channel volume anywhere past 12:00.
Measured voltage at the tip of the FS & got -14.5v, but when the FAT panel switch is depressed the meter goes to 0? Now I can't seem to get the tip to read -14.5v in any combination? Can I disable the panel switch & just keep the FS?

Thanks for any help with this.

[52 Bill]

The footswitch jack is supposed to cut off the panel switch whenever a plug is inserted into it. If the panel switch is causing something else to happen then the switch function of the jack is not working. Inspect the footswitch jack and wiring and see if the circuit breaks when a plug is inserted.

[Twin]

Good stuff here 52 Bill. As of now the FS jack is NOT deactivating the panel switch... So maybe I have a shorted jack? I double checked the wiring & solder points even after I reflowed the joints & that still doesn't cut out the switch... I don't really need the panel switch since I don't fiddle with the amp much when playing, but the FS is a must have for the boost. Is there a way I can eliminate the panel switch from the circuit without having to pull the PCB? If so, which leads need to be focused on?
Thanks!

[52 Bill]

There is a normally closed switch contact on the footswitch jack. If you look at the jack you can see how it works. Check the jack and the wires going to it and to the small jack board as well.

I think that it will be easier if you fix the amp correctly than to try and modify it to make it work.

[Twin]

Sorry, I was wrong... At a second look, plugging in the foot switch does indeed disable the panel switch... Also the FS jack is functioning as it should as well as the panel switch. However, anytime the FAT circuit is engaged(either via panel switch or FS) there is a notable malfunction when the Volume pot is dialed up past 2 - there is a "clunk" in the speaker & the amplified volume is reduced(more than half, perceivably) & there is also a transient static noise that increases the further the Volume knob is dialed up. When the knob gets to about 12:00 a high pitched oscillation sets in & the further up the pot is cranked the more audible/worse the oscillation gets.