Fender Blues Jr. fat switch poppinˋ

[Zouto]

Hi, the fat switching circuit on a Fender Blues Jr., cream Pcb., 2002, creates loud popping noise when switching by panel switch or by footswitch.
Any ideas how to fix it?
Thanks for help and hints
Zouto

http://schems.com/bmampscom/fender/B...e%20Manual.pdf
scroll down to 21-March-01 RevC manual

[nickb]

My first suspect would be that either C3 is leaking or R9 is open (or bad connection to R9). Also note there is an error in the schematic in the the bottom end or R9 is connected to ground. So with the amp on and FAT off, first check for 100K ohms or so between the drain of Q1 and ground. Repeat with the meter leads swapped to show if leakage current from C3 is affecting readings.

[Enzo]

Is R9 open? Is Q1 leaking DC?

[Zouto]

My first suspect would be that either C3 is leaking or R9 is open (or bad connection to R9). Also note there is an error in the schematic in the the bottom end or R9 is connected to ground. So with the amp on and FAT off, first check for 100K ohms or so between the drain of Q1 and ground. Repeat with the meter leads swapped to show if leakage current from C3 is affecting readings.

Thanks Nick, with amp on and Fat off, I get 1M2 between drain of Q1 and ground, with swapped meter leads 1M7. Since C3 is the Bright Cap on Volume Pot, it is not clear to me, how it should interact with the Fat switching circuit. What´s the best way of checking leakage of Q1?

[nickb]

Thanks Nick, with amp on and Fat off, I get 1M2 between drain of Q1 and ground, with swapped meter leads 1M7. Since C3 is the Bright Cap on Volume Pot, it is not clear to me, how it should interact with the Fat switching circuit. What´s the best way of checking leakage of Q1?

Look at page 7. C3 is on the FAT cct. The version on page 11 shows that cap as C4. Your readings show R9 is open.

[Zouto]

Look at page 7. C3 is on the FAT cct. The version on page 11 shows that cap as C4. Your readings show R9 is open.

Thanks a lot, I‘ll swap R9, probably C4 too to fix the circuit, version p.11.

[Zouto]

Culprit found: swapping open R9 fixed it, no popping anymore.
Thanks for help
Zouto