Troubleshooting Boss TW-1 T Wah

[aviherman5]

Hi all!

I'm having a bit of trouble with my Boss TW-1 T Wah.

The peak knob works fine, but the sensitivity knob and toggle switch does nothing at all. The pot reads fine (42k but should be 50k) out of circuit and the trim pot only raises and lowers the frequency.

It seems my envelope is non existent 

It is the 14 pin IC model, and the schematic is as follows


Any ideas for how to move forward?

Thanks so much!

[Slowpoke101]

The best way forward is to take some voltage measurements on the IC pins and post them back here. Fault finding these old pedals can be a bit tricky but not too difficult.

[bartimaeus]

ideally you could scope the output of the envelope follower section to see what it's doing.

it's a good sign that the Trim pot is working, that means the problem is somewhere downstream.

the sensitivity knob controls the input to the follower, and it's connected right after the input so it's probably working.

the Drive switch is after the envelope follower. so if the drive switch isn't working, then the problem is probably between the Drive switch and the Trim pot.

first, i'd use continuity test to see whether the Drive switch works (with power disconnected). if the switch is broken and doesn't connect to either side, then it makes sense that the pedal isn't working.

next, i would put the pot back in circuit, then check the IC and resistors directly after the Drive switch.

[aviherman5]

Hi,

Thanks for the suggestions. The switch has continuity and is working properly and agree that the problem lies downstream.

Will post IC voltages when I get home!

[bartimaeus]

if the drive switch works, then i'd bet it's the 1uF tantalum cap. a quick recap will fix a lot of 70s/80s pedals