A little help on troubleshooting an AW-2 Auto Wah?

[mort]

schematic can be found here: BOSS AUTOWAH AW-2 Service Manual free download, schematics, eeprom, repair info for electronics

The symptom is the pedal is generating no effect. The pedal passes guitar signal in bypass and when engaged, though there is a 'tonal difference' between the two. None of the controls have any effect on the signal.

Can someone give me a hint on where to start with it?

I've got it out of its shell, and I'm assuming I will need to jumper the output jack's ground lug to a ground in the circuit since it doesn't have the shell to ground to...


Edit: After loosening and re-tightening the jack and pot mounting hardware, the unit does pass some effect. The effect doesn't seem as pronounced as it should be. The rate and depth controls seem to be working, but the "manual" and "sens" controls only seem to slightly alter a wee bit of treble.

[Mark Hammer]

First, verify that the wah itself works.

Tack some wires to the pads where R50 and C9 are connected to IC1b. The wah changes frequency as the value of the photocell in parallel with those components changes resistance. If you wire up a 500k pot in parallel with that photocell, resistor, and cap, you can mimic what happens when the photocell is made to change resistance. If making the resistance get bigger and smaller produces an appropriate wah sweep, then you know the problem is NOT with the wah section itself.

Assuming the wah part is fine, the question then becomes why you can't get much sweep. You will note that the manual sweep, the envelope portion, and the LFO all go through a common pathway of IC3b and R26, which then feed Q7/Q6, and in turn makes the LED get brighter and dimmer.

BUT, you will also note that R27/R28/RT1 provides a bias voltage to Q7, which the envelope, manual, and LFO add to. If the bias is set wrong, then the LED may not display the appropriate changes in illumination, and the photocell also not have the appropriate resistance changes that make it go wah.

It is possible the trimpot has been knocked/jarred away from its original setting. To adjust it by ear, you would want to set the depth to minimum, and manual to minimum, and the sensitivity reasonably high. Then, as you strum, adjust the trimpot until you hear a nice sweep.

[mort]

It is possible the trimpot has been knocked/jarred away from its original setting. To adjust it by ear, you would want to set the depth to minimum, and manual to minimum, and the sensitivity reasonably high. Then, as you strum, adjust the trimpot until you hear a nice sweep.

That was it. Nice full effect with the trim pot dialed back a little from where it was set. I played with a few settings and found a nice spot. Thanks a bunch.

[Mark Hammer]

Excellent! Thanks for the followup. It helps to shape future advice to others.