Electric Mistress issue... "Color" knob doesn't work

[kvandekrol]

I'm trying to debug an Electric Mistress... This one follows the GGG schematic exactly:

http://www.generalguitargadgets.com/diagrams/dmistsc.gif

It flanges, and the Rate and Range controls work, as does the Filter Matrix - but the Color control doesn't have any effect. I traced it with an audio probe and it's passing signal through, it just doesn't seem to be getting where it's supposed to go?

Let me know what voltages I could post to help out... this thing has about 50 IC pins in total, so I didn't want to post all of them straight up! :) Where should I be looking? What failure points are nearby that could be causing this? (All the electrolytics are brand new.)

[PRR]

Does Feedback Trim do anything?

Do you have signal at Feedback Trim pot lugs? At Color pot lugs?

[kvandekrol]

Yep, I do have signal at the lugs of the Color knob and the Feedback trim. FB trim is set at around 12k and moving it up or down changes the signal strength, but even at the lowest resistance I still don't get any effect from turning the Color knob.

[Thomeeque]

Do you have signal at pin 3 of IC1? Does volume of this signal change when turning Color pot?

[kvandekrol]

Yes and yes. The color knob affects the signal on pin 3 of the 741, but not the signal on pin 6.

[Thomeeque]

Yes and yes. The color knob affects the signal on pin 3 of the 741, but not the signal on pin 6.

Wow, that is really weird I'd say - try to replace IC1, maybe it is defective in some very unusual way.. Or maybe yet try to measure DC voltages at IC1's pin 3 and 6 first..

[kvandekrol]

Amazing - that was it. Thank you so much! I dropped in a TL071 and it sounds beautiful.

I've been building, repairing and modding for only about a year now and haven't yet experienced a bad opamp, so I sort of subconsciously believed opamps were infallible... it probably would have taken me awhile to find that one on my own.

[Thomeeque]

Amazing - that was it. Thank you so much! I dropped in a TL071 and it sounds beautiful.

I've been building, repairing and modding for only about a year now and haven't yet experienced a bad opamp, so I sort of subconsciously believed opamps were infallible... it probably would have taken me awhile to find that one on my own.

Well, making functional (besides defective 741, which may not be your fault) Deluxe Electric Mistress only after one year of experiences, that's pretty impressive, hats off

Congratz, T.