Mu-Tron Bi-Phase mod help - pedal rate range too high - internal trim control?

Started by bholder, March 02, 2023, 04:29:56 PM

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bholder

Hi, I finally have a Mu-Tron C-100 expression pedal to go with the Bi-Phase, and everything seems to work as intended, but pedal control of the Sweep Generator A sweep rate is in far too high a range to be useful to me - it goes from "way too fast" up to "FM bird chirp" territory, and I want slow variable swooshies.

Is there anyone out there who can help me decipher the schematic to figure out what section implements the pedal rate control and adjust the rate range way downwards?  I used to be pretty good with schematics, but that was in college 40 years back sigh (used to do the Craig Anderton stuff for fun). 

bholder

There's an "LFO Low Speed Adj" trimpot at R12b 10K that looks very promising (following from the pedal input jack on the lower left, towards the right:




Now that I look closer, though, that looks like it's on the "Man" or manual leg of the manual / pedal switch, so that's not it.  Following the other Pedal leg of that switch leads down to a couple of transistors I don't understand, but then further down there's another trimpot - labelled R25 10K Pedal Offset Adj - that looks like it? 

Any Bi-Phase experts out there?

bholder

Well I found and tweaked R125 (not R25) all the way down to its lowest setting - too far - no the regular sweep is inaudible.  I hope I can find the "sweet spot" without too many more trial and error tries.  I can't figure an easy way to check the operation without taking the 15 minutes or so to put all the pots and screws and knobs back on.