Calibrating Trimpots on Mutron Biphase Clone

[seten]

Just finished up my biphase clone schem here: https://www.pedalpcb.com/product/duophase/

I'm trying to calibrate the trimpots for the widest sweep possible and attempting to understand what exactly theyre doing to the circuit. It seems that Trimpot 1 controls how dim the LED is at its dimmest, counterclockwise it is almost completely off at the bottom of the sweep. It does not seem to affect how bright it is at its brightest on side B, but on side A it does - not only does turning it clockwise make the low lower, but when the sweep is at its fullest and i quickly crank it clockwise the led definitely gets brighter. Could that be a result of tolerance between the two trimpots? Seems like it shouldnt be behaving that differently in the two identical cirucits.

Second, trimpot 2 seems to me that its controlling how bright the LED is at its brightest. Fully counterclockwise yields the brightest peak, which doesnt make sense to me because in that position there is the least amount of resistance between the LED and R45 which is then connected to ground. Why is that? I would think that the more signal leaking to ground the dimmer the LED would be.

[PMowdes]

The pedal PCB version isn't the same as the original unit,  I think it is just a pair of mutron phasers on one PCB.

The calibration procedure for the original unit can be found here

https://drive.google.com/file/d/1WDIcJKwfED5GuEUQh-Zo87qZaRinuMxV/view?usp=drivesdk

Whether it gives you much help is another matter

[digi2t]

I would find that a bit aggravating to have a build doc for a build with trimmers, and no info as to what they do, or in this case, tuning instructions.

To be fair, I know first hand how difficult it is to write these guides up. Every build we've done to date that requires trimming or tuning, I've written up guides for. But, the worst part is trying to write them in a clear and concise way, that no one will misunderstand. Regardless, there's always one or two that come up with questions which undermine all the effort you put into writing the doc. OK, OK... I was kidding on the "undermine" angle. In reality, it only helps sharpen up the doc, but when you've spent 8 hours writing it up, it can feel like a thorn on a rose.

So... in a nutshell;
- The two 1K trimmers set the waveform of the LED's. Using the guide that PMow linked to, use an O'scope to view the waveform, and to trim out any flat spot on the peaks. You can do this with the LDR's exposed, since you're defining how the LED's slope will operate. In the DEFX tuning guide, refer to "Voltage calibration step" - steps 5 and 6. Q41/POT42 and Q51/POT52 pertain to the transistor/1K trimmer pairs on the Lola, so translate accordingly to the Duo-Phase.

- The two 10K trimmers set the phase sweep center. This needs to be done with the LED/LDR's covered, or in total darkness. In the Lola guide, this is covered in the "Phase center calibration steps". While using your ears will get you roughly close, again, an O'scope is the desired tool if perfection is the quest. In the Lola guide, POT41 and POT51 pertain to these trimmers, so translate accordingly.

Since there are less trimmers in the Duo-Phase, disregard the other trimmers mentioned in the Lola guide.

MOST IMPORTANTLY!! - Be sure to refer to the Lola's "Calibration configuration" before adjusting anything. The controls structure seems to be identical between the Lola and the Duo-Phase, so make sure that you set the Duo-Phase as indicated in the Lola's guide before twisting trimmers.

I hope this helps.

[Rob Strand]

MOST IMPORTANTLY!! - Be sure to refer to the Lola's "Calibration configuration" before adjusting anything. The controls structure seems to be identical between the Lola and the Duo-Phase, so make sure that you set the Duo-Phase as indicated in the Lola's guide before twisting trimmers.

That's a really cool document.    No doubt a sheeeet load of work.

[digi2t]

MOST IMPORTANTLY!! - Be sure to refer to the Lola's "Calibration configuration" before adjusting anything. The controls structure seems to be identical between the Lola and the Duo-Phase, so make sure that you set the Duo-Phase as indicated in the Lola's guide before twisting trimmers.

That's a really cool document.    No doubt a sheeeet load of work.

Yeah, but a lot of credit goes to Mike Both. His input was instrumental in helping decipher the existing tuning information. Not to mention his sharing information on component selections. He made putting that doc together quite a bit easier.

[Rob Strand]

Yeah, but a lot of credit goes to Mike Both. His input was instrumental in helping decipher the existing tuning information. Not to mention his sharing information on component selections. He made putting that doc together quite a bit easier.

Believe me, I appreciate the efforts. These things don't happen without a lot of people pitching in and the information is hard to come by.

Deciphering the existing set-ups is a tedious job.  Every time I see one of those vintage pedal repair videos where someone just turns the trimpots makes me cringe.   As time goes on less and less units are actually set-up to the factory specs.  Like you mentioned before, you can only go so far by ear and often it's not what the real thing was. 

For some pedals I've actually analyzed the audio from youtube videos to back-engineer the set-up.  Sometimes finding large differences between units.