Delay Biasing for Dummies?

[brucer]

Hi all.

Wondering if there's such a thing as general, but effective instructions for biasing an MN3005 delay ... preferably without an oscilloscope?  If you know of something like that, then I'd really appreciate it if you could point me in the right direction.

Asking because I built up electricb's Analog Delay (Maxon AD-900 / https://drive.google.com/file/d/0B8HPHm7M8U3zNTZrSDZnMG5Ub3c/view), but there are no biasing instructions.  He was kind enough to provide some reference documents:

- Instructions for MN3008 version from freestompboxes: https://drive.google.com/file/d/1MME5E1GFaOBb-6xN6fMEdc4ghryU8rEh/view
- Link to diystompboxes thread about MN3008 version https://www.diystompboxes.com/smfforum/index.php?PHPSESSID=39ktmc3jnq3q5k4f6tb482kss6&topic=109774.20
- MB Dirtbag Deluxe BOM: http://www.madbeanpedals.com/projects/Dirtbag/DirtBagDeluxe.pdf

... but I'm not getting the values indicated in the instructions for the MN3008 version pretty much at step 1 (i.e. I turned "Clock" trimmer to 7KHz ±200Hz, but don't get 110 to 140KHz when I turn the Delay pot CCW.  And step 2 requires an oscilloscope ...

Thanks!

PS.  Lame question #2: how do I find the mid-point of a 25 turn trimmer after it's installed on the board?  They just seem to endlessly spin in either direction?   

[alanp]

One dead-lazy way is to audioprobe one of the output pins (or the opamp after the output pins), and adjust bias until you get the biggest possible signal out of it. (If you have a Loophole, or Boss looper, or something, this makes it a lot easier.)

[electricb]

Yes, without a scope you definitely need to use an audio probe to set the bias and balance of each delay chip. The precision trimmers are annoying for sure, but I think it's useful to get an audio probe going and keep turning until you get some signal from the delay line, then dial it in precisely from there.

It sounds like you got the clock frequency right, don't worry about the values when the delay is CCW at the moment.

[brucer]

Thanks for the suggestions guys.  Much appreciated.

Proof I'm a dopey beggar ... it never occurred to me to use my test probe and a looper pedal for biasing. 

With those in play, I think I got the "Clock", "Bias 1" and "Bias 2" trimmers dialed.  Maybe even the "Balance 1", "Balance 2" and "Repeat" trimmers?  No self-oscillation at full CW position on the delay time, mix and repeats pots, but lots of repeats, starting at similar volume to the original signal before fading out.  So that's good, right?

Still, I'm embarrassed to say I have no idea what the "Level" trimmer does since I can't detect an audible change in signal at any test point or at the circuit output no matter how much I tweak it.  I've scrutinized the schematic and it looks like it sets the signal level from one MN3005 to the next (?), but I've very little confidence in my circuit interpretation skills ...

Would any of you familiar with delay circuits ... or maybe even this one ... be able to shed light on the function of the "Level" trimmer and how to set it (link from OP: https://drive.google.com/file/d/0B8HPHm7M8U3zNTZrSDZnMG5Ub3c/view)?

Thanks again!

[alanp]

Proof I'm a dopey beggar ... it never occurred to me to use my test probe and a looper pedal for biasing. 

Yeah, at one point, I had a screwdriver in one hand, my probe in the other, and I was trying to strum my guitar with my foot. You either get mad and give up, or come up with something less stupid.