Deluxe Memory Man not self oscillating... which trimpot controls feedback???

[scott_v]

I just got back my 90's Deluxe Memory Man (it has the built in power trannie and 3-prong plug) from the repair shop, the tech replaced the monostable vibrator IC, resoldered some bad solder joints but then also went ahead and adjusted the trimpots for "optimal performance".  Wish he didn't do this last part (or to this extreme) as in doing this it seems that he adjusted it so that the feedback pot when fully clock-wise no longer allows the unit to go into self oscillation, it just comes so close... its one of those things that i love about this pedal being able to do.

Can someone familiar with these older DMMs tell me which of the 5 trimpots controls the feedback? from another thread i learned that it is a 100K trimpot that controls feedback??? I would like to avoid lifting the entire board out of the enclosure, as the trimpot adjustments can be made right on the bottom, but there are no identifiers on that side...  I know its best to use a scope for this work but think I can do this one adjustment by ear if its just the one trimpot that needs to be tweaked (I read that it could be another that also controls the delay gain?), i just don't want to start guessing on these one by one if i really don't have to, and marking the original settings as i know how sensitive all these trimpots in these things are.  I would take it back to the technician but if its simple to do myself, and rather save the hassle... i do have some basic knowledge around simple circuits.

i posted this at the ehx forum also but thought i'd get some input here first, its abit slow there...

attached is a board shot from a very similiar DMM (location of all the trims are the same in mine)
http://home.earthlink.net/~drasp/dmm_board_back.jpg

[rustypinto]

None of the five trims directly control the feedback amount. However, a properly adjusted DMM will self-oscillate, no problem. There are two "gain" trims; they adjust the varying gain levels that BBDs would (or wouldn't) produce. These allow you to either boost or attenuate the signal coming out of each BBD (usually the later because BBDs have a problem with clipping). And the last key point is that the higher the amplitude of the signal going through the delay line, the more repeats you will get from the feedback loop.

Can't say for sure, but i would ask your tech if the 'optimal' adjustment was dropping the gain through the delay line to keep distortion to a minimum (and ask him if he used a signal generator/ oscilloscope or not). If you do it TOO much, you're just sacrificing signal level (and as you found out, the amount of repeats). You have to realize that DMM's are quite prone to clipping, and thats what the external GAIN control is there for.

If you want to bump up the internal gain yourself for the self-oscillation thing, very gradually turn each one, alternating between the two. And sorry, i can't remember which direction is boost, and which is attenuate, just mark where they are set before you start turning. Pluck a signal through the unit while you adjust, and it should start to self-oscillate like you want.

From your picture, here is the order of the trims (from left to right): post-Gain (BBD2), post-Gain (BBD1), bias (BBD2), balance, bias (BBD1)

Also, be very careful around that power switch in the upper right-corner of the box, thats got 117VAC!

[scott_v]

Thanks Rusty Pinto this should help me on what to carefully adjust and mark the position. I've also noticed that the signal is perceived to be "weaker" the level and blend pots need to almost be 3/4 CW and I compared with this unit with another Deluxe Memory Man (an older 80's version w/ built in power and 2-prong plug) that I have and that one has a strong output "level" and "blend" as the level of those pots don't need to nearly be as high. 

This one that was just repaired also does something weird, i tried adjusting the "blend" pot to a good level and it sounds like its also sucking the life out of the input signal "level". So what you've stated and recommended makes sense, boosting and attenuating the gain trimpots for the BBDs.

I'll be careful of that power!

[scott_v]

OK I did an adjustment , an ever so slight one on the 2nd trim pot from the left (post-gain BBD1) and that did the trick! it only took a miniscule turn CW to make it start to oscillate with the feedback pot turn about 90% its rotation.  So i just did abit more of a turn (maybe an 1/8 of rotation) on that same trimpot and now it does that nice runaway oscillation at a little over 75% of the rotation of the feedback pot.  It also sounds like it recovered some of the delay gain on the blend pot and now the volume change from switching to the bypass to effect sounds more better.  I also tried to slightly adjust the 1st trim pot on the left (post-gain BBD2) and I didn't hear anything different so i left it where it was adjusted by the tech who repaired it... its not "perfect" as i think on the 3 different deluxe memory mans that i've had in my life, all of them pretty much self-oscillated at just about 60% of the rotation of the feedback pot, but i kinda like this better now as i have greater control on the delay repeats as its using more of the pot rotation before the runaway oscillation starts...

thanks again!

[scott_v]

oops double post.