Ive got an old multimoog that has been messed around with quite a bit by the previous owner.
Ive downloaded the micro/multimoog service manual and tried for hours to get the tuning procedure down, and its getting pretty close to being in tune but there are still quite a few problems im having, as well as terms i dont really understand that they talk about in the service manual.
First, lets go over stuff i dont understand…
-What the hell is ‘zero beat’ and how do i measure it? (i do have access to an oscilliscope and a pretty accurate digital tuner if either of these help.)
-It looks very much like the previous owner was messing around with the source chip temperature adjustment and keyboard CV trim pots inside the synth.
The CV im not to concerned with, but i really dont understand the procedure for fixing the source temp.
-What is trill trim? Ive messed with it, but i cant hear any audible difference.
-I dont fully understand the relationship of osc range, osc scale, and osc hi for tuning. I think i’ve kinda figured out osc range and scale, but osc hi is a mystery. hearing a fresh explanation of the 3 might make these concepts ‘click’ for me.
one of the biggest problems i am seeing on this is probably related to osc range. When im tuning on LO-F, i seem to end up slightly flat as i go up octaves until by the time i am at HI-F i am a full semitone flat.
Tweaking the osc range seems to help, but i have the trim pot turned all the way and it is still flat. Is this a bad trim pot or am i missing another adjustment somewhere?
Anyways, i’d really appreaciate some advice from someone who’s done this before so i can get this beastie back in tune and do some recording with it.
That’s a BIG problem. Take it to a reputable synth tech. Temperature adjustment is not something for the novice, and no other VCO calibration can be made until this one is set right.
A very good example why analog synth trimpots should NOT be adjusted unless you know what you are doing.
Beating occurs when the oscillators are close to being in tune and you close in on unison tuning between your reference tone and oscillator; when you reach unison the beating will stop (zero beat).
Please don’t take this the wrong way, but the mere fact that you’re asking this question is a pretty good indication that you probably shouldn’t be attempting this procedure.
This trim is not accessable from the back panel (like some of the others are) because it needs to be calibrated by a qualified tech. There probably aren’t even very many techs who are qualified to calibrate it. So, it was a big mistake for the former owner to even touch it, and it will definitely need to be serviced now, if this has effected the overall tuning at all.
The trill trim is for calibrating the LFO. Again, no offense, but why would you mess with it when you don’t know what it does?
According to the manual, only “osc hi” and “osc scale” are interdependent. Unfortunately, because of the damage that has probably already been done, it is likely that your Multi cannot now be calibrated sufficiently from these trims.
The good news is, this damage is most likely entirely reversable if you can find a good tech. But the more you mess with all of these trims, the more damage that someone will eventually have to pay to have fixed. Trimpots only have a limited life, and the more they’re arbitrarily (or even meaningfully) adjusted, the quicker they will fail. In my experience, they are very sensitive and VERY delicate.
This is probably not what you wanted to hear, but from one ambitious user to anaother, I think it’s the best advice.
Ok, i had my local moog specialist have a look at this beastie for me to find out what is happening, and you were partialy correct.
Due to the fact that i couldnt tune it from any of the external trimpots, the multimoog definitely needed to be looked at by a moog specialist, but the tuning problems had nothing to do with the chip temp setting. (which as it tunred out wasnt too far out of whack to begin with.)
The way he explained chip temp is that as long as it is HIGHER than what the maximum expected air temperature would be inside the keyboard then it is set fine. The chip temp is a heater meant to maintain the chip at a higher temperature than the chip would normally heat up to on its own so that room temperature wont throw off your tuning. It is generaly better to set this to a higher value than lower.
Anyways, it wasnt that far off the correct setting but he dealt with it anyways.
The main problem was with some of the resistors within the oscilator boards had begun to deteriorate to the point where they were so far out of calibration that the trim pots were not in range enough to compensate. In other words he had to massage the physical components to get the oscillators back within ‘tuneable’ parameters.
Anyways, multimoog is sounding great now.
The ‘glide’ function is still broken, but that doesnt matter much cause i can control it externaly through my roland MPU-401 and put the pitch-CV through a moogerfooger CP251’s lag processor to give me a far supperior replacement to the multimoog’s OWN glide feature
It would be nice to get the internal glide feature working again, but i can live with it like this for now.
What is wrong with it is that any setting higher than 2 causes the gate to stay up. Very strange… anybody else seen this happen?
