Mellotronists

Power supply stability certainly does assure that tuning is stable as well. The big Moog had rather dirty DC due to the large distribution harness, so ‘lytics spread about the harness terminal strips cleaned things up greatly.

The first and second incarnations of the mini VCO card had tempco resistors in the exponential converters to keep the scale (volts per octave) from changing grossly with temperature. Without tempcos, the unit wouldn’t be practical (the reason that the Moog 951 keyboard has a “Scale” knob, for the old 901’s). But other parts of the circuit are also affected by temperature so until everything evens out over a few minutes, there will be some pitch drift. The third incarnation of the mini VCO used ua726 transistor pairs for the exponential converters, which have an internal heater that takes the chip die up over the ambient temp and stays there. They come up to temp pretty quickly. The rest of the parts are tighter spec’d than the early cards also so typically a mini with s/n 10,000 or higher will be in tune within a matter of seconds and stay there. You can tell the incarnation by the bend/mod wheels and front panel:

First version – clear wheels, metal front panel, small lettering, full markings around all knobs

Second version – smooth white wheels, plastic front panel, larger lettering, partial markings around all knobs

Third version – ribbed white wheels, plastic front panel, larger lettering, partial markings around all knobs

The PSU’s are the same on all, but the PSU caps deteriorate over time so all units will vary depending on this. I’ve taken a PSU out of a stable unit and moved it to a less stable unit, the the stability followed the PSU. The difference was not so much the tuning drift, but rather the purity of the VCO tone (the amount of tiny warble). I suspect the amount of drift is due to loose tolerances in the VCO components. Old resistors and caps have higher temperature susceptibility than newer parts. Carbon resistors are bad, metal films are good, and newer machines had better resistors as the suppliers improved their parts. But minis are prone to board swapping, so many machines have a mix of old and new.

The Micro and Multimoogs used CA3046 transistor arrays for the exponential converters, and spare trannys in the chip are used as heaters for the same effect. However it takes longer for these units to come up to temp than the 726-equipped minis, something like five minutes. After that they’re dead on. I think the Moog 921’s used the same trick.

But back to Mellotrons… let’s see, need to find something relevant to the list… OK, I just got back #1213 which was on loan to Ryo Okumoto for the new Spock’s Beard album (along with my Pro Soloist). Tapes used were Ian flutes, Sad Strings, and Russian Choir. He told me that he used the Pro Solost all over the place too.