[sdiy] uA726 temp setting

[Kevin Lightner]

Can anyone help out on this problem?

I have a Sonic Six in for service.
Nothing new here, but generally I don't replace the ua726.
In this case, the unit came from another service center and needed one.
They also played with the heater trimmer and I have no idea where it 
should be set.
With that said, I replaced it.
Normally, if I replace the trimmer, I unhook a leg and measure the 
setting, set a multiturn cermet to the same value and reinstall. Moog 
published no spec in the service manual on how to set the 726 temp 
though. It's not a "set to 55mv" or "measure the room temp and apply 
this formula" type thing- they only say that it can be used as a 
rough scaling control, otherwise shouldn't be messed with and that if 
it IS changed, only make tiny changes.

Now here's the thing.
Most ua726 designs use one per oscillator as part of their expo converter.
The Sonic Six uses two transistors separately, one for each oscillator.
The spec that Fairchild published shows for the heater pin (6) is a 
62K resistor to ground for the mil-spec version and 75K to ground for 
the commercial version, same as what the Minimoog later osc boards 
use.
Roland Sys700 vcos use a 100K.
According to the Minimoog manual, a 75K equates to a 78.4C heat 
(173.1F) . Quite toasty.
But the Sonic Six has a 220K trimmer in series with a 100K resistor, 
or 100K to 320K of range.
Apparently running it cooler than others. Weird.

So, does anyone have any idea what this should be set to?
I don't care how it's expressed: resistance, voltage or current.
One would guess the setting to be somewhere 150K or over, or they 
would have installed a 100K trimmer (or expect 726 tolerances to be 
very different from each other.)

Any clues would be nice.
Getting a Sonic Six to track well is hard enough when the temp IS set 
correctly, but it's almost impossible if it's off.

Thanks.
-- 

Kevin Lightner

Myself: http://www.synthfool.com
Service and sales: http://www.moogmusic.com/service.php
Sales: http://www.cluboftheknobs.com