[sdiy] uA726 temp setting

[Peter Grenader]

Oddly enough, the726 datasheet doesn't mention anything about how to
translate heater drive at the temp adjust pin (which governs the internal
current amp Q3) to actual temp.

if it's any help, Don Buchla had that pin tied to ground through 82K ohms
with a part powered form +/-15.  A suggestion:  why don't you have a look at
the minimoog schematic that used that part - did they have a trimmer or a
fixed resistor?  If it was a trimmer, do they specify a setting?  I suspect
not...

The key of course is not how hot it is, just that it's set so that ambient
temperature of the operating environment has little effect on the trannies
pair in the 726 and this is done by elevating the internal temp of the 726
to above the room temp.  So it makes sense that Moog opted to slap a trimmer
in there as obviously . You'll know if it's not hot enough because it's not
going to track - so possibly (?) screwing around till you find that elbow is
the key. 

- P


Kevin Lightner wrote:

> 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.