Minimoog old style VCO

[Magnus Danielson]

From: Rotwang the Mad Inventor <polarn-p at acc.umu.se>
Subject: Minimoog old style VCO
Date: Thu, 28 Dec 2000 21:28:44 +0100 (MET)

> Hello!

Hi there!

> Recently, I bought a Mini-Moog cheap. Very cheap. The catch; it need
> service and the VCOs are of the old, less stable kind. Anyone of you
> Synth-DIY:ers have a pre-#10175 (or whatever that magic number
> was) Mini-Moog?

That's the magic number allright. Did you forget to bring the Minimoog
service manual with you?

> I have dismantled my Mini, and found that the VCOs are of a traditional
> relaxation-oscillator type, with those notorious tempcos in the CV summer
> feedback. My Mini was drifting wildly, and I found out that someone had
> replaced two of three tempcos with ordinary 1k resistors! :(
> Then I noticed the poor construction/layout of these early boards; the
> tempcos and the exponentiator transistor-pair (CA3046) are not thermally
> coupled, and they are not even close to each other on the board! No wonder
> these VCOs had temperature stability problems! (shame on you, Bob! ;) Now,
> my idea is to get myself new tempcos, and rearrange things a
> little; placing the tempcos in direct contact with the 3046 capsules with
> some silicon paste and epoxy glue. This would greatly improve at least the
> thermal stability of the VCOs, don't you think? A good thing about having
> these old VCO boards is that there is no need for worry about those 726
> heated expo pairs breaking down! 

When Patrik said this I was really supprised, I did not know of this
"misstake". I mean, today we speak alot about thermal coupling, is
this maybe the classic misstake from which everyone learned from?

I personally think that moving the tempco's over to the
expo-transistors and termopaste couple them should do alot to help
stablize them. It should at least reduce the expo's temperature
deviation.

I think you should drop an email over to Kevin "Synthfool" Lightner,
who has experiences with early Minis. I recall him commenting on
having S/N 2 up on his bench...

> Any more experiences with old-style Mini-Moog VCOs? Comments? Ideas?
> When I get back from christmas vacations I'll fire up the soldering iron
> and get things going... 8-)

Let me know and we can share the experience... ;)

If you have a suitable thermometer we could actually do a pre/post fix
comparision.

> 	Moog Christmas!
> 
> 	/Patrik above arctic circle (need temperature compensation!!! :)

Yeah, but those Americans is having a cold sweep right now, so they
are competing with you temperature wise...

Cheers,
Magnus