[sdiy] VCS3 heat

[Magnus Danielson]

From: Tony Kalomiris <tonyk38 at sympatico.ca>
Subject: Re: [sdiy] VCS3 heat
Date: Thu, 30 Jun 2005 13:00:14 -0500
Message-ID: <5.1.0.14.0.20050630125114.00bd5e20 at pop1.sympatico.ca>

> Hi mark,
> 
> Glad to hear you've got a stereo VCS3 again.The hot spots are normal for 
> this version of the A board.

Indeed.

> Sounds like you got a very early design, before they moved Q110 to a heat 
> sink and modified the circuit around that "hot" resistor (25 ohms 5 W).

He could have an early one without that transistor! ;O)

> I wouldn't worry too much about it. Even though it's a poor design,as Magnus 
> Danielson wonderfully described, it shouldn't create a problem.I owned and 
> used one for many years with the charred pcb.

If you know that it will burn that amount of wattage, and service it up for it,
then it is certainly not a problem. The problem is really that the design was
not really made for the heat originally and has undergone a number of variants
to combat the heat. In the original Mk I design there is a resistor between the
two capacitors at the diode-bridge (off-board). However, this makes it a bad
"source" so you can reduce envelope-modulation of oscillators by shorting it.

While the Mk I PSU can have strange failure-modes, I am bone-headed enought not
to rebuild it into something else (like the Mk II which is more traditional).
Smaller patches to make the original design less heat-concentrated and similar
small deviations is nicer for everyone. Especially when done with some care.

> If you're still concerned just keep a smoke detector near by ;) j/k

He... on my Synthi-A the PSU had burned open, so it was running the full
electronics on the unregulated power (42V rather than 21V between the +12V rail
and the -9V rail). There was a slight hum as a warningsign but nothing else to
give it totally away. I have a MkII so it "should" only be the caps at the
diode-bridge. But no, it proved to be the transistor driving the transitor
giving the +12V that had burned into a conductor. Noise was quite, but
everything else kind of worked. Once I had located the broken transistor it
all came back to normal operation, including the noise. So, even such faults
can be of fairly limited damage. Could be nice to know.

They are nice machines, but they are not always so very cleverly designed with
our modern eyes looking at it.

Cheers,
Magnus