[sdiy] Formant troubles: uA726's with a very short lifetime

[Oren Leavitt]

Hi Martijn,

Sounds to me like something is producing a destructive (to the uA726) transient voltage when the synth is switched off.
Watch the power supply voltage levels as you switch the synth on and then off and see if there is anything that looks spooky like momentary polarity reversals.

That might be one possibilty...

My 2 bits,
Oren

-------Original Message-------
From: Synthusiast <synthusiast at gmx.net>
Sent: 06/04/03 02:40 PM
To: Synth DIY <synth-diy at dropmix.xs4all.nl>
Subject: [sdiy] Formant troubles: uA726's with a very short lifetime

> 
> Ok, last one for today:

Hi list,

I'm having troubles with the uA726's used in the core of the Formant
VCO's.
This system I have here has 2 VCO's and both uA726's were dead when I got
the unit. The power supply and every component around the uA726's turned
out
to be ok. So I replaced one of the dead uA726's with a 'new' one. This
worked very well for a couple of hours. Then the synth was turned off for
a
while. After the synth was turned on again, the uA726 got very hot and
completely stopped working. Maybe it was a bad uA726 or I overlooked
something in the VCO circuit, anyway, I tested another uA726 in the other
VCO. Same story: chip worked well for a while, but after repowering the
uA726 it stopped functioning and the casing got very hot.

Before I put in another uA726 and see it go to dust, what could be the
cause
of this very short lifetime of the uA726's? Any other Formant owners
recognize this problem? What did/would you do to prevent this from
happening
again? Any help is greatly appreciated. :)

TIA,
Martijn

>