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

[Jim Patchell]

	Here is a debugging suggestion...get a pair of 2n3904's (assuming that 
the 726 is a dual NPN), glue them together and wire them in.  They will 
work good enough as an exponential pair to make sure that the oscilator 
works, and if they go poof, you are out only 25 cents.  I would 
definately not plug in anotther 726 until you understand how the first 
one died.

Tim Parkhurst wrote:
> I could be SOOOOOOOOOO wrong on this one but I'll chime in anyway. ;)
> 
> Is it possible you have a bad cap in the VCO? Could be a bypass cap that you
> haven't checked or perhaps even the timing cap itself? My theory is: Maybe a
> very old and almost dead (shorted) cap is pulling a lot of current through
> the 726. Lord knows, caps can and do short when they get old. If you have a
> marginal cap in the VCO, maybe the 726 can keep up with the current demands
> when it is brand new, but the process of cycling the power drains the charge
> from the cap and makes the 726 attempt to charge it up again.
> 
> Again, I could be really wrong on this one, but I was just trying to think
> of what could be pulling enough power to make the 726 run hot enough to
> croak. Any input from the EEs out there?
> 
> Tim Servo
> "Imagination is more important than knowledge." - Albert Einstein
> ************************************** 
> 
> 
>>-----Original Message-----
>>From: Synthusiast [mailto:synthusiast at gmx.net]
>>Sent: Wednesday, June 04, 2003 2:41 PM
>>To: Synth DIY
>>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
> 
> 
> 
> 
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