[sdiy] EDP Gnat envelope oddity

[Tim Stinchcombe]

Found my notes (though one set was little more than a load of scribbles on a scrap of paper)!

One fault with one of my Gnats _was_ in the VCA envelope circuit - it was a blown 4016 switch chip.

The blown LM3900 turned out to be in my _Wasp_ (the chip concerned was U12), which has sections in both the VCA envelope and filter envelope, where the circuits are similar'ish to the Gnat, but some of the capacitors are _bigger_, which may explain my 'too much current' thought (?).

Tim
__________________________________________________________
Dr. Timothy E. Stinchcombe, CMath, MIMA, CEng, MIET, MIEEE 

53 Winstonian Road
Cheltenham
Glos, UK
GL52 2JD

email: timothy at timstinchcombe.co.uk
http://www.timstinchcombe.co.uk

Tel: +44 (0)1242 571747
__________________________________________________________




> -----Original Message-----
> From: Tim Stinchcombe <tim102 at timstinchcombe.co.uk>
> Sent: 07 August 2020 11:09
> To: 'Gordonjcp' <gordonjcp at gjcp.net>; 'synth-diy at synth-diy.org' <synth-
> diy at synth-diy.org>
> Subject: RE: [sdiy] EDP Gnat envelope oddity
> 
> > The next problem is that the VCA envelope is not doing much.  Now its
> > timing capacitor checks out okay even though it's the same no-name
> > green 2μ2 cap, so I replaced that also with a 1μ capacitor, while I
> > wait for bits to arrive.  This hasn't actually solved the problem, is
> > that although the voltage on pin 13 of the LM3900 goes up nice and
> > high, the output - and indeed the voltage at the junction of the two resistors
> where it goes to the VCA OTA - barely changes.
> 
> I'm sure one of my Gnats was DOA which was due to a bad LM3900, and I have a
> distinct memory from the time that the IC may have blown as there is nothing in the
> circuit to stop the cap delivering more current than the LM3900 is happy with.
> However, whether it was in this same area of the circuit I cannot recall and
> unfortunately I am not sure where my notes from the time are stored (I quickly
> flipped several files, but couldn't locate them).
> 
> As the LM3900 offers so many PN junctions from input pins to ground, it would be
> worth measuring these with a DVM and comparing to a new chip, and any gross
> anomaly might quickly point to the problem...
> 
> I must have notes _somewhere_ - I shall continue to rack my brains as to exactly
> _where_ they are.
> 
> Tim
> __________________________________________________________
> Tim Stinchcombe
> 
> Cheltenham, Glos, UK
> email: tim102 at timstinchcombe.co.uk
> www.timstinchcombe.co.uk
>