[sdiy] EDP Gnat envelope oddity

Tim Stinchcombe tim102 at timstinchcombe.co.uk
Fri Aug 7 12:09:17 CEST 2020

> 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 Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at timstinchcombe.co.uk

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