[sdiy] Roland JX-3p vca - noise

[Richie Burnett]

> ...What exactly
> is the purpose of the muting transistors? Just to make sure the signal
> goes to ground if there's no actual synth signal coming through - kind
> of like a gate?

They are there to prevent loud thumps when you switch the instrument on and 
off.

The muting transistors short the audio outputs to ground from the moment the 
instrument is switched on, and keep them shorted out for a second or so, 
until all of the preceding audio stages have reached their steady-state 
operating points.  After this delay they are turned off, and *should* be 
transparent to the passing audio! Similarly, as soon as the instrument is 
turned off they clamp the audio outputs to ground, so that you don't hear 
any glitches when circuitry malfunctions as the power supply rails decay 
away.

Tony, do you have any idea how or why these transistors fail?  They're well 
protected from outside influences, and don't appear to dissipate any power, 
so I'm guessing it has to be a latent manufacturing defect?

-Richie,