Analogue systems

>
> Folks,
>
> I've got a bit of a strange problem that has cropped up with one of my
> RS-15
> chassis.  This is an older chassis that I bought used and it is filled
> with
> audio processing modules (filters, VCAs, comb filter, delay, phaser,
> etc.).
> At odd times, all of the modules will produce a low 120Hz or so sound and
> no
> inputs will work.  No other patch cords, except output, need be connected
> for this to happen.  Power cycling the chassis usually fixes the problem
> for
> awhile and then it will suddenly come back.  It is localized to just this
> one chassis and all the modules seem to work otherwise.
>
> Anybody had this happen on their system?  Any ideas?

Yup, I've got an older cabinet I bought used from the UK (AS logo on the
wrong side!) that has issues (working on that) that manifest in similar
ways.

The culprit? Power supply. If you've got a multimeter or something like
that  to measure voltage, check out your +/-12 rails. In my cab with these
problems, if the hum is happening the rails are low, around 10.2v.

It seems like my PS boards (this old cab has two) can't handle even a
small module load. Two modules each are usually OK, three modules and the
rails start oscillating between the 12v and 10.6 or so. Four and they
steady down around 10.2.

Ouch!

When it's oscillating, the increase in noise is exactly correlated to the
dropping voltage.

I'm guessing I'll have to replace the PS boards with a newer one.

-Brandon