Doepfer

> Hi all,
>
> I'm using one of the little A-100 MNT bus boards and for some
> reason its started playing up and I was wondering if anyone has
> any idea where to look ?
>
> The problem is that regardless of what module I have plugged into
> the bus board (eg. A-110, A-106-6, etc) something seems to be
> interfering/modulating the output of the modules in unusual ways.
> For example, if I have an A-106-6 Xpander filter plugged in and
> self resonating, the sinewave sounds as if I've got something
> FM'ing it even though I have no other modules connected. If I
> then plug in an A-178 theremin module touching it seems to affect
> the A-106-6 without using any patch cables.
>
> This also happens if I plug an A-110 in (in this case the range
> switch doesn't seem to work properly). Of course if I plug these
> modules into another case they all work fine.
>
> The only thing that looks suspicious is that 2 of the 4
> capacitors on the bus board (the ones near the heat syncs) look
> as if they've expanded slightly compared to the ones further away
> from the heat syncs (the top of these caps sticks up slightly).
> I'm fairly sure they're of different values so maybe this
> appearance is normal ?
>
> If anyone has any ideas I'd appreciate it.
>
> Cheers
>
> Simon

Sounds like a 50/60Hz hum on the power supply. Maybe the external
transformer is not sufficient (9V AC and min. 400mA required). Another
explanation would be a defective smoothing capacitor on the pc board. The
different size of the capacitors is OK because they have to withstand
different voltages in the two stages of the voltage doubling circuit
(capacitors with higher voltage have a larger case). Usage of a improper
power supply (i.e. more than 9V AC) will destroy the capacitors because the
voltage is doubled in the circuit and the capacitors withstand only 25V.
E.g. a 15V AC supply will damage the capacitors in any case.

Best wishes
Dieter Doepfer