ARP Omni capacitors

Mark Smart smart at
Fri Oct 18 19:11:19 CEST 1996


I just bought a partially hosed ARP Omni and am in the process of bringing
it back to life. Thanks to Sean Costello, I now have a service manual for
it also. Like most Omnis I have read about on the net, this one had notes
stuck on when I got it. This was due to blown 22uF 25V tantalum capacitors
in the keying voltage circuits which allow each note to decay separately.

My question is this: what causes these capacitors to blow out on so many
Omnis? I thought of several possibilities:

1. Heat. There is a lot of circuitry in this thing and it has no fan.

2. Reverse voltage. Maybe the circuit is designed in such a way that the
   tantalum caps occasionally (or often) get backwards voltage. I know from
   experience that tantalum caps don't like this very much. The
   communications boards I work on at my job have tantalum caps in a
   power-on reset circuit. If these caps are installed backwards, they will
   work fine for several hours before they toast. Of course, if you connect
   them backwards directly to a power supply, they will create an explosion
   which Beavis would enjoy greatly.

3. Overvoltage. Maybe the circuit puts transient voltages on the caps that
   exceed their 25 volt rating.

I ended up replacing all the caps on my Omni with 50 volt non-polarized
electrolytic capacitors. My reasoning behind this was that these caps would
be less susceptible to damage if it is being caused by overvoltage or
reverse voltage. I also plan to install a fan in it to combat the heat
problem. I know that tantalum caps are generally considered higher quality
and more impervious to heat than electrolytics, but I couldn't bring myself to
pay upwards of $1.50 apiece for 50 capacitors. The electrolytics were like
18 cents each.

So far these have worked fine. There were and are also quite a few blown
CMOS chips in this thing which may also be victims of heat. I recently
brought a friend's ARP Quadra back to life, and it also suffered from
numerous blown CMOS chips. We put a fan in it also, and so far it has
worked ok. Not enough time has elapsed since fixing either of these
keyboards for me to know for sure that the heat was the problem. I would
appreciate input from you synth experts out there.

Since I am becoming so familiar with the Omni, I have thought of making a
sideline out of buying busted Omnis (there are a LOT of them out there) and
reselling them at a profit. Of course I'll keep one for myself since they
sound so damn cool.

Thanks in advance!

*     Mark Smart                               *
*     Network Technician                       *
*     University Communications Inc. (UCI)     *
*     smart at                      *

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