[sdiy] Oberheim OB-Mx PSU question

[Peter Grenader]

The hum Sebastian is experiencing is due to the fact that the OBMX was
released in 1993, and at that point Gernam AC was runing about 220.  I've
been told now it's floating at around 235.  It's transformer is not designed
to handle this additional voltage, thus it hums. The same thing happens to
our old subwoofers we (tried to) sell there.  Now we have equipped them with
235 - 240 volt transformers and the problem is gone.  You can imagine how
difficult it is to try to sell a subwoofer that hums.

And this isn't only 235 volts, it's 235 at 50 Hertz. The bulk of the problem
comes from the conversion to 50 hz, where approximately 25% more material is
required in the Xformer.

While the procedure below may work, it seems extremely complicated.  The
short form solution here I think is best - converting the unit back to 115
and using an external step-down to condition the input voltage.

> Respect to the hum derivated from electro-mechanical activity,
>> (vibration) I have a method that don't recommend to beginners. If
>> you are not sure to have a happy-end don't try it.
>> When the hum reaches really annoying levels dismount the transformer
>> paying special attention to the original connections (write down
>> colors, positions, etc.)
>> Search for shop with a hand operated hydraulic press (motor servicing,
>> tyre assembling/disassembling, engines repairs, etc). Once dealt its
>> use work by yourself (don't let the operator make the job, probably
>> he/she don't know exactly what you wont and ends up with a ruined
>> transformer)
>> Start applying a small pressure at corners (0.5 to 1 ton) paying
>> special attention where the transf. seats, the objective is to
>> compress the core not the transformer itself :-) see ASCII draw