[sdiy] Oberheim OB-Mx PSU question

[fmg]

My English is not good but I hope you can understand me

I have never seen an OB and never open one but I guess the procedure
is the same for any transformer PS.

Once you changed the voltage operation from 110Vac to 230Vac, and
wish the equip. work in the same safety area you also must change
the input fuse for a one with a half current (Ampere) value. ie: if
the original is a 3A you must put in the unit a 1.5A one. This simply
comes from the Ohm's laws and ensures you that the fuse will blow if
an overload accur. (If the fuse don't "fuse" then the transformer
primary coil will "fuse", and transformers generally tends to be
slightly more expensives than fuses).
The 3A by 1.5A change was an example, I really don't know wich value
uses you model, may be someone in the list can help us here.

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

      pp                pp
      |    ---------    |
     \|/  |         |  \|/
     ---------------------
     |                   |   pp=press point
     |                   |   sp=seat point
     ---------------------
     /|\  |         |  /|\
      |    ---------    |
      sp                sp


Release the press and apply a slightly more pressure at central
zone (1 to 1.5 ton). Without releasing pressure slide a small
cuneiform piece [*1] of a non magnetic material (wood) in the room
now created between the core and the coil pack. (You already must
have with you this piece, previously made at home)
Release the press and go home to mount the transformer. You can
also spend some time talking with the operator about the weather
or the economy, this will help you in the case you need more
"pressings" in a near future.

I never check how many dBs this method reduces the hum but if
you are careful in all steps you will end up with a really
silent power supply, at least from the mechanical point of view.



[*1]- or wedge, I don't know how name it in English


Fabio Gonzalez
e-mail: 1984 at softhome.net
address: S34º28'32.5" W058º36'57.2", Earth


Sebastian Kuehnl wrote:
> 
> --- Tony Allgood <oakley at techrepairs.freeserve.co.uk>
> wrote:
> > Hi Seb and all,
> >
> > OK, that plate near the actual inlet socket aperture
> > is removable
> > without too much force. I actually can't remember
> > how its actually
> > removed, but it took me only seconds to work out how
> > to do it with the
> > unit in front of me. [Now if it were very hard to
> > get off I would have
> > remembered this all in detail :-) ]
> 
> I figured it out and it was easy as you say, but too
> complex for my English to describe :^o
> 
> > There is a little switch underneath that simply
> > selects between either
> > 110 or 230V. The new voltage setting will appear in
> > the window once the
> > plate goes back. And its not paper but a white piece
> > of printed plastic
> > and part of the switch itself.
> 
> It's a piece of plastic that just needs to be removed
> and inserted 180 degrees flipped. The OB is up and
> running now. BUT...
> 
> > Also change the fuse inside to half the original US
> > fitted value... I
> > think you'll need a 250mA from memory.
> 
> The inscription on the lid says "use only with 250
> volt fuse", and that's what's in there, so I thought I
> shouldn't change anything. The local computer shop
> told me they'd never seen anything like this, and
> advised leaving the fuse where it is.
> 
> However, while the instrument runs, it hums like a
> refrigerator. The 50 Hz sound comes from the PSU's
> corner, is not audible in the signal path, and is
> completely static (that is, no phase cancellations or
> frequency changes were observed during several
> minutes).
> 
> Conclusion?
> 
> Thanks again, any help appreciated.
> 
> Sebastian Kuehnl
> 
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