[sdiy] Oberheim OB-Mx PSU question

[harrybissell]

Excellent suggestion, because most of the hum in the stacked core comes
from loose laminations... which your method will at least help.

Another solution is to switch to a toroid transformer... which have lower
hum because of the different core... and less magnetic leakage... but this
is
for experienced people only.

H^) harry

fmg wrote:

> 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
> >
> > __________________________________________________________________
> >
> > Gesendet von Yahoo! Mail - http://mail.yahoo.de
> > Möchten Sie mit einem Gruß antworten? http://grusskarten.yahoo.de