Nearly there with IBM 029 KeyPunch

Brent Hilpert hilpert at
Thu Jun 11 15:45:10 CDT 2009

Roger Holmes wrote:
> > From: Brent Hilpert <hilpert at>
> >
> > My remaining concern might be that even with the new C bringing the
> > V up,
> > the regulation function of the supply may have been lost if the
> > transformer
> > is no longer functioning in the regions it was designed to.
> Yes, but relays are unlikely to be damaged by a few volts too much.

Not a concern about damaging the relays, rather that the voltage may vary with
different load conditions (number of relays energised / state of the
machine). How consequential that issue is in this 'small' machine, I don't
know, but IBM did bother to provide it with a regulated supply, although that
could have been motivated by supply-side concerns.

> My concern is the current in the intermediate winding could damage
> itself.
> So far so good but I am charging a much larger capacitance for a
> slightly longer period so I would think the current would be higher.
> To get 48 volts I am going to have to add about another 6 uF, so about
> 48 F instead of 15, so very roughly three times the current, assuming
> (and its a big assumption I can't justify), that the voltage is the
> same.

The other thing that changes with f is the inductive impedance of all the
(other) windings of the transformer. Ferro-resonance transformers do not rely
only on the resonance principle, magnetic/inductive issues are also very
involved. I'm not at all convinced that simply changing the C is adapting the
supply for the new frequency (50 Hz). That you are having to go so far off the
theoretical new C would tend to support this.

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