[Milsurplus] BC-348 PSU questions

[Ian Wilson]

The center-tap of the transformer and the -ve side of the caps all go
to one point. This point connects to the B- connection (not the chassis) of
the receiver.

There is a hand-written note that says 'loud hum' so perhaps there is more
to what's going on here than just the PSU wiring. Will look tomorrow in more
detail.

73, ian K3IMW


On Sun, Sep 27, 2015 at 6:45 PM, Bob Camp <kb8tq at n1k.org> wrote:

> Hi
>
> Without a schematic it’s hard to be sure. Your symptoms sound a lot like
> the cap(s)
> on the rectifier are returned (“grounded”) to the wrong point.
>
> Caps should go straight across the rectifier. The other stuff goes
> *around* the caps, not in
> series with them.
>
> Bob
>
>
> > On Sep 27, 2015, at 9:35 PM, Ian Wilson <ianmwilson73 at gmail.com> wrote:
> >
> > I am working on a BC-348-O which has had some hammification,
> > including an AC C-L-C PSU built into the dyno space - classic 5Y3 supply
> > but using Si diodes instead. The B- was returned correctly to the choke
> > whose internal resistance generates negative bias for the 41 audio o/p
> > tube.
> >
> > I rebuilt the PSU, replacing the diodes and the electrolytics with
> something
> > more modern and somewhat higher value.
> >
> > There are two problems with the result:
> >  - the B+ is much too high (about 330V - this is with about 45mA load)
> >  - there is a large amount of hum
> >
> > Looking at the -ve voltage developed, it has a large amount of hum on
> > it. However, I don't see how this can be avoided: surely the diode
> > currents pass through the (300 ohm) resistance in the B- lead? Perhaps
> > there is a problem with the choke (has about 600 ohm DC resistance; no
> > idea what the inductance is).
> >
> > I can increase the capacitor values a lot. However, why does this large
> > AC component not cause a problem in other BC-348's that I've worked on
> > with (presumably) similar power supplies? This is the only radio that has
> > exhibited this problem.
> >
> > Grateful for any enlightenment.
> >
> > 73, ian K3IMW
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