[Milsurplus] BC-348 PSU questions

[Ian Wilson]

There is clearly something odd about the transformer and/or the wiring
in the PSU (this is an existing addition that I am rebuilding).

Measuring resistances between the two hot wires (diodes removed from
circuit for the moment) and the centre tap, I see wildly different values
(360ohm and 60ohm). That can't be good. Perhaps there's an internal
short somewhere.

Going to scrap the transformer and choke (which also has issues) and
see what happens when I connect an external PSU that I know works OK
with BC-348's.

73, ian K3IMW


On Mon, Sep 28, 2015 at 5:52 AM, Ray Fantini <RAFANTINI at salisbury.edu>
wrote:

> Having issues with my web pages right now and looking for a new server to
> host them on but I have a video on YouTube that shows how to incorporate
> the AC power supply into a BC-348Q along with the schematic located at:
>
> https://www.youtube.com/watch?v=YKRez8euQU4
>
> I decided not to use modern silicone diodes but opted for a 6X4 being the
> vacuum tube rectifiers give a small amount of internal loss to help reduce
> the B+ On a radio that’s over seventy years old there is no point on
> running super high B+ voltages. I used a transformer that was rated at 230
> volts at 60 Ma and in addition to that I also run an autotransformer before
> the input to all my oldest radios and only supply them around 100 to 105
> Volts AC being at the modern AC line voltages you will also see things like
> the filament circuts running at higher voltages then the recommended 6.3,
> another advantage to using a tube rectifier is that it allows the rest of
> the tubes the opportunity to warm up and avoids applying  an unloaded
> higher voltage to the radio during start up. An unloaded solid state supply
> may be dumping four or four hundred fifty volts on the set before the tubes
> come up and bring the voltage down. Another option may be to install a
> center tap interrupt switch in the secondary of the power transformer that
> way you can keep the B+ off until the radio warms up and this will also
> serve as a standby switch if you use the radio with a transmitter.
>
> Ray F/KA3EKH
>
> -----Original Message-----
> From: milsurplus-bounces at mailman.qth.net [mailto:
> milsurplus-bounces at mailman.qth.net] On Behalf Of Ian Wilson
> Sent: Sunday, September 27, 2015 11:07 PM
> To: Bob Camp
> Cc: Military Surplus Mail List
> Subject: Re: [Milsurplus] BC-348 PSU questions
>
> 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
> > > ______________________________________________________________
> > > Milsurplus mailing list
> > > Home: http://mailman.qth.net/mailman/listinfo/milsurplus
> > > Help: http://mailman.qth.net/mmfaq.htm
> > > Post: mailto:Milsurplus at mailman.qth.net
> > >
> > > This list hosted by: http://www.qsl.net Please help support this
> > > email list: http://www.qsl.net/donate.html
> >
> >
> ______________________________________________________________
> Milsurplus mailing list
> Home: http://mailman.qth.net/mailman/listinfo/milsurplus
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Milsurplus at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>