[Milsurplus] BC-348 PSU questions

[Bob Camp]

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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