[Milsurplus] BC-348 Power Supply/Biasing

Michael D. Harmon mharmon at att.net
Sat Jan 1 22:11:20 EST 2022


Maybe I'm over-thinking this issue!

All my experiments to date have been with the freshly-built power supply 
just laying on the bench, fed with suicide clips.  I have not installed 
it in the radio.  There are several things I need to fix in the radio 
prior to actually applying power.  According to the documentation, both 
the original dynamotor and the Hallicrafters EP-298 both provide a 
filament winding (with one side grounded) and a high-voltage winding 
with the + and - connections floating.  Since the B- power supply output 
is NOT grounded to the chassis (and everyone says NOT to connect it to 
ground), then how can I use the chassis as a common point to measure the 
various voltages in the radio?  There has to be a "mystery connection" 
there somewhere!

According to the schematic, the B- terminal on the power supply feeds 
terminal 6 on the output transformer (choke winding).  The other end of 
the choke is at terminal 5, and from there, there is a 50 ohm resistor 
to chassis ground.  When I jumper the B- output to terminal 6, and 
jumper terminal 5 through a 50 ohm resistor to the frame of the 
transformer, and turn on the power, I get the B+ voltage between the B+ 
and B- terminals on the power supply, but also all the way down the line 
to the resistor - no change.  I have no idea as to how much current 
needs to be drawn from terminal 5 to read 12~18 volts bias.

Like I said before, I'm missing something here.  Maybe it's just my 
"old-timer's disease" catching up with me, but there's something I'm 
just not seeing here!

BTW, the graphic sent by Al Klase did NOT come through.

Thanks!
>
> Mike Harmon, WB0LDJ
>
> mharmon at att dot net
>


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