[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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