[Milsurplus] BC-348 Power Supply/Biasing

[Jim Whartenby]

MikeAgreed, B- current flows through line item 108-2, a 50 ohm resistor to chassis ground.  It can be found on the schematic just to the right and a little above the dynamotor.  This resistor develops about 50 ohms X .06 amp = 3 volts bias for the audio PA.  245 volts is the rating of the hallicrafters power supply.  AN08-10-112 calls for 224 volts @ 0.7 amp from the dynamotor but that current has to be a misprint, should be 70 mA .  
245 volts is about 9% high and should not be a problem, IMHO.  Most voltage readings will appear high anyway since most VOMs of that period were 1kohm/volt and loaded the circuit which resulted in a lower then actual reading.  If you meter reading is in the ballpark, good enough!  Just about all of the resistors are +/- 10% so if you are within that tolerance, all is good.
Have fun with the radio,Jim

Too much agreement kills a chat.  E. Cleaver

-----Original Message-----
From: hwhall at compuserve.com
To: mharmon at att.net <mharmon at att.net>; milsurplus at mailman.qth.net <milsurplus at mailman.qth.net>
Sent: Sat, Jan 1, 2022 5:00 pm
Subject: Re: [Milsurplus] BC-348 Power Supply/Biasing

The separated plus & minus HV is correct.  A "chassis ground" is established between resistances in the radio's circuits.  Something similar is often seen in solidstate gear that requires plus & minus DC rails & a ground.
I'm working on a very pretty BC-348-Q right now. Has a DIY HV supply in place of the dyno. Making sure all is okay before I try to light it up. 245 VDC may be a little too high.  I'm wanting to see about running this one at a lower voltage so maybe it can play without a recapping. It's too near pristine for me to want to do that right now.  I've seen online talk of running as low as 150 VDC & working okay.

WayneWB4OGM

-----Original Message-----
From: Michael D. Harmon <mharmon at att.net>
To: milsurplus at mailman.qth.net
Sent: Sat, Jan 1, 2022 3:46 pm
Subject: [Milsurplus] BC-348 Power Supply/Biasing

I recently picked up a BC-348Q which I would like to get working.  It 
had no dynamotor or power supply, but everything else was there.  
Actually, it's in pretty good shape for an 80-year old radio!  All the 
tubes (except for one 6SK7) even tested OK on my TV-7.  There were a lot 
of loose wires, but I believe that I have them all identified now.  The 
filaments had already been rewired for 6.3V operation.

I scrounged up a surplus transformer and built a power supply and 
mounted it on a piece of heavy-gauge aluminum cut to fit the original 
dynamotor opening.  With a 4000-ohm 20-watt resistor (245V @ 60 mA) on 
the output, the supply puts out 245V.  I have used the drawing for the 
EP-298 Hallicrafters supply as a design model, and the plate supply is 
isolated from chassis ground - no chassis connections at all.

I'm obviously missing something here!  Where is the ground return?  If 
both the B+ and B- output connections on the power supply are isolated 
from ground, How is a voltage developed between either one and chassis 
ground???

I've been dealing with this issue so long my brain is fried!  Please 
somebody help!

Thanks!

Mike Harmon, WB0LDJ

mharmon at att dot net

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