[Milsurplus] BC-348 Power Supply/Biasing

[Al Klase]

Jim

I'm not so sure about that.  B- "comes up to chassis ground" through 
inductor 101-B which is part of the output transformer assembly.

I hope this diagram comes through.  Some of these lists are living in 
the 20th century re: attachments.


73,
Al


On 1/1/2022 7:16 PM, Jim Whartenby via Milsurplus wrote:
> Mike
> Agreed, 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 
> <https://www.brainyquote.com/authors/eldridge-cleaver-quotes>
>
>
> -----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.
>
> Wayne
> WB4OGM
>
> -----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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-- 
Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net
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