[Milsurplus] BC 348 Question: in re lifted B- lead

[Al Klase]

Gang,

I'm glad Robert broke the ice on this one.  The power supply arrangement 
in the BC-224/348 is not at all unusual.  It's common practice in a lot 
of receiver designs to return the most negative point in the power 
supply to ground through an impedance (resistor, or choke, or both).  
The voltage drop across this impedance is used to provide a negative 
bias voltage to some of the receiver circuits.  Grounding B- leaves the 
radio operating without bias.  This is usually not a good thing.

Once the dynamotor assembly is removed from the receiver, you're left 
with some wires with spade lugs that may or may not be in the right 
relative position.  The one you want to connect the negative terminal of 
the power supply to is the one that goes to the ungrounded end of the 
choke in the "Audio Pack Assembly"  (choke and output transformer) on 
the top of the chassis above the I/O connector on the rear.

I was hoping I could just tell you to connect to pin-X of the Audio 
Pack, but the pin number varies with receiver model.  The one I looked 
at is a BC-224H, and the connection in that case is pin-1.  However, 
according to my schematic, in BC-348Q it's pin-6.  Other schematics in 
my possession don't indicate pin numbers.  There does seem to be a 
sticker with schematic on each audio pack.

So, it's an exercise in schematic reading and ohm-meter usage.  I f you 
do it properly you should end up with a bias of about -8 volts (or more) 
on the audio output tube control grid.  Check it!

Hope this helps,
Al



WA5CAB at cs.com wrote:
> I can't vouch for the original BC-224 but all models from BC-224-A through 
> BC-348-S have the B- above ground.
>
> If you mean if the radio is modified with an AC operated rectifier supply, 
> you simply connect the B- from the rectifier supply to the same point where the 
> dynamotor B- connected.  So B- would be floating in the power supply.  Filter 
> capacitors, however, should still go to chassis, not the B-.  If using a 
> conventional center-tapped HV winding and a full wave rectifier, the center-tap 
> would go to the B- connection in the radio.  If using a bridge rectifier, the 
> negative terminal of the bridge would go there.
>
> If you try to use a substitute dynamotor (more DM-28 filter-bases turn up 
> than do the actual dynamotors) you must use one that has the B- brush holder 
> insulated like the B+ is.  Or take two to make one.
>
> In a message dated 3/8/2009 3:58:27 PM Central Standard Time, 
> bluebirdtele at embarqmail.com writes: 
>   
>> Now I'm starting to build up a file on the BC 348.
>> Can someone tell me which versions have the B- line off of ground and why?
>> What do you do to the power supply wiring to compensate for this?
>>
>>     
>
> Robert Downs - Houston
> wa5cab dot com (Web Store)
> MVPA 9480
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-- 
Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/