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

[WA5CAB at cs.com]

As Al says, in the JNQ it's pin 6 in the Audio Pack.  In most of the others 
it's pin 1.  However, I have a bunch of the audio packs for the EMOPS and HKLR. 
 I normally ID which they go to (functionally if you ignore the component 
numbers the latter two are interchangable) by noting whether the schematic on the 
top ID's the transformer, choke and capacitor as 101A, B & C or 123A, B & C.  
However, I recently noticed one that was different from what I've come to 
think of as the norm.  I'd have to go and locate it again to see whether it was a 
101 or a 123 but whichever it was, it was pin numbered as the other one 
normally is.  It was also one of the early units with no low Z tap on the audio 
transformer secondary.  So I concluded that during the production life of the 
receivers, some dummy decided to change the pin numbers.  So you really need to 
go by the paper diagram on the top of the audio pack.  

In all models except the BC-348-JNQ, the B- filter choke goes to ground 
inside the pack.  In the JNQ it does not, and the bias is also applied to the mixer 
grid.  But in all cases with the audio pack still installed in the radio, the 
dynamotor lead or audio pack pin you are looking for will have a resistance 
of 25 ohms or less to ground.  But not zero.  If the paper diagram is gone and 
you can't differentiate whether the pack pin is the choke or one of the taps 
on the output transformer secondary (which also has one end grounded), you can 
ID the latter by plugging in a headset, grounding one lead from an analog 
ohmmeter (set to Rx1) and touching the other lead to pins on the audio pack.  The 
one or two (on later units) pins connected to the output transformer output 
winding will cause a click in the headset.  The one (or with JNQ two) connected 
to the choke will not.  The two that could cause a click depending upon 
ohmmeter setting but that show high resistance to ground will be the audio 
transformer primary.  And a DVM won't usually work for this test.  DVM's generally 
aren't much use in working on tube radios.

In a message dated 3/8/2009 7:04:20 PM Central Standard Time, al at ar88.net 
writes: 
> 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