[Boatanchors] An "Undocumented" BC-224 mods? Any sound familiar?

[WA5CAB at cs.com]

Phil,

Several models of BC-224 and BC-348 used a VT-48 as the audio output tube, 
not a VT-152.  If the only schematics you have all show VT-152, then you 
probably don't have a schematic of how the set you are working on was originally 
wired.

Also, it's been mentioned here and over on Milsurplus numerous times that 
unlike most common dynamotors, the negative HV brushes in the DM-24 and DM-28 are 
not grounded.  B- exits the dynamotor and goes through the filter choke in 
the audio pack assembly.  In some models, the other side of the choke is 
grounded.  In some, it goes to ground through a 50 ohm resistor.  And the top of the 
resistor supplies negative bias to some other circuits.  The top of the choke 
goes through a decoupling network and supplies negative bias to the VT-48 or 
VT-152.  The audio tube cathode is grounded.  Whoever put the AC supply in it 
just wasn't smart enough to simply wire it like the dynamotor.

All models (I think) had panel lamp dimmer pots which were mounted near the 
top of the panel to the right of the tuning dial window.  There are several 
variants of what that circuitry looks like behind the panel.  One has a fixed 
resistor bolted to the back of the panel and electricaly in series with the pot.  
However, come to think of it, that resistor wouldn't be there in a BC-224.

Where is the S-Meter?  That's the most glaringly obvious modification, and 
unfortunately the one that given the meter sizes used when all of this was being 
done, invariably ruins the panel.

I've no idea what the DPDT switch might be for.  Remote control of a 
transmitter maybe?  One of the holes to the right of the dial window might be where 
the dimmer pot was.  Original diameter was about 3/8".  If it has paint inside 
the hole, it's original.  Where some of the other original holes were depends 
upon what model the radio was.

In a message dated 10/7/2005 11:04:58 AM Central Daylight Time, 
beaconeer at sbcglobal.net writes: 
> I'm still working on this BC-224.  It's a bit of a headache, but I think 
> I'm up to the task ;-)
> 
> Anyway, it's been more heavily "modded" than I first realized.  (Other 
> than just the AC power supply where the dynamotor usually resides).
> 
> 1.  The VT-152 output tube has been replaced with a VT-48 tube and the 
> circuit modified with what is apparently a cathode resistor and bypass 
> capacitor to ground. (I haven't determined the tube pinout of the VT48 
> to be sure, but that's what it looks like)
> 
> 2.  There is a variable resistor mounted inside the radio, adjacent to 
> the "S" meter, not yet sure what it does, but the bracket certainly 
> isn't original!
> 
> 3.  A DPDT toggle switch  is mounted to the left, top side of the panel 
> and the six wires are routed across the chassis and disappear under the 
> added on power supply.  NOT at all sure what this switch is for.  The 
> owner of the set doesn't know what this switch is for either and has 
> said I may remove it if I wish. 
> 
> 4.  There are also two "toggle switch sized holes" drilled in the upper 
> right side of the radio, between the dial escutcheon and the Meter, but 
> contain no controls.
> 
> Does any of the above mods sound familiar to anyone?
> 

Robert & Susan Downs - Houston
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