[ARC5] BC-946 Tuning capacitor removal and panel straightening

[Mike Hanz]

I went out and measured the coils on one of my 239kHz Z-4 BCB BFO cans.  
The large coil (L-13) measures ~1,200uH, the small one (L-12) ~140uH.  
The BFO a "Tuned Plate" oscillator, so the conditions described in 
references like Figure 24-2(A) in the Radiotron Designer's Handbook 
would apply.  The coils are pi wound with Litz wire.  I've never tried 
this, but a possibly fruitful approach to replication might be to take a 
85kHz BFO coil and pull off ~420 turns on the large coil - or until you 
have ~1200uH - then try it in the circuit.  The grid coil is not 
particularly sensitive to ratio, but one should probably reduce its turn 
count proportionately to keep its resonant frequency well above 239kHz - 
110 turns pulled off should do it.  If you decide to wind your own from 
scratch, then Gordon White's superb article in the January 1967 CQ issue 
contains the following gems on the BFO coil:

L12 - 70 turns 5-41 Litz
L13 - 213 turns 5-41 Litz
0.172" spacing

73,
Mike

On 3/27/2011 4:22 PM, Bob Macklin wrote:
> What is the frequency needed?