[Milsurplus] BC-620-F on Ten?

[W7QHO at aol.com]

In a message dated 1/6/05 9:00:50 AM, roger at new-gate.co.uk writes:


> A local military radio group I'm a member of has just acquired a good
> condition BC-620-F with the PSU, mounting tray, box of crystals and handset.
> A quick search shows that the rig's top frequency limit is 27.9 Mc, is there
> any possibility of using it on Ten Metres without serious modification, as
> we would like to have it as a working exhibit?
> 

Roger,

Interesting question. 

The BC-620, covers 20 - 27.9 MHz, roughly the old US "Tank" band.   A very 
similar set, the BC-659 covers the 27 - 38.9 MHz "Artillery" band and would be a 
better choice for 10M operation as pointed out by WA5CAB.   At the Ft. 
MacArthur Military Museum in San Pedro, CA, we have been using three BC-659s 
successfully in the 10M amateur band for some time now.   We have no direct 
experience with the BC-620, but similarity in the designs would suggest that 10M 
operation with this set might be feasible as well.

The LO in the 620 triples in it's plate circuit and gives an injection 
frequency to receiver mixer which is 2.88 MHz below the selected receiving 
frequency.   To receive on 27.9 MHz, for example, would require a crystal frequency of 
(27.9 - 2.88)/3 =   8.340 MHz.   Following the same formula, operation on 29 
MHz would require a crystal frequency of (29-2.88)/3 = 8.7067 MHz.   
Alternatively, the LO might give satisfactory performance on the 4th harmonic with a 
crystal cut to (29 - 2.88)/4 = 6.53 MHz.   Successful operation in either case, 
of course, would also require that the LO plate and all other relevant receiver 
and transmitter circuits have a sufficient range of adjustment to accommodate 
the new, higher operating frequency.

However, the primary challenge encountered in operating either of these sets 
today is what to do about BA-41 which provided the 60- and 25-volt potentials 
required by the AFC circuit and -4.5v bias for the receiver audio amplifier.  
This special battery fit inside it's own box on the transmitter/receiver 
chassis .   The -4.5v for the receiver is no problem, but the other voltages would 
require a considerable pile of currently available 9 and 1.5v cells to 
replicate.   Not an impossible solution, but one ends up with the chassis festooned 
with made-up strings of batteries.   Works OK but rather untidy.

What we came up with instead is an FET circuit that replicates the function 
of the original DC amplifier (triode part of V12).   Uses only a couple of 9 
volt batteries and a few other parts and hides neatly inside the original BA-41 
compartment. Can provide circuit details if you would like to give it a try.

The deviation on these sets is on the order of +/- 30KHz.   Don't remember 
what the restrictions on modulation index were in the UK but over here we are 
constrained to operation above 29 MHz with these sets.   

Hope this is useful.

Dennis DuVall,   W7QHO, ex. G5BSW, DA1IN
Glendale, CA