HW-12A Help Needed - A Different Approach

[Stu Lyon]

Hi Doug,

Although I don't have a HW-12A (or any Heath SSB transmitters left for 
that matter) I have fixed a few.

I assume you have the schematic of the HW-12A:  
http://bama.edebris.com/manuals/heath/hw-12a/   sch1.gif , sch2.gif , 
_*sch3.gif*_ .

The  PA grid circuits are all about the same and are adjustable in the 
same way. The trick is to measure the PA grid current w/o unsoldering or 
breaking the circuit.  What you will need is a  0-1mA analog meter in 
addition to the front panel meter. (no digital meters here).

With clip leads, place the meter across (in parallel) with R73, 47K. 
*(sch3.gif)* One lead is the center tab of the bias adjust pot *(-)* and 
the other the junction of  R72, 6.8K, 2W *(+)*. You now can measure the 
PA grid current w/ a full scale of the meter since R73 is fully shunted 
by the meter. Offset the balanced modulator enough to measure some grid 
current. Now adjust the coils for increased grid current while turning 
down the "Tune" pot to keep the meter off the peg.

Should work.
73, Stu W6CUX
One-half century a ham.

Davies, Doug A FOR:EX wrote:
> Since I didn't receive any replies to my first posting, I'll try a
> different approach.  Would someone who is familiar with the HW-12A tell
> me if there a way to determine the proper setting of coils L2 and L3 in
> the HW-12A.  These coils, the 12BY7 driver grid and plate coils
> respectively, were supposed to be pre-set at the factory and not require
> readjustment.  However, on the rig I have, the tops of the covers have
> been opened up and I'm sure someone has fiddled with them them.  
>
> There is nothing in the abbreviated manual I have that specifies the
> value of grid or plate current for the 12BY7 driver tube.  With the
> meter switch in the TUNE/OPERATE position, full scale meter reading is
> 300ma.  Can I use this to set these two coils?  This is all new ground
> for me so any and all suggestions are appreciated.
>
> Doug  VE7DRF
>
>
>   

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