[Collins] REMOVING B+ FROM 75A4 FILTERS

[Gerald]

On Sun, 2005-07-17 at 10:25 -0400, Magoo wrote:
> Can anyone point me to where I can find the information on removing B+ from 
> the filters of the 75A4?  I looked through the compendium available from 
> Electric Radio and did not see anything there.
> 
> Thanks.
> 
> Bill, VE3NH 
I presume its wired like an IF transformer now with the tube plate to
one pin on the input end of the filter and then a bypass capacitor on
the other input pin with a resistor or RF choke to the IF tube B+
supply, possibly also a resistor to the IF tube screen. There should
also be a tuning capacitor, probably close to 100 pf, and it could be
variable.

To get DC off the filter you need to unhook all those parts and
connections from the input end of the filter. Rewire those to a couple
terminals of a terminal strip (like the Mouser catalog number 158-1003).

You can leave the tuning capacitor hooked to the filter. Connect the
filter terminal that was bypassed to ground. Connect a silver mica
capacitor say 470 to 1000 pf from the other terminal of the filter to
the tube plate wire. That capacitor could be a NPO ceramic 1KV voltage
rating too. Connect an RF choke from B+ terminal strip end to tube plate
end. Probably 15 to 25 mhy though the 75S-1 used 2 mhy and the KWM-2
used 10 mhy. Perhaps a J.W. Miller 70F223 or 70F102, Mouser catalog
number 542-70F223 or 542-70F102. For the coupling capacitor a dipped
silver-mica such as the CDE FD102J03 (Mouser catalog number
5982-19-500V1000) or CDE FF102J03 (5983-16-1KV1000) should be fine.

Be sure to repeak the input tuning capacitor if it is variable at the
filter pass band center.

That's how I'd do it.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer