[ARC5] Switching the dreader B+ line.

[Leslie Smith]

Hello All,

Jim asked me why not use Dave Stinson's scheme to put a "command"
transmitter on the air.  In my reply, I omitted the most important part
of the discussion - a circuit diagram of the SCR-274 transmitter.  I'm
certain most reading this list will have that - but not all.

SO I send that now.
Looking at J64 (the connector on the rear panel of the transmitter) we
can see that the B+ line 

(1) to the PA comes in via pin #7, (to the 1625 circuit).
(2) to the screen supply comes in on pin J64.4 
(3) the supply to the oscillator from pin J54 pin #3 and K-53.

The HV supply (to the PA,  J64.7) is switched in the BC-456 modulation
circuit.  (See J64.7 then PL154.10 and then K-52 - fed from the B+ line
on the dyno.  (I don't supply this part of the circuit.)   So the actual
switching for the PA B+ line takes place in the BC-456.  Closing a key
in the +24V heater line pulls in the relay, and the B+ supply.

It would be possible, of course, to leave the PA B+ permanently "on"
because the oscillator is switched in the transmitter, K-53.    I'm sure
some will do that - but a safer method is to switch the B+ line "on"
using the key only when the transmitter is "on-air".  I'm want to do
that, using a FET rather than a relay.  I'm simply duplicating the
function of K-52, but with a solid state device.

Some will say that switching the B+ line with a relay is safer than
switching with a FET.  That is correct, without doubt.


Les
-------------- next part --------------
A non-text attachment was scrubbed...
Name: BC-457 transmitter, cct.jpg
Type: image/jpeg
Size: 126848 bytes
Desc: not available
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20170402/72727785/attachment-0001.jpg>