[Collins] E. F. Johnson T/R switch

[Dr. Gerald N. Johnson]

On Tue, 2007-05-22 at 18:58 -0500, Smith Bradford wrote:
> Has anyone tried to incorporate the E. F. Johnson T/R switch into the 
> Collins S Line series 3?  The goal is having true break in keying. In 
> experimenting  with the idea I have run into a problem with hearing the 
> tone oscillator until  K1 drops out. The result, is after you let off 
> the cw key, you hear the tone generator at low level, then the K1 drops 
> out and you hear the receiver audio.
> 
> I currently don't see any practical way to prevent this. I have no 
> intension's of making major modifications to the S Line. And the tone 
> generator is doing what it is suppose to do in cw, that is key the K1 
> relay via the attached triode and provide cw sidetone. I removed the cw 
> sidetone cable from the receiver but the tone generator is still audible.
> 
> I am curious how the 750 cps tone generator is making it's way into the 
> audio path to the speaker. I defeat the receiver mute by place the 
> receiver function switch in the operate position instead of the normally 
> used standby position. The only other path I see is the tone generator 
> going into the balanced modulator. But during cw operation the rf signal 
> doesn't pass through the balanced modulator.
> 
> Smith Bradford
> W9HAK
> 
The grid of the audio cathode follower is almost grounded (33 ohm
resistor) through the mode switch. That should drop that audio signal
about 80 dB, enough to meet emission standards but well above the noise
in the receiver.

V3 IF amp is not used on CW. Pulling it should reduce the feed through.
Taking a 6U8A and clipping off pins for the triode section (1, 8, 9) of
V2 should help cut feed through. Makes for slow mode switching when SSB
is wanted. Grounding the RF input to the balanced modulator should be
effective in reducing that feed through. Loading that point with a
resistor should reduce feed through. I'd try something like 500 ohms to
1K. It will reduce the drive on SSB but I think the balanced modulator
input impedance should be fairly low. But adding a resistor should cut
the stray coupling (common point of C187A/C187B) through the mode
switch.

Making sure the keying voltage goes far enough negative to cut off the
mixers may help too.

Sometime there has been an article I think on making the S-line do QSK,
but I don't remember where now. Probably QST or ham radio, about 1964 or
'5.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer