[AMRadio] Receiver muting

Donald Chester k4kyv at charter.net
Thu Jan 9 13:10:20 EST 2014

> Most of the receivers I have use mute by opening the ground to the RF gain

> pot, a reason I'm not a fan of Hammarlund. It is easy to soft mute them 
> with just a pot across the open ground.  The BC348 opens B+ to the 
> converter, it would have been nice if the 'Test' button on the 375 keyed 
> the TX without muting the RX.  Has anybody tried putting resistance 
> across the mute line to let the convertor work just a little bit so you 
> can hear a sidetone?  ...While just calling CQ is not a problem, 
> answering one is. Anybody have an easy solution to zeroing the TX freq. in

> a situation like this?   You can be sure nobody tunes around looking for 
> an answer to their CQ anymore, most have a fit if you are not dead zero 
> beat.

I have always found the easiest way to mute a receiver is to break the audio
line to the speaker or headphones during transmit, using a relay connected
to the main T/R system. You want to make sure the AF output stage remains
loaded to avoid blowing the output transformer, however. The surest way to
do this is to short out the speaker line, rather than simply opening it.
Just make sure the  relay contacts are low resistance enough to do the  job.

For me, that isn't an issue, since I use an outboard audio amplifier with my
receiver; it is easy enough to short out the low level audio line between
the receiver and the amplifier with about any kind of relay contacts.

One problem with killing the B+ as is done on many older receivers, like the
pre-War Nationals, is that the HF oscillator and BFO are powered down and
tend to cool down during your transmission, and then when you come back to
receive, the receiver is off frequency and you have to wait for it to  drift
back. Not a problem with AM, but with CW and slopbucket that poses a

My CW sidetone oscillator went out on me several years ago. I tried monitor
keying by disabling the receiver mute, but that was unsatisfactory, because
the audio levels of the signal picked up from my transmitter and that of the
other station were inevitably far different, requiring me to fiddle with the
gain control each time. Plus, it didn't work on the rare occasions that I
didn't operate zero-beat with the other station. I tried using my R-1000
general coverage receiver, with a short piece of wire for antenna. That
worked great for calling CQ, but when answering another station, it was a
cumbersome, confusing mess to try to zero-beat my VFO to the other station
AND tune the R-1000 for a satisfactory side-tone in time to answer the
station. I could imagine trying to work a CW QuaRMtest that way! The result
was that I almost quit operating CW, since I find it hard to send with the
bug without monitoring my keying. I finally fixed the sidetone oscillator,
after finding on the internet the schematic of the HAL keyer I was using for
side-tone. The keyer itself never worked  worth a crap, so I stripped off
all the keyer logic stuff from the PCB, fixed the power supply regulator
problem, and used a makeshift  logic probe to find the dead IC that was
preventing the side-tone from working. Evidently, a lightning surge had
toasted the HAL even while everything was unplugged from the power mains.

The 75A-4 has a built-in mute function, but I never could get it to operate
to my satisfaction, so I don't use that.


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