Hello all,
I am new to this forum and I am looking for some help.
I have a Hammarlund HQ-140X that I use with a Johnson Valiant II.
The Valiant has PTT.
I am using a Dow Key relay for the antenna switching and for muting the receiver.
The problem is that when I go from transmit to receive, the receiver makes a loud annoying pop in the speaker.
Using headphones is impossible.
Anybody have any ideas for reducing or eliminating this noise?

Thanks,
Don W9BHI

Don,

Receivers which interrupt all (or part in the case of the HQ-140X) of the B+ feed tend to pop or make other noise when switching. I prefer biasing receivers into standby via the gain control circuit, look at a Hallicrafters SX-101 diagram as one example of how this works. The SX-88 uses a slightly more complex version which provides an auxiliary sensitivity control which sets gain during standby so the receiver can be used to monitor the transmitter.

You could easily add this method to your HQ-140X by inserting a 10K fixed resistor (or a 10K variable pot if you want adjustable standby sensitivity) between R39 and R40. Bring a pair of leads out that are connected to each end of this added resistor and use them to short the resistor when in receive mode. Set up this way your standby/receive switch would remain in the receive position.

Rodger WQ9E

Are you shorting or opening the speaker leads with this method? If opening I would add a load resistor to prevent excessive voltage from being developed by the unloaded output transformer and I wouldn't use the shorting method with a SS output stage without a series resistor. If the rest of the receiver is still active, then even with the antenna disconnected you are going to have a lot of drive to the audio stage with most receivers; very few have sufficient shielding to not pick up quite a signal while transmitting.

Rodger WQ9E

I'm gonna have to go with "tubenut" on this one.
I've been doing what he suggests for the last 55 years and haven't blown up any audio transformers or anything else for that matter.
I've always left the receiver running as removal of B+ always seems to induce a bit of drift
after you unkey and re apply high voltage again.
I use the extra contacts on my dow key to open the audio to the speaker. I have an 8 ohm
one watt resistor across the audio output at all times just in case. Eight ohms is just enough protection but doesn't seem to effect the audio output to any noticeable level.
Everyone has their favorite and this one works for me.

I did the same thing with the HQ-129X which is essentially the same design. I never used the mute circuit on the receiver. Never had any issues with years of use in that configuration even without a separate load resistor.

Curtis Eickerman

I went with WQ9E's suggestion.
It works great with no noise when switching.
P.S.
Dont' ever ask the same question on more than one forum.
W9BHI