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[Elecraft_K3] Momentary RX quieting when transitioning from TX to RX, but only sometimes...


K Van Horn
 

Dear Joe,
 
This sounds like the classic symptom of a T/R relay sticking, which can be caused by dirty relay contacts(slide a new $1 bill across or between contacts 5 or ten times) or the specific relay starting to fail. If you use Full QSK, especially during a contest weekend, you can exceed the normal relay lifetime as specified by the relay manufacturers in 1 contest weekend, which is why some amplifier and radio manufacturers have tried to use other schemes for T/R switching(like PIN diodes.) This is not a perfect solution either, as a static or lightning strike within 50 miles can damage the semiconductor diodes(like on the Alpha 87 series of amplifiers.) If you already have an amplifier which uses PIN diodes, you should always unplug your antenna from the amplifier when it is not being used. Switching a coax to the “Ground” position is not an acceptable alternative to unplugging the coax from the amp or radio(contrary to what the switch manufacturers tell you.) My guess is that you have a T/R output relay becoming defective, most likely in the amp due to the higher power levels involved. Hope this helps!
 
73, Kirby, k7ec
 

Sent: Thursday, 20 November, 2014 14:42
Subject: [Elecraft_K3] Momentary RX quieting when transitioning from TX to RX, but only sometimes...
 
 

My K3 is typically used as a driver for my SB220. About 20-30% of the time, when transitioning from TX to RX, the K3 receiver becomes momentarily deaf, as if the RX sensitivity is turned way down. Full recovery to max sensitivity is had within about 100mS or so (guesstimate, I don't have an oscilloscope to measure this).

 

I'm concerned that I may be somehow bleeding high RF power into my receiver as the SB220 is coming out of TX mode. If this is the case, then long term reliability of my K3 front end may be in question.

 

Any ideas as to the possible root cause of this?

 

Thanks in advance for any suggestions.

 

73 Joe N9OK


joe_n9ok@...
 

Sorry about the delayed response. Finally had a chance to get on the radio and check this out. My SB220 was given a thorough overhaul last year, including replacing the T/R relay, so this didn't cross my mind.

I ran an experiment, transitioning from transmitting to receiving 40-50x, both with and without RF power, on numerous bands. Result: the problem has temporarily seemed to go away. No definitive results were seen.

Will have to run this test over a period of time. I'll post again if this shows up more.

73 Joe N9OK


Randall K Martin
 

This is a very old topic, but it is the closest match to my problem. One of my K-3's is not returning to full RX after transmitting. I am a contester, and this behavior showed up this past weekend in ARRL SS SSB. It happened earlier this year, but only for a brief time. This weekend, it became consistent. I don't think the amp is the culprit, because if I tap gently on the K-3, the receive signal comes back. If I don't transmit on that K-3 for a while, the problem goes away. Might be heat related in some way.

I don't have a schematic, but assume there is a sealed T/R relay that I need replace. Has anyone in the group done this particular surgery, or do I need to send it to Elecraft?

73 Randy K0EU


Brian K0DTJ
 

Elecraft uses pin diode T/R switching. There’s no relay. I’d suspect a loose coax connector or marginally failed coax as the culprit. GL!

Brian, K0DTJ

On Nov 23, 2020, at 12:59, Randall K Martin <rkmassoc@...> wrote:

This is a very old topic, but it is the closest match to my problem. One of my K-3's is not returning to full RX after transmitting. I am a contester, and this behavior showed up this past weekend in ARRL SS SSB. It happened earlier this year, but only for a brief time. This weekend, it became consistent. I don't think the amp is the culprit, because if I tap gently on the K-3, the receive signal comes back. If I don't transmit on that K-3 for a while, the problem goes away. Might be heat related in some way.

I don't have a schematic, but assume there is a sealed T/R relay that I need replace. Has anyone in the group done this particular surgery, or do I need to send it to Elecraft?

73 Randy K0EU


Keith_WE6R
 

I have seen the center socket of the Ant 1 SO-239 become loose and cause this.
Otherwise it is usually a bad joint in the antenna system.  

If you think it is in the K3, there is no T/R relay, but there are some other relays it might be;
When the RX says quiet, try switching below 12w, change bands, switch to Ant2 or RX antenna.
That could narrow it down.
Keith WE6R Elecraft Tech.


Michael Kopec
 

I have had this problem twice with my K3s/KPA500/KAT500 combo. Both times I traced it down to the PL239/SO239 connection at the antenna Balun. The SO239 center was loose and not making good contact with the Pl239 center pin. de Mike K8NS

On Tuesday, November 24, 2020, 05:21:16 PM EST, Keith_WE6R <keithtrinity@...> wrote:


I have seen the center socket of the Ant 1 SO-239 become loose and cause this.
Otherwise it is usually a bad joint in the antenna system.  

If you think it is in the K3, there is no T/R relay, but there are some other relays it might be;
When the RX says quiet, try switching below 12w, change bands, switch to Ant2 or RX antenna.
That could narrow it down.
Keith WE6R Elecraft Tech.