[Hallicrafters] Xmtr noise getting into Rcvr

[Jim Brannigan]

Also from "The wayback machine"
Electronic TR switches had a tendency to cause signal suckout in the
receiver.  The transmitter output circuit would act as a trap.
The solution was to try various lengths of coax to minimize the condition.
QST's of the 50's and early 60's have many articles on TR switches.

Jim


> At 10:13 PM 4/14/2005, J. Steven Cochrane wrote:
> >...Hallicrafters HT-46, SX-146, R-48 and an E. F. Johnson T/R Switch.
> >...the SX-146 starts receiving a 20 db over nine noise,
>
> Steve,
>
> The following is from memory and advice passed on from others. Even though
> I have a couple of the Johnson T/R switches here, I have not yet put them
> into operation.
>
> The noise is coming from the finals of the transmitter through the T/R
> switch.  To confirm this, turn off the transmitter or as you did turn it
to
> standby to see if the noise goes away.  Likely there is nothing wrong with
> your T/R switch.
>
> The cure is either:
>   - do not use the T/R switch
>   - Bias the transmitter finals really off during receive.  You have to
> modify the transmitter at least a little bit to do this.
>
> Last night I was reading an ARRL Hints and Kinks, I think from the
> 60's.  It contains at least one scheme to solve this problem if I remember
> right.  The QSK fellows back then really worked on this problem and had
> many solutions to it.  I think one scheme is to add a clamp tube to the
> transmitter if it does not have one to pull the screen voltage very low
> during receive.
>
> You may note that in the Johnson T/R switch instructions, it says to use
as
> short a coax as possible between it and the transmitter.  I'm not sure why
> this is, but folks who've used T/R switches report that it is good advice.
>
> You will notice that the Johnson T/R switch runs VERY hot.  A small fan
> will solve this and make those few parts in there last longer, especially
> the power transformer.
>
> The thing has a selenium rectifier if I remember right, and replacing that
> with a modern diode (e.g. 1N4007) may cure some noise.
>
> >  4. The receiver has great sensitivity and selectivity except when the
> > transmitter Operation Switch is placed in the USB or LSB position.
>
> I am not familiar with the HT-46, but I expect that the final is biased
> well up into standing plate current to establish linear conditions in the
> final.. the final tubes are acting as strong noise generators in this
> condition.
>
> >... possibly adjusting the T/R switch (if there is any adjustments) which
> >may be over amplifying any noise generated in the finals of the
> >transmitter and allowing that to get into the receiver when in the
receive
> >mode.
>
> The T/R switch has no adjustments. You might want to check all ground
> connections inside it, and *solder* any terminal strip ground tabs and
tube
> socket ring ground tabs to the chassis.  This will take a LARGE solder
iron
> - the Weller pencil won't do it!
>
> >Has anyone had this problem and know a cure, preferably a simple one?
>
> Yes, many folks have solved it, but it's like elephant soup. It sounds
> simple but may not be:
>
> Elephant Soup:
>
> Put some elephant meat in a pot with vegetables.
> Simmer 'till done and season to taste.
>
> Roy