[Hallicrafters] Xmtr noise getting into Rcvr

[Roy Morgan]

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


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Cell 301-928-7794
Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan at nist.gov --