[Hallicrafters] SR-400A Preselector tracking problem

[Roy Morgan]

On Jan 27, 2010, at 3:00 PM, Alexander N Gerli Sr wrote:

> Hi, gang!
>
> My SR-400A is working, but the Preselector doesn't track properly.  
> Tx and rx
> do NOT peak at the same setting, as I believe they should.

Sandy,

I find a similar thing when I use an antenna tuner with my (rice box)  
transceiver.  The tuner is a Millen 92201 Transmatch Junior, but that  
likely does not matter.  I get a louder received signal with different  
settings than the settings that give me lowest SWR during transmit.

My guess is that the receiver input circuit is not resistive at 50  
ohms. This is assuming that the transmitter works best (shows 1:1 SWR)  
when the tuner presents a resistive 50 ohm load to the transmitter.  I  
have not measured the impedance at the input to the tuner when it is  
tuned for 1:1 SWR. That would tell me the output impedance of the  
transmitter.

Conjecture and guesses aside, we do know that HF receivers do not  
often present a resistive 50 ohm load to the antenna feed line, may be  
50 ohms at some frequency but usually are not over the rest of any one  
band.  The R-390A receiver specification calls for input impedance  
from (something like) 25 ohms to 250 ohms (might be off on this..   
from memory.)  I would not try to guess what the SR-400A actually is,  
though it would be an interesting exercise to measure it.


Roy

Roy Morgan
k1lky at earthlink.net
529 Cobb St.
Groton NY, 13073
Home: 607-898-3607
Cell: 301-928-7794