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[TenTec] Isolation of diode T/R Switch on Corsair

To: tentec@contesting.com
Subject: [TenTec] Isolation of diode T/R Switch on Corsair
From: Joe White <jwhite001@bellsouth.net>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Sun, 7 Jun 2009 16:35:13 -0700 (PDT)
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
What level of isolation or attenuation should the output diode T/R switch on a 
Corsair I Low Pass Filter Board have?  I've been measuring the one on my 
Corsair by looking at the voltage out at the antenna connector and comparing 
that level to the voltage on the RX ANT jack with the RX/XCV switch in the XCV 
position.  I'm seeing voltages at the RX ANT jack that are 1/1000 of the 
ANTENNA output voltage on 80 and 40 meters, and only 1/165 at 10 meters.  These 
tests were made using a 50-ohm dummy load connected to the ANTENNA connector.

The reason I ask:  

The Corsair I have had a close encounter with lightning before I bought it.  
Most of the damage was repaired, but the original owner missed the damaged 
diodes in the final T/R switch.  Apparently after making the partial repair, he 
attempted to use the radio and, since the output T/R switch was non-functional, 
the output power from the final was fed back to the PIN diode T/R switch on the 
Low Level Driver Board, blowing the two diodes that make up that switch.  That 
was the condition of the radio when I bought it. I've replaced the components 
that I found to be bad (mainly 4 T/R switch diodes and a shunting transistor).  
I have used the radio at QRP levels since then with no problems, but I've 
always been skeptical to run full power at the higher frequency bands, not 
knowing if I was getting sufficient isolation to protect the T/R circuitry on 
the Low Level Driver Board.  

Recently, I cranked up the power and the receiver went deaf, so I thought I had 
damaged things again, but it turned out there was no problem with the radio -- 
whatever caused the receiver to go silent was something else in my setup.  The 
radio still seems to work fine, and I've been testing it on the bench at full 
power.  However, I'm concerned that I have not totally repaired the output T/R 
switch and bad things could happen if I attempt to use it with a mismatched 
antenna.  Can anyone tell me if the results I reported in the first paragraph 
are typical of a properly functioning Corsair?  .. or is there a better way to 
confirm the operation of the output T/R switch?

   Thanks and 73,

        joe, WA4GIR
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