Can't be a short in the cable (my first suspicion), so I'd say it's the RX
front-end breaking into oscillation when it's looking straight into the
duplexer. Try a 3 dB pad on the RX input & see if that keeps the RX
sensitivity stable when fully connected to the duplexer (minus 3 dB of
course). Another option, if you happen to have one lying around, is an
isolator. Just connect it to the RX in the reverse direction that you'd
normally connect it to the TX (input side towards duplexer, output towards
RX). That will keep the RX seeing 50 ohms everywhere. I had to use that
method on a Kenwood TM-742 that wouldn't behave below 440 MHz (front-end
oscillation) when a pass cavity was put in line.
Bob NO6B
At 3/21/2005 05:25 PM, you wrote:
>OK, this is making me nuts. I have changed jumpers about 3 times
>thinking I have a cable problem but I am convinced it is not the
>cable. Here are the symptoms:
>
>220 Hamtronics receiver (very old, no labeling on the board). Tunes
>great on the bench (.15uv 12db sinad). Installed into rack still
>tests great. Hookup to the duplexer and sensitivity goes to crap
>(1.5uv).
>However, if I back the PL connector (I know, not my repeater, just
>helping) on the cable to the rx out of the T connector on the
>duplexer, the sensitivity improves dramatically. As soon as the
>ground on the connecter touches the duplexer the signal dies.
>
>I have done this without the tx transmitting but connected and
>disconnected with no difference. My service monitor is hooked up to
>the t connector on the antenna port of the duplexer.
>
>I could see if this happened with the antenna connected to the
>duplexer. I would guess it is overload from other rf desensing the
>receiver. But this is happening with the service monitor hooked to
>the duplexer.
>
>BTW, this is a 4 can tx/rx duplexer that was sent to tx/rx to be tuned
>and it came back doing the same as when it left so I don't think it is
>the duplexer.
>
>I don't know enough about this but, could this be a problem with the
>receiver and how it couples with the duplexer? I hate to order a new
>receiver without being relatively sure that it is the problem.
>
>Thanks for any guidance!
>
>
>
>
>
>
>
>
>Yahoo! Groups Links
>
>
>
>
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