[ICOM] Pro II vs. Pro III RX-ANT input circuit (was "Smoked ProII")

[Augie (Gus) Hansen]

On 2/5/06 7:23 PM, "Adam Farson" <farson at shaw.ca> wrote:

> On researching this topic a little further, I noted that the 756Pro and
> 756Pro2 RF Unit boards had a fuse bulb (DS111) in the RF signal path from
> the RX-ANT jack to the ANT/ANT-R selection relay. This has been omitted in
> the Pro3. Interestingly enough, though, the Pro3 RX-ANT circuit incorporates
> an RF sensor (D101, Q103, Q104) which operates the ANT/ANT-R relay (RL-101)
> when RF is applied to the RX-ANT socket, thereby breaking the RF-ANT signal
> path. This feature is lacking in the Pro2.

Hi Adam,

After seeing your message I checked the circuit diagram and did a live test
on my newly acquired Pro III to verify the behavior you described. With only
100 watts output, my RX antenna, a low receiving wire that is too short to
qualify as a beverage, didn't cause any problems. At 1KW, the rig (P3 + PW1)
put out a brief burst of energy and then stopped transmitting. After
disconnecting the RX antenna, the rig  was able to put out full power.

The fast-acting circuit did its job of protecting the transceiver front end,
but it is not a substitute for a "front-end saver". You didn't imply that it
is, but I want to clarify this point for anyone using an RX antenna with the
Pro III. The protection circuit is effectively a fast, self-recovering
"fuse".

To be able to operate at high power with an RX antenna connected, one still
needs to use an external relay to at least short the RX input (SPST
contacts), or better yet, open the RX antenna lead and short the input (SPDT
contacts).

Thanks for the tip.

Gus Hansen, KB0YH