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

[Larry Benko]

Hi Gus,

While the Pro3 protection circuitry did the job in a single transmitter 
environment it unfortunately won't help if another radio in your shack 
is the transmitting and going to damage the Pro3.  Even an external 
relay won't help in the case of a second transmitter.  In that case you 
need some external circuit that is not based on the Pro3 transmitting 
for protection.  Heck in the case of a second transmitter being the 
problem both the standard antenna and the RX antenna inputs are 
susceptable.  Of course this is why contesters use single band filters 
external to radios and 1/2 and 1/4 wave coax stubs.  What I wish Icom 
would do is to publish a maximum spec that will not cause damage such as 
+30 or +35dBm.

73,
Larry, W0QE

Augie (Gus) Hansen wrote:

>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
>
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>  
>