[AMRadio] Protecting receiver (was WTB: Coax relay, 110VAC, aux contacts)

[Rick Poole]

At 08:10 PM 11/18/2011, Don Cunningham wrote:

 >>>>>
>Take a look on Phil Salas' website (don't have the URL in my head, but
>Google his callsign and you'll find it).  Scroll down his "articles" to "A
>T/R Switch for your Boat Anchor Station".
<<<<<

Good morning, Don.

Yes, I saw that, and it is a very neat way to do it, unfortunately it 
will not work with my receiver.

The Hammarlund HQ-180A does not do muting by completing a ground; the 
mute relay contacts actually complete the B+ to the RF amp, the 1st 
mixer, and one other stage (don't have the manual right here in front 
of me).  The manual doesn't discuss it in the text (all it says is 
"The muting connection is compatible with the Hammarlund HX-xx 
transmitter" as though no one would ever think of using a Hammarlund 
receiver with someone else's transmitter) but from the schematic, it 
appears that if you want to use the muting function, you put the 
Send/Receive/Cal switch in the Send position, then when the muting 
relay is open you are muted, and when it is closed you can 
receive.  Putting the Send/Receive/Cal switch in the Receive position 
bypasses all of that and your receiver is active full time.  At least 
that is my interpretation of the schematic, which is not easy to read 
or follow in some places.

This relates back to something someone else said about being exposed 
to 110VAC on the relay coil connections, a shock hazard... with the 
Hammarlund, the actual B+ to the first few receiver stages is routed 
out the accessory connector and back in again, posing yet another 
shock hazard since it could be easy to get across that at the wrong time, too.

Now I'm wondering if removing B+ from the RF Amp stage, and putting 
back to back 1N4148 diodes in parallel across the receive antenna 
connection, is enough protection for the receiver front end, or do I 
have to go find a 4PDT 110VAC relay?  In particular, the 1N4148 
diodes clamp the input signal to no more than 0.6 volts peak to peak; 
is that voltage level enough to damage anything?

Rick WA1RKT