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

[Geoff]

On 11/19/2011 7:36 AM, Rob Atkinson wrote:
> How are you all doing the sequencing of the relays? rx muting
> (shorting the antenna lead to ground) should be the first thing to
> happen and the last thing to reverse.

Where it me, I'd incorporate a few relays and have them operate in this 
order.

1)Kill the low-voltage in the tube-type receiver(*)
2) switch the Antenna
3) apply grid drive to the final
4) apply B+ to the final/modulator stage

(the last two are probably already being done in the rig, but not always...)

> I am building (if I can ever get the time for it) my change-over
> system on a wood board with velcro on it so I can keep swapping parts
> in and out and keep adding relays as needed.

Always a good idea to get a good working prototype up and going before 
you build the finished product.

73 = Best Regards,
-Geoff/W5OMR


(* = I see lots of people talking about muting the receiver, by opening 
the contacts on the speaker.  Only a couple of people have suggested 
shorting across the antenna terminal to reduce input, but that still 
leaves an active audio circuit.  Removing the low/screen voltage 
effectively makes the vacuum tube receiver inert.  One idea could be to 
open the center tap on the low-voltage supply transformer.  Another way 
to protect the front end is pull the voltage off of the first RF stage.  
PLEASE tell me you're doing something to protect the front end of the 
receiver by -some- method, other than just making/breaking the speaker 
wires.  How long does it take your AVC to recover after you unkey like that?

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