[Amps] T-R switching

[Steve Thompson]

kg7hf at comcast.net wrote:
> Greetings,
> 
> I preface this with I am not a EE (I write code for a living) ;)   
> 
> With the help of a few books, I have attempted to design and build a circuit, but I am having trouble making it work consistently without destroying various components.  I have at least two problems that I know of. 
> 
> http://home.comcast.net/~kg7hf/Project2/TR_schematic.JPG
> 
> The first is that either T17, or Z1, fail after some time, 30 minutes to an hour which results in the TR switch staying in the keyed position.  
> 
> The second is somewhat of a stranger issue, perhaps they are related.  This circuit is going into a small solid state amplifier circuit 5 - 10 watts in will produce 50 - 100+ watts output.  When the TR is "working", it seems if I excite the amp with 1 to 5 watts, the TR circuit relay RL17 switches on and off extremely rapidly, as if RF were getting into the relay and causing it to switch on/off, this produces almost a buzzer type effect.  As I increase the exciter power above 5 watts, the relay starts holding and behaves normally (until I get an all out failure).  I've tried what I think is obvious, ferrite beads, and 0.1uf cap from the vcc to ground at the relays, still no change in behavior .
Sounds very much like rf is getting into things. I can't see how 
Z1/T17 are being damaged by dc in the circuit if the control input 
is a switch to ground as you show. I suggest you add 10nF disc 
ceramics on the supply line, switching input, relay coils and each 
base-emitter junction.

Other thoughts - it's good practice to have some resistance 
base-emitter on T17 to kill the effect of any leakage. 10k or 100k 
should do. I think you can probably make R5 and R6 higher in value 
and save on dissipation. I'd try 1k and 10k respectively.

Your circuit designing looks better than my code writing :-)

Steve