[Top] [All Lists]

Re: [Amps] T-R switching

To: <>, <>
Subject: Re: [Amps] T-R switching
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Thu, 30 Aug 2007 10:13:58 -0400
List-post: <>
> 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.

You might have a wiring error, but you have also broken 
basic rules of interfaces. You don't have any significant 
impedance (resistance) in the path from the external switch 
wiring through the zener to the base of the transistor. You 
also do not have bypassing.

You don't have a pull-down at the base of T17, and have no 
bypass capacitor. It's a perfect diode detector with a very 
low impedance to your external SW SPST2 and TP17.

All of these are major problems.

> 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 .

Try bypassing at semiconductors (around the B and E 
junctions) and using decoupling resistors.

There are other issues. Look at the resistor size of R5 and 
R6. 90 ohms (which is a weird value) is far too low. Why 
would you want to have ~100mA of base current on T1???

You also depend on T17 saturating and having the collector 
of T17 drop below .6 or so volts to turn off T18 and  T1. 
Not good. Looks like the design considers all components 
perfect parts and no noise on lines.

73 Tom

Amps mailing list

<Prev in Thread] Current Thread [Next in Thread>