[Amps] vacuum relay failure

[R.Measures]

On Nov 22, 2004, at 7:02 AM, Pete Smith wrote:

> I modified my SB-220 about 5 years ago with vacuum relays on the input 
> and output.  It has had a lot of hard contest use since then without 
> incident, but this weekend, after about a half hour, I noticed that I 
> was intermittently getting no output from the amp, even though the 
> transceiver looked like it was putting in normal drive.  It seemed to 
> change from one switching cycle to the next -- sometimes it would go a 
> dozen QSOs with no problem, but on the 13th nothing was getting out.
>
> About an hour after this started, the input relay (a Hi-G d'Italia 
> surplus job) started not closing intermittently, leaving me with no 
> drive either, so I shut the thing down altogether.
>
> I have replacement relays on the way, but would like to tap the 
> experience on the list -- does this sound like just contact failure,

Yes, Pete.  A vacuum relay equipped amplifier that is used with a 
modern transceiver is at risk of hot-switching if it does not make 
faster than the transceiver.   Cu-contacts are especially vulnerable to 
erosion from hot-switching.  Since modern transceivers typically begin 
to produce RF in 5 -  8mS, only a few vacuum relays are capable of 
switching fast enough -- and even then, a speed-up circuit is needed to 
initially overwhelm the L in the relay's coil.  Satisfactory relays for 
this job are the Jennings RJ-1A and the Kilovac HC-1.  In order to get 
enough speed-up, 3 - 5x the coil V needs to be available during make.
http://www.somis.org/D-a-07B.jpg

> ...

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org