Thanks to all those who took time to offer suggestions and advice.
Picking things up in no special order -
(i) the Herc II has to key up in around 5 ms. On published specs that
rules out open frame relays, even if there is enough space (there
is'nt) because its not practicable to generate the over-voltage drive
needed to get a really fast closure. The thing has to go over at 12v.
(ii) there seems to be a slight divergence of opinion, but I reckon
from the manual that its not switching hot. The interlock circuit is
supposed to prevent the OMNI firing RF into the Herc until it sends an
OK down the control wire TX enable. It does that after a delay timed
to ensure the Herc relays have closed to the TX state. If it hot
switched 5 amps of RF I guess it would wreck the vac relay pretty
fast (and what would the arcing do to the keyed signal ??)
(iii) the RF voltage is not that high as long, as the SWR is normal
(say below 2:1). Relay insulation should not be a problem as long as
there is no tracking and a reasonable inter-lead space on the relay
(iv) yes, thanks. I will definitely check Surplus Sales of Nebraska.
I have their catalogue from Dayton last year. Maybe I'll get lucky
and pick up a spare at Dayton this year.
(v) degaussing sounds neat; I've no idea how to do it with a tiny
ceramic metal vacuum relay. But looking at it, I do not think its a
magnetic reed. I go along with the best guess that the internal
seal failed and that the thing sticks, more or less at random.
TT suggested mechanical stimulation ("try hitting it with something").
I did just that with a sealed solenoid gas valve in our central
heating unit, and it worked quite well for a while. I used to get up
early and thump it with a two pound hammer, then go back to bed until
our home warmed up. Now I can't really see that as even a temporary
cure for a relay that sticks at random in the middle of a QSO.
"QRX whilst I hammer my relay ...".
(v) I still like the 12v Herc II, but I miss the glow from those
811-A's. Now the open frame surplus 28v relay in that amplifier
would start a truck, but fast it is not. Strictly PTT. I can always
lug it and its 1500v power supply down from the attic, I guess.
Thanks again - John G3JAG/QRP
Consultant in product forgery - Asia and
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