Thanks Web - I have now tried the idea and it works. As you remember
my Hercules II was intermittently producing no RF output, only a high
SWR, so I needed to find out if the vacuum relay coil was getting any
drive voltage. If the drive was there then the relay must be sticking.
I needed to monitor the relay coil with the Herc in its normal
location, and without drilling holes. It was rather easier than I
I disconnected the Herc II speaker lead at the rear panel phono
connector. I then tack soldered a 1k resistor to the positive coil
lead on the vacuum relay and routed a wire from the resistor to the
now vacant phono connector. Then I had to make a rush trip to the LED
mine because none of my junk box LEDs would work. I hooked a new LED
to a few feet of shielded cable and a phono plug. I connected a
15 amp supply (that is enough to test the Herc without RF drive) and
keyed it up. The LED followed the keying. Great ! I could now put the
covers back and re-install the linear in the shack.
Sure enough, after firing up I saw the LED following the keyer, but
the RF output varied from normal to zero, the zero being signalled by
a high SWR. The relay coil drive volts are there, but the relay does
not always follow.
So the relay is kaput ?? I phoned TenTec. Yes, they have seen this
before. Yes, the vacuum relay has probably gone soft and is sticking.
Yes, they will send me a new one. That will be - wait for it -
$137.41, plus airmail. OUCH ..........
There MUST be a cheaper solution. The relay does not hot switch; it
only needs to close very fast and carry around 4 amps of current after
closes. It is not in a high voltage circuit. I have a catalogue
showing tiny 6 and 12v PC mount relays that will handle 2 amps
and close in well under 5 ms. They are $3 each and are good for over
100k operations. Maybe two in parallel would do the job - the
available relay drive current would be more than enough to zap two at
once. I guess its time to get brave ....
On 20-Feb-99 Web Williams wrote:
> What a wonderful idea! Never thought of it! Thanks for coming
> upwith a
> useful service tip- you might want to forward it to one of
> those ham magazines who pay for good hints- bet you'd make a
> few unexpected pounds! 73, -Web in Myrtle Beach, SC USA
>> The best idea I have is to temporarily
>> route some (thin) coax to the relay coil and monitor the actual
>> relay keying voltage with a meter or LED.
Consultant in product forgery - Asia and
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