Well, the new fan came today - an Orion, would you believe. I soon had
it fitted. Result - slightly noisier than the original one - oh dear!
While checking everything over with the new fan I ran the Herc up to 400
Watts output. I was alarmed to find that after 10 seconds or so the fan
speed began to drop, and it would have stopped completely if I'd left it
any longer. I began to suspect the speed control circuitry.
I powered up the speed control circuit from a bench supply and began
looking around with the 'scope. The drive to the fan was a DC level with
"spikes" on it at the rotation speed. The spikes didn't surprise me, but
the RF oscillation at some speeds did. I tried decoupling various parts
of the circuit but nothing fixed it.
So then I put the original fan back on - no RF oscillation. So it looks
like there's something about the load presented by the Orion fan that
the driver circuit doesn't like.
I adjusted the speed control potentiometer so that the control loop was
just inactive with the amp cold. Then I checked, by putting my soldering
iron close to the fan control diodes, that the fan speeds up when they
get warm. Then I put it all back together ... without the finger guard,
which makes it a lot quieter.
The combination of slightly slower idling speed and the absence of the
finger guard has made a definite improvement.
Oh, remind me about that missing finger guard before I next reach under
the amp to retrieve my pencil, will you ;)
73,
Steve G3TXQ
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