[TenTec] Hercules II fan noise

[Steve Hunt]

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