Try moving the finger guard further away from the fan. A small increase in
spacing is often a big help.
----- Original Message -----
From: "Steve Hunt" <steve@karinya.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Thursday, August 28, 2008 12:57 PM
Subject: Re: [TenTec] Hercules II fan noise
> 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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