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Re: [Amps] Cooling 77SX

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Subject: Re: [Amps] Cooling 77SX
From: "Paul Christensen" <>
Date: Sat, 7 May 2016 11:15:31 -0400
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> As it sits now, the fan is always on at the maximum amount and other 
> than when I'm on RTTY, the air temp from the exhaust rarely gets over 
> 110 degrees.


It might not seem obvious, but the 77's blower runs at approximately 75V and
its speed can be adjusted with the sliding tap on R4, a 75-ohm/25W
adjustable WW resistor.  The blower is in series with the 50V transformer
winding in a bucking configuration.  There's a benefit here in that as the
amp demands more current, blower speed actually increases.  

I would not substantially alter the current blower configuration.  I suppose
you could try adding a relay that activates simultaneously with the amp key
line such that the new relay's contacts either short across R4, or in the
alternative, the contacts engage another WW resistor in parallel across R4.
In any event, be careful not to reduce blower speed below the required air
flow volume needed for the 8877.

Paul, W9AC

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