[Elecraft] K3 KPA3 Fan Upgrade Question

[Guy Olinger, K2AV]

Hi, Joe,

The 37 C (98 F) you quote does not seem the least bit unreasonable.  IMHO it 
seems quite mild.  That's 85 degrees Farenheit below the thermal  safety 
cutout temperature.  That's coffee thats gone cold to a lot of people.

The manual states that the PA will drop out on safety when the PA temp 
reaches 84C (183 F) .   That's hot water temp in bygone days before people 
started suing tank makers for getting burned when they turned on hot water 
only and stuck their hand under it. Coffee temp before that lady sued 
McDonalds and won.

 My ambient room temperature is 27 C today.  65 F you quote is a bit chilly 
for me sitting down without a jacket for any period of time. So my numbers 
run a little higher than you would get in your chilly room.  Thought I would 
do an experiment along your lines:

I zero beat a broadcast carrier on 40m where my SWR is 1:1 and hit the tune 
button putting out 50 watts, and just left it on steady (brick on key).  The 
PA temp settled at 60 C (140 F) after 3 minutes with fan at speed 3 which is 
an entirely safe running temp. The exhaust air seemed only mildly warm. 
Coffee cooling down but OK.

Just to see what would happen I ran the power up to 100 watts and did the 
same thing -- 100W brick on key, watching very carefully for a temp runaway. 
Had never tried that before.  Temp settled at 70 C (158 F) after 3 minutes 
with fan at top speed, and the air was decently warm. Nice coffee temp.  I 
HAD thought that 100 watts steady might trip the temp cutout, but it was 
nowhere near 84 C.  25 degrees F below the cutout temp.

When in CW contesting doing runs at home, driving my 3-1000 with 50-55 
watts, I find that the fan is running on its lowest speed and the PA temp 
around or below 40 C.  I would say this is quite like your experience.  When 
driving a band filter to a 3CX1200A7 running 110 watts for a 48 hour CW 
contest out at the multi-op I get upper 40's C with fan speed 3. I figure CW 
contesting is like a 40% duty cycle at highest, or 110 times .4 = 44 watts 
overall, plus some extra degrees C for a warm room with 4 amps going.

I would say that your new fan's inability to come on at the lowest fan speed 
shows that it does not qualify for operation in the K3.  That is regardless 
of their published CFM spec. It is entirely possible that the temperatures 
will actually run higher with the replacement fans in the K3 during moderate 
operating because they are not spinning or not obtaining the RPM for the 
next air speed at the set voltages and will only deliver the higher cooling 
when the K3 is calling for fan speed 4.

The K3's low fan speed voltage will have been calibrated to desired air 
movement with the particular fan stock in the K3, as all DC fan motors do 
not start to move at the same voltage. Google "DC motor deadzone".  Using 
the stock settings for the fan ( PA nor) will also minimize bearing wear, 
particularly that they will not turn at all during long periods of listening 
only.

73, Guy.


----- Original Message ----- 
From: "Joe Planisky" <jplan at jeffnet.org>
To: "elecraft reflector" <elecraft at mailman.qth.net>
Sent: Wednesday, May 13, 2009 11:25 AM
Subject: Re: [Elecraft] K3 KPA3 Fan Upgrade Question


> Apparently not all K3s are created equal, thermally speaking.
>
> On my K3, it takes about 2:45 of 20 wpm CW at 50 watts into a dummy
> load to raise the PA temp from 18C to 37C (the fan turn-on temp.
> Ambient temp is about 18C.)
>
> 73
> --
> Joe KB8AP
>
>