[time-nuts] Replacement fan in SR620

[Charles Steinmetz]

Jarl wrote:

>In my SR620 the fan is a Delta DBF0624H.  It is a 60x60x25 mm fan, 24V /0.11A.

Mine, too.  Does anyone know the airflow rating of the Delta fan?  It 
does not appear to move as much air as it should (at least not if the 
idea is to hold the interior at a fixed temperature) -- every SR620 
I've used has the fan running at full speed by the time it's been on 
for 10 minutes, unless the ambient temperature is less than 17-18C.

On the one hand, it would be nice to move more air so the internal 
temperature is more tightly regulated; but on the other hand, it 
would also be nice if the fan were quieter.  It is unlikely we can 
have both.  The fact that the original fan seems marginal counsels 
against replacing it with one that moves less air.

The thermal design of the 620 is far from optimal.  The thermistor is 
located in a "tunnel" between the interior of the instrument and the 
exterior, with the fan blowing directly across it through the 
tunnel.  So, the fan startup is an ugly process of fits and starts as 
the instrument warms up.  Also, the air inlets seem to be too small 
and the internal airflow was not properly designed to circulate 
cooling air where it needs to go.

I have toyed with the idea of cutting a slot maybe 4" long and 1/8" 
tall in the right wall of the chassis at the rear, above the four 
TO-220 devices mounted there -- and perhaps another slot about the 
same size in the top cover above those devices.  Also, maybe 
attaching some internal baffles to the top cover to channel airflow 
where it is needed.

I installed a terminal strip on the GPIB connector mounting screw and 
relocated the thermistor there in my SR620s (in the general vicinity 
of the oscillators).  (However, note that stabilizing the internal 
oscillator temperature is not really very important for most of us, 
because time nuts generally use an external time base.  In that case, 
it is probably more important that the temperature of the triggers 
and interpolators is held constant.)

With the thermistors relocated as I have described, the fans start up 
as they should (monotonically, speeding up smoothly from stopped to 
full speed without any fits and starts).  They still reach full speed 
in 10 minutes or so, so at the end of the day I don't think I've 
really changed anything except the aesthetics of the fan 
startup.  (IMO, the change is worth it just for that, but there does 
not seem to be any operational improvement.)

Perhaps SRS did not intend to regulate the interior temperature of 
the SR620 -- maybe they just wanted it to warm up faster (if you did 
away with the thermistor and had the fan run full speed whenever the 
counter was on, it would presumably take longer to warm up).

Anyway -- does anybody have an old Delta catalog or datasheet that 
specifies the airflow rating of the original Delta fan?

Best regards,

Charles