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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
