492 fan replacement - speed control - update

Edward Prest

Update. I now have installed a pabst 624hh 24 volt fan which runs steady at 22v. I use the -17v and +5v lines with a 100uh filter. No speed control. I have other rf issues so I didn't check much for interference. The instrument gets warmish after a while and the power supply itself at the rear runs almost cold. It's good enough that it cools properly now and I can wait a while longer for it to warm up compared to the original fan. The power supply got many original caps replaced and new thermal compound on the heat sinks.

The fan circuit allows for a rack mount fan and provides 9v to 13v depending on temperature to a unmarked 3 pin connector. This line runs the original 3 phase motor drive circuit. This control may work well if using a 12v fan but with 24 volt fan it's way too slow. The connector for rack mount fan is buried in the fan controller pcb and is unmarked.

The Pabst brushless fan is specified for 18 to 24v but it still runs at 9v so it's very easy to throttle. At 22v it's not too noisy but you hear it clearly.

Edward Prest

FYI With a room temp of 22C and almost an hour warm up I measure the back power supply and sides of the instrument temperature around 31C. The bottom where the air exits is 37C. This was measured with a hand held IR meter.