wilber@alice.UUCP (01/29/88)
Lenny Tropiano writes: >... I bought two fans, >my next project is to remove the foam in the left side and replace that >with another fan. (Hey, what's a little more hum?) :-) I'll keep everyone >posted. After reading all the tales of woe about fans dying and then the 3b1 going into self destruct mode, I've decided that maybe I ought to put in a second fan myself. The only thing holding me back is a (slight) concern that the little bit of extra power drawn by the fan might push the power supply over the edge. Is there any one with that expen$ive technical manual who can tell me how much power the 67 Mb disk and the various boards use, and how much power the 3b1 can supply? (Okay, so I'm paranoid, but it seems that aside from cooling the other weak link in the 3b1 reliability chain is the power supply.) Alternatively, are there *AC* fans of the right size that could be used? (Such a fan could draw power directly from the wall without going through the power supply.) Bob Wilber {ihnp4, mtune}!gauss!wilber or wilber@research.att.com
motteler@umbc3.UMD.EDU (Howard E. Motteler) (02/01/88)
Concerning adding fans in a 3b1: Fan power is not straight off the 12v supply. I have heard that it is dropped through some thermal sensing circuitry. Or it could just be a resistor. (I have the hardware manuals, but they don't include any info on the power supply.) With the stock fan, I measured (as I recall) about 9-10 volts with an open (cool) box. Due to the (sensor or resistor) drop, if you add more load, the voltage to each fan will drop slighly. The 67 meg miniscribe draws 2 amps at 12 volts (quite a bit...), and I didn't want to add too much extra load. What I did was to buy a PAIR of the radio shack 3" DC fans. These draw a bit less current, about 120 (each) milliamps as opposed to about 200 for the stock fan. I also put a ten ohm half watt resistor in series with each, so both fans are running very quietly on about 8.5 volts. Total air through the machine is about the same or slightly increased, and noise is slightly less. If you need more cooling you could probably leave out the extra resistors, and one new fan in parallel with the stock fan should also be ok, especially if the current power supply is the same as that with the old 2 fan verions. Extra cooling will allow a supply to put out more current, up to a point. As someone else mentioned, if you want really moby cooling, you could replace the DC fan with a 3" AC fan, or even 2 AC fans (available from radio shack or any electronics supply house). This would make more noise, but takes some load off the supply while providing more cooling. Howard Motteler