All of a sudden my macintosh plus starts rebooting, upon startup or even sometimes randomly when its idle...but mainly when it uses the floppy drive I think. I got it to start fine for a while, now sometimes I cant even get it to go through the boot process. I heard it could possibly be the power supply, but I'm kinda afraid, and clueless on how to open it. :-(
Rebooting on floppy drive startup is caused by the voltage regulator drifting. The +5 volt power supply has drifted down to something between 4.5 and 4.75 volts or so. When the drive starts up the additional load drags the "+5V" supply too low and a reset circuit is triggered. Normally this reset circuit is there to keep the logic in reset until the power supply voltage reaches a proper operating voltage. But it also resets the circuitry in the event the voltage drops low to prevent an undervoltage condition from causing a malfunction that could corrupt data. When it resets the floppy drive turns off, the power supply voltage returns to an adequate level and it reboots.
The fix is fairly easy but you have to know what you are doing.
Take off the rear case.
Disconnect the cable to the floppy drive.
Attach a voltmeter to the +5V line.
There are several axial lead electrolytic capacitors on the logic board near teh power supply. At least one of these has 5V on it, this is the easiest place to connect the meter. Make sure however you have the +5 and not the -5 line.
On the analog board is a hole that I believe is marked with the DC symbol, a horizontal line with a dashed horizontal line below it. This is a potentiometer for adjusting the +5V regulator. Insert a non-conductive tweaking tool into this and slowly adjust it bring until the meter reads 5.00 V.
Reconnect the floppy drive and see if it boots.
If you don't understand the instructions, find someone who does. Doing it wrong could damage something. And there is high voltage circuitry exposed while working inside so you don't want to touch the wrong thing.
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