On Jun 28, 2011, at 5:57 PM, Tom Norris wrote: > > On 28 Jun, 2011, at 11:27 AM, Jeff Walther wrote in reply to poopoopoopoo's > msg: >> >> On Jun 27, 11:56 pm, poopoopoopoo <[email protected]> wrote: >>> I picked up a very clean Mac Plus with all the trimmings - 40mb CMS >> >>> I did a bit more housecleaning before I reassembled it. It booted >>> right up but after awhile now it randomly reboots. I am beginning to >>> suspect bad RAM. >> >> Most likely the power supply is marginal and every time an event draws >> a little too much power the voltage dips below the level that will >> support the CPU, and it resets. >> >> See this document for details: >> >> <http://68kmla.org/files/> >> >> In brief, you'll want to measure the 5V supply with a voltmeter and if >> low, measure the 12V supply (they're linked rather than individually >> regulated). If they're both low, then you can adjust the internal >> pot. to raise the 12V back up to nominal and get the 5V back up as >> well. If the 12V is nominal and 5V is low then some components need >> replacing. If adjusting the pot. won't bring the voltage back up to >> nominal, then some components need replacing.
A common form of this problem is the computer starts up and when it goes to boot from the floppy it restarts, due to the floppy loading down the power supply too much and triggering a protection circuit that forces a reset if the 5V supply goes too low. You need to adjust the 5V supply till it is 5.00V. The 12V supply isn't very critical on voltage but the 5V is. There are three axial lead (leads on opposite ends) electrolytic capacitors all together at the center rear of the logic board. IIRC, one is +12V, one is +5V and one is -5V. Touch the (-) lead of a voltmeter to the metal chassis and check the + lead of each capacitor until you find one showing +5V (more or less). Then connect the meter across this cap and tweak the power supply adjustment pot on the analog board until it reads 5.00V. > Clark Martin Redwood City, CA, USA Macintosh / Internet Consulting "I'm a designated driver on the Information Super Highway" -- ----- You received this message because you are a member of the Vintage Macs group. The list FAQ is at http://lowendmac.com/lists/vintagemacs.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To leave this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/vintage-macs Support for older Macs: http://lowendmac.com/services/
