(totally unrelated) Does anybody have the time or patience to rebuild dead PSU's? If so, I have a few laying around that you could have. I've been kind of hesitant to throw them away, just in case I find such a person.
On 8/19/05, Jeff Walther <[EMAIL PROTECTED]> wrote: > >From: "Powermac" <[EMAIL PROTECTED]> > >Date: Fri, 19 Aug 2005 01:43:44 -0400 > > > > > >----- Original Message ----- > >From: "Jeff Walther" <[EMAIL PROTECTED]> > >To: "Vintage Macs" <[email protected]> > > >> Although, as old as that machine is, anyone who wants to keep one > >> running probably needs to learn the basics of power supply repair--at > >> least enough to replace the capacitors every decade or so. > >> > >> Jeff Walther > > > >So what are the basics for PS repair (is there a PDF or book on the > >subject)? > > I wish that I knew. :-) I can replace most components skillfully, > but have few skills in diagnosis. > > In most cases, it seems to be that power supplies fail from worn out > electrolytic capacitors. So a shotgun approach that often works is > to replace all the electrolytic caps in the thing. Also, look for > discolored (from heat) spots and bad solder joints. > > I think that the way a technician would diagnose such a unit is to > develop a basic understanding of what voltage is expected where in > the PS either by having a schematic or working one up by examination. > Then start measuring voltages either from teh input towards the > output or vice versa. > > If you start at the output, you'd measure the output to see if it's > correct. If not, move back behind the last stage of components and > see if the voltage has the proper form there. Keep working backwards > until teh voltage has the proper form for that stage. The failure is > probably in front of that stage. > > If you start at the input, you'd follow the AC current in, make sure > it's reaching the first component properly and then work your way > forwward as in the above, until you find the stage where the voltage > is not as expected. > > Of course, you need at least a multimeter that can handle AC and DC to do > this. > > Before the advent of switching power supplies, a typical power supply > would have a transformer as the first component. That is a bunch of > coils of wire around an iron core. 120VAC would go in and a lower > voltage such 18 VAC would come out the other end. Or possibly it > would have two or more taps such that 18 VAC and maybe 8 VAC would > come out. > > Then there would be a rectifier stage to convert the AC to a rough > DC. This DC would have a lot of level variation in it. > > Then there would be a filtering stage to smooth out the DC into > something useable. This was usually done with a bunch of capacitors. > > There might also be a solid state DC-DC voltage regulator in there to > really nail the power output. And there could be filter elements > included in any of the stages. > > On such old style power supplies its fairly easy to know what to > expect and to measure the levels to see if they look okay. > > Switching power supplies are somewhat different animals and frankly, > I don't remember the little that I read about them. IIRC, they use > a solid state component that takes a higher than desired voltage and > switches open and closed very fast. The duty cycle of this switching > depends on the relationship between teh desired voltage and the input > voltage. Then the output is smoothed out with some filtering > (capacitors again, I think) and a lower voltage is achieved. > > I'm not sure how you'd check that the switching component of such a > supply is switching properly. It's output should look something like > a fast square wave and I guess you'd need an oscilloscope to check > it. Of course, if the input to the switcher is good, and output is > bad, and you've already replaced the filter, then the switcher is > probably the place to look? Like I said, I just don't know enough > about diagnosis to be confident. > > Jeff Walther > > -- > Vintage Macs is sponsored by <http://lowendmac.com/> and... > > Small Dog Electronics http://www.smalldog.com | Enter To Win A | > -- Canon PowerShot Digital Cameras start at $299 | Free iBook! | > > Support Low End Mac <http://lowendmac.com/lists/support.html> > > Vintage Macs list info: <http://lowendmac.com/lists/vintagemacs.shtml> > --> AOL users, remove "mailto:"; > Send list messages to: <mailto:[email protected]> > To unsubscribe, email: <mailto:[EMAIL PROTECTED]> > For digest mode, email: <mailto:[EMAIL PROTECTED]> > Subscription questions: <mailto:[EMAIL PROTECTED]> > Archive: <http://www.mail-archive.com/vintage.macs%40mail.maclaunch.com/> > > iPod Accessories for Less > at 1-800-iPOD.COM > Fast Delivery, Low Price, Good Deal > www.1800ipod.com > -- Vintage Macs is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Enter To Win A | -- Canon PowerShot Digital Cameras start at $299 | Free iBook! | Support Low End Mac <http://lowendmac.com/lists/support.html> Vintage Macs list info: <http://lowendmac.com/lists/vintagemacs.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[email protected]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive: <http://www.mail-archive.com/vintage.macs%40mail.maclaunch.com/> iPod Accessories for Less at 1-800-iPOD.COM Fast Delivery, Low Price, Good Deal www.1800ipod.com
