On 2/6/02 10:15 am, "Glen Niewand" <[EMAIL PROTECTED]> wrote:
> I have a 8500 that has a blown powersupply. I have found the problem, it is a > blown disc cap. Does anyone know where I might get a copy of the scematics > for these things? Or does anyone mind looking inside their powersupply to > find out what size my cap would be?? I've repaired quite a few, but never found a disk cap short. There are no schematics available. Is the defective device in a sleeve possibly sealed with white silicon rubber, and is adjacent to the 2 large black reservoir capacitors, possibly 470 or 680 uF 200 or 250V? I ask as a common fault I see is the PSU autoswitching to 115V and blowing 2 disk metal oxide varistors and the internal mains fuse. MOV's look similar to disk capacitors. Each reservoir capacitor has a MOV connected across it, check the print underneath to verify. If so these are 150V AC 200VDC types, they conduct and short at 200V DC to protect the inverter from overvoltage. If this is the case and you are in Australia and never likely to use 115V, just unsolder the triac on its heatsink that connects to the centre of the 2 reservoir caps and replaced the MOV's and fuse. Don't get muddled up with the similar triac that shorts out the surge limiter resistor after start up. Other things to check. If there are 47K resistors across each reservoir capacitor, check these with one end lifted. If one is open it will cause a misbalance and can cause the MOV's to blow. Lastly, check the 2 reservoir capacitors for a solid bulge in the top, but ideally with an ESR meter. If one has gone high ESR/low in value it too can cause a misbalance. >From memory there are 2 manufacturers of the PSU, the internal designs are completely different. My notes cover the basics of both. Remember the 2 reservoir capacitors can under some fault conditions store around 170 V DC across each, so take care and discharge with a bleed resistor before getting too involved. If the MOV's are shorted this is the result of a fault, not the actual fault. If you are unsure of the correct safety precautions and any fault finding techniques or spares suppliers, replace the PSU with a used one or an exchange PSU. Steve Bell -- PCI-PowerMacs is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Refurbished Drives | -- Sonnet & PowerLogix Upgrades - start at $169 | & CDRWs on Sale! | Support Low End Mac <http://lowendmac.com/lists/support.html> PCI-PowerMacs list info: <http://lowendmac.com/lists/pci-powermacs.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/pci-powermacs%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
