reforming PS caps w/primary-side components

From: Dwight K. Elvey <dwightk.elvey_at_amd.com>
Date: Tue Jun 3 13:40:00 2003

Hi
 I think the rule is 0.5ma per cubic inch of computer
grade capacitor and 0.25 ma per cubic inch of normal
capacitor. As an example: 2" dia X 3 inch = 9.4 in^3
                 Cap voltage rating of 10V
                 use 2.2K limiting resistor and
                 10V supply.
 Of course, if the capacitor is on a 5V rail, you'll
need to disconnect one end.
Dwight


>From: "Luc Vande Velde" <luc_at_e2t.be>
>
>For years I use a high-voltage DC power supply with current limiting for
>this. Works very fine...
>Only a few, weird designs, need the AC voltage to start.
>This is also a very safe way to repair the things (safe for the unit and the
>technician)
>
>gr.
>
>Luc
>
>
>-----Original Message-----
>From: cctech-admin_at_classiccmp.org [mailto:cctech-admin_at_classiccmp.org]On
>Behalf Of Robert Maxwell
>Sent: Tuesday, June 03, 2003 5:21 PM
>To: 'cctech_at_classiccmp.org'
>Subject: RE: reforming PS caps w/primary-side components
>
>
>-----Original Message-----
>>Anybody want to offer advice on reforming filter
>>caps? I'm planning to power up some stuff that
>>hasn't seen voltage in a long time. My plan is
>>to put several low-wattage light-bulbs in series
>>with the thing, to limit the amount of current it
>>will get, and hopefully allow those old caps
>>to reform instead of, say, exploding in my face.
>>...
>>Now the questions:
>>
>>Could putting the light-bulbs (or whatever) in
>>series on the primary side of the transformer
>>actually produce the desired effect? IIRC, a
>>step-down transformer divides the voltage but
>>multiplies the current. So I'd have to limit the
>>current on the primary side that much more to
>>keep the secondary current down, right?>
>>
>>Also, what kinds of things could be damaged by
>>getting less voltage than they were designed for?
>>I could imagine hard disks spinning too little to
>>lift the heads from the surface, for instance.
>>(This is moot, because there are no hard disks
>>in any of the equipment I'm looking at.) How
>>about CRT's? Could too little deflection make
>>the beam hit something it shouldn't? Anything
>>else?
>>
>> Thanks,
>> Bill.
>
>Bill,
> (1) Don't use a light bulb - they have a very low resistance when cold,
>and will cause the inrush you're trying to avoid. The best recommendation
>I've seen is to use a "variac"-type variable transformer to reduce the
>powerline going in. Start at minimum, and turn things up slowwwwly, watching
>current draw. Beware gotchas like LSTTL drawing heavy current at around 2.1
>volts supply. Actually, with a meter across the capacitor supply, stop at
>about one volt, so any semiconductors in line can't get terribly excited: if
>there's no apparent short as things stabilize, turn the voltage up.
> (2) CRTs aren't a hazard from undervoltage (I ran a sick Zenith TV on
>85VAC for years using a Variac). It's overvoltage that causes X-Ray risks.
>Just to be safe, disconnect anything mechanical, to avoid problems like the
>hard-drive one you mentioned - some types of motor overheat if undervolt-ed.
>
>Bob "HW-Hacker" Maxwell
>How many people do you know that sawed their computer in two?
Received on Tue Jun 03 2003 - 13:40:00 BST