Cap Juice?

Discussion in 'General Electronics Chat' started by pntrbl, Aug 29, 2008.

  1. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
    123
    0
    I've dug up an old ATX supply out of an E-Machine to convert for bench use and found a bunch of a whitish looking waxy substance all over one corner of the board. Lower left hand in the pic.

    [​IMG]

    As far as I know this was a working supply when the machine was retired, but all of that stuff being in close proximity to a pair of big electrolytics makes me wonder if I shouldn't move on ....

    As always, any advice appreciated.

    SP
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Hard to say from the picture, but that looks like RTV (silicon rubber). Have you probed it for consistancy?
     
  3. RmACK

    Active Member

    Nov 23, 2007
    54
    0
    Unless it's wet, it will indeed be some sort of silicon or other adhesive. Will be rubbery or hard. Used to take mechanical stress of physically large components rather than relying on the solder joints.

    Cap juice tends to be wet, toxic & brown or clear. It also usually comes out the top of caps after they have bulged a bit.
     
  4. HarveyH42

    Active Member

    Jul 22, 2007
    425
    5
    I've only had the mis-fortune of working on one Emachine computer, thought it was pretty cheaply built. It was a real pain to fit a hard drive into. Everything had kind of a fragile feel. Check and see if it solid, like the mentioned silicon, could be to hold the the big caps on the board, so the cheap copper traces don't break... Looks applied, not spillage or leakage.

    Haven't done a conversion, but doesn't seem like a big deal. Something to do with adding a load resistor if I remember right. Give it it try, worst you'll get is a little smoke, maybe get lucky and some cool flames...
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    I played with few 250W ATX supplies, and those didn´t need the load resistor.
     
  6. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
    123
    0
    Thanx for the replies guys. I have since found a 350 watter out of a Dell that looks to be much more substantial than the 200w E-Machine supply. It certainly doesn't have any putty holding it together! :rolleyes: I'm gonna go with the Dell and relegate the other to parts duty.

    Thanx again.

    SP
     
  7. RmACK

    Active Member

    Nov 23, 2007
    54
    0
    Generally the feedback is much more heavily weighted to the 5V rail than the 12V rail so if you pull much from the 12V it will sag badly, I've seen as low as 8 or 9 volts. Loading the 5 volts will help with this. It all depends on what you're trying to do.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You will need to load the 5v side in order to get proper regulation.

    I have a 250W ATX supply that I pulled out of an old Compaq for a bench supply. Adding a 10 Ohm 10W power resistor that I picked up at Radio Shack across 5v/ground did the trick; before adding the resistor the +/-12v was around 8v; after adding the 10 Ohm load I measured +11.75V and -11.5V. Increasing the load on 5v will improve the regulation on +/-12v, but I just wanted it to be in the ballpark.

    To keep the power resistor reasonably cool, I used a couple of nylon zip-ties to strap an old 486 CPU heatsink to it. The heat won't hurt the resistor; I just don't like heat building up close to wiring.
     
    Last edited: Aug 29, 2008
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    In my case, the regulation was connected to the 3.3V side.
    But beacuse all the rails are from one tapped winding, I don´t think it really matters.
     
  10. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
    123
    0
    I tie-wrapped a 10 ohm 10 watter in there across the +5v and she's a-workin' jus' fine.

    The only goofy thing is I put a power switch on the green wire which was supposed to act as a DC On/Off. It has no effect at all. With the green wire grounded thru the switch, or not, I still get all the voltages.

    SP
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    That's odd. :confused:
    The purple/violet wire is standby +5v. I use that to supply a yellow LED pilot light (via a 220 Ohm resistor) so that I know the supply is alive/120VAC is connected when the power switch is off.

    My green wire is connected to the other side of the SPDT power switch. Ground is common. That way, when I throw the switch the yellow pilot light goes off, and the green wire is grounded.

    I have a couple of green LEDs connected to +3.3v and +5v so that I have a quick verification that those voltages are online and working.

    I suspect that you've pinched your green wire in the case somewhere.
     
  12. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
    123
    0
    Good call on the green wire Sarge and I wish it were true, lol, but no, it's as clean and unblemished as the day it was born. No pinch.

    I hate to admit it but the SPST switch I got at Radio Shack has got me confused and maybe that's where the problem lies. I had to go and buy one with a built in LED, 275-014's the number, but when I got it home I found 3 terminals on it instead of the two I was expecting.

    At one end it's labeled Power. The middle terminal is labeled ACC. And the other end is Ground. Not knowing what to make of this I put my meter to it and found the only two that ever get electrically connected are Power and ACC. Probing from the Ground terminal I did get the LED glowing a bit now and then from the battery in my meter, but no connection to either of the other two.

    So I put the green wire and common(Blk) across Power and ACC and in fact the green does become connected to the common when I flip the switch up. No LED .... but I'm just about disgusted with that thing anyway!:D

    Running it with the case open now and I find the green wire at 3.5v until I flip the switch and it drops to 0v. I don't know how I did it but even with green at 3.5v I still have all voltages available.

    Who knows? LOL! If there's a way to mess it up you can count on me ....

    SP
     
Loading...