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Thread: 486 Power Supply Failure

  1. #1
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    Default 486 Power Supply Failure

    While using my 486 ATX computer, the power supply failed. It has a HIPRO HP-200PP. All the voltages went away. I could easily check the fuse, the full wave bridge and large filter capacitors, all were good. So something in the switching circuits must have failed. Something that affects all the voltages. I was looking around the net for a schematic, but have not found anything. Does anyone have a source for this power supply? Thanks Mike.

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    Well... no luck with finding a schematic yet, but after looking at the CB it looks like the 200 volts from the initial supply is switched thru a transformer. There are two 2SC3832 NPN transistors that do the switching. I removed them and found them to be OK. But since all the voltages are missing, I think this maybe where the trouble is. Apparently there is a Pulse width modulator, TL494 that controls these transistors. Maybe tomorrow I'll replace the transistors and see if there is a switching signal being applied to the bases of these transistors. Maybe this chip is bad. Mike

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    Check ALL of the capacitors, even the tiny ones buried down under things. If the entire secondary side of the supply went away, it's most certainly something wrong with the primary side.

    When you said you checked the large filter caps, how did you check them? You need a tester that can at minimum get a capacitance and ESR reading. If the capacitance is within 20% of the cap rating, it's most likely fine. The ESR also needs to be low, more than 3-4 ohms and I'd consider it suspect. The tiny 5x10mm caps going bad can also cause the PSU to fail, because they're often used for the switching and monitoring circuitry.

    Other things to check are things like resistors, especially big 1w+ types, because they can drift in value or go open/short. I've had to repair several failed supplies where one or more big power resistors failed.

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    I checked the capacitors, by just measuring the voltage on them, approx. 175 volts dc. They are rated at 200V. Seems OK. Since the last post I ordered some new tl494 pulse width modulators. I replaced the one on the CB. Now I can heard the switching circuit buzz, and when first energized the output voltages are there be quickly reduce to about 1/2. I see that there is a quad competitor LM339 Not sure how this is used, but maybe it is a problem. Without a schematic, I'm reduced to guessing.

    I'll look at the passive elements next, Thanks for the help, Mike

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    I checked as many of the passive elements and the transistors and diodes, but did not find anything that looked bad. I passed some time reading about computer power supplies. I found an old post on the Forum that mentioned that there is a low current trip on these units. This is something I did not realize. So, since after replacing the TL494, I did hear the switching buzz, maybe this low current lockout was preventing my unit producing voltages. I found a 3 ohm power resistor and placed it on the +5 volt wires. Amazingly the unit came to life and all the voltages were present. So, apparently, my problem was a bad TL494 pulse width modulator and a little lack of knowledge. This is great that I learned something, but 'my' biggest personal problem is trying to remember this for the next time. Thanks for the help. Mike

  6. #6
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    Quote Originally Posted by Mike_Z View Post
    I checked the capacitors, by just measuring the voltage on them, approx. 175 volts dc. They are rated at 200V. Seems OK.
    This is not how you check capacitors. Checking what voltage is across the capacitor does not verify if it's good or not, you need an LCR and/or ESR meter.

    https://www.ebay.com/itm/264960990462

  7. #7
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    I've been looking at different Capacitor testers. Can these be used on capacitors in circuit or do they need to be removed? Mike

  8. #8

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    Any capacitor needs to be removed from the circuit before an accurate test can be performed.

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    They also need to be discharged, else you'll most likely blow up your meter.

  10. #10
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    A very cheap way to check capacitors is one of the little LCR-TC1 testers. Accurate enough for a go-no go test and can identify all sorts of components.

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