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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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ATX power supply repairs
Of those brave souls who have ventured into trying to fix ATX supplies,
what are you mainly finding wrong with them and how are you testing them? I know its hardly worth economically doing it. But its the challenge i guess. Most of the ones i have scrapped have not shown the typical cap failure. I have resoldered a few and replaced fans in some. bob |
#2
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ATX power supply repairs
On Mon, 23 Nov 2009 16:42:30 -0600, bob put
finger to keyboard and composed: Of those brave souls who have ventured into trying to fix ATX supplies, what are you mainly finding wrong with them and how are you testing them? This may help you understand the inner workings: http://pavouk.org/hw/en_atxps.html - Franc Zabkar -- Please remove one 'i' from my address when replying by email. |
#3
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ATX power supply repairs
"bob" schreef in bericht ... Of those brave souls who have ventured into trying to fix ATX supplies, what are you mainly finding wrong with them and how are you testing them? I know its hardly worth economically doing it. But its the challenge i guess. Most of the ones i have scrapped have not shown the typical cap failure. I have resoldered a few and replaced fans in some. bob Back in 2002 I wrote the following posting and I did so again in 2007. I did not many repairs ever since. The components required for the repair became more expensive then a new supply. Meanwhile I found that some Czech, Pavel Ruzicka, put a schematic of a 200W ATX supply on the net. This can be a great help though a little bit late for me. Just Google for it. Be aware that the schematic is an example of an ATX supply. You may find a lot of differences out there though the general idea will be the same. At least most of the times. begin I repaired several PC supplies for a hobby, but if your time is valuable, buy a new PS. First of all read the sci.repair.faq. Especially the parts on safety and SMPS. Most of the times the fault is found between the mains connection and the transformer(s). 1. In the most simple cases only the fuse is blown. After replacing this fuse, connect the PS to the mains using the serial lightbulb trick. - If the bulb burns brightly, you know that the old fuse had a good reason to quit, so the case is not simple anymore. The first thing you have to do now is to find the short circuit. The most suspected components are the mains rectifier, the filter capacitors and one or more of the power transistors. Use eyes, nose and an ohmmeter to find scene of the crime. Remove and check the suspected components. Replace defective components except for the power transistors at this time. It makes no sense to continue until you fixed the short circuit - If the fuse is good but the PS still dead, you can start to check the voltages. 2. Check the voltage between pin 3 and pin 9 of the ATX-mainboard connector. This should be 5V. - If not you have to check the voltages on the mains side. Otherwise it will be wise to check the voltages on the mains side as well (3-5). Then continue reading up to point 12, not to miss some explanation. Continue at 12. 3. The AC-pins of the mains rectifier should show the mains AC voltage. - If not you may have an interrupted trace or mains filter. 4. Between plus and minus of this rectifier you should find about 310V DC or 325V DC depending on your mains voltage. I call it the primary power voltage. - If not you may have a faulty mains rectifier. - If the voltage is much lower (analog meter) or jumping around (digital meter), the large filter capacitors (p.e. 470 muF, 200V) are also suspected. 5. Both filter capacitors mentioned above are in serial. The midpoint should be at half the primary power voltage. - If not, the mains rectifier, the filter capacitors and the parallel resistors (parallel to the capacitors) may be defective. Another suspect is a third capacitor (p.e. 1muF, 250V) that leads from the midpoint to a transformer. Explanation: ATX-PS's usually has three power transistors at the mains side. One connected to a small transformer, the other two connected to a larger transformer. You can recognize the pair of transistors best by finding the emitter of one of them connected to the collector of the other. First you have to deal with the one transistor and the small transformer. (Go to 8 if you removed this transistor already.) 6. Check the voltage on the collector of the transistor. - If this voltage is zero or very low there may be an interruption between the collector and the primary power voltage. - If this voltage is below the primary power voltage or jumping, there seems to be switching activity. You can check this with an AC voltmeter on a secondary coil of the transformer. The reading will not be correct, but if you find an AC-voltage you have to continue checking the secondary rectifier and regulator. - If this voltage is the primary power voltage the transistor does not conduct. 7. Check the voltage on the base of the transistor. - If this voltage 0.6V the startup resistor may be defective. Otherwise the transistor may be gone (most likely.) 8. Disconnect the PS from the mains and take the safety precautions to discharge the capacitors. 9. Remove the suspected transistor and check it with an ohmmeter or a transistorchecker. Most of the times you will have to provide a new transistor. (Beware! Even a transistor that looks good under test conditions may malfuntion in the actual circuit.) This is also the time to remove, check and replace other fried, exploded or discolored components near the transistor/transformer combination. 10. Re-power the PS using the serial light bulb. - If the lamp is burning brightly you have a short circuit in your PS. Most likely your (new) power transistor is conducting due to too high a continuous base-current. You have to dive deeper into this part of the circuit until you find the couse of this problem. - If the lamp is dim or not burning at all you can re-check the voltage between pin 3 and pin 9 of the mainbord connector. Finding 5V you can continue, otherwise you have to restart your investigation of the small transformer/one power transistor part of the circuit.(Point 6.) It makes no sense to continue until this part of the PS functions correctly. 11. Disconnect the PS from the mains when you are done so far. Explanation: For the next part of the repair procedure you have to provide some load to the PS. This is simply because of some PS's will not function well without load. You may use an (old) main board. Someone ever told me he uses 12V car bulbs, one on the +5V and one on the +12V. I prefer a huge and heavy old harddrive. Those old basalt blocks (we use to strengthen our dikes) consume a lot of energy. The one I use, provided enough load to all the PS's I ever repaired. 12. Replace the power transistors you may have removed earlier. Reconnect the PS to the mains using the serial light bulb. Check the voltage between pin 3 and pin 9 of the main board connector. Connect pin 14 of the main board connector to pin 13. This will switch on the main part of the PS, the part with the two power transistors and the large trafo. - If your load start to work, check the voltages of the several power connectors. When they have the correct values your PS is on air again. Check it out by removing the serial light bulb. - If (even after removing the serial light bulb) some but not all of the values are correct, you have a problem. You have to investigate the failing voltages from the secondary coil of the transformer till the connector. Quite a challenge. - If the light bulb is burning brightly you have a short circuit. Most likely your power transistors are gone so you have to check (and almost sure replace) them and their surrounding components, especially the start resistors. Pay also special attention to the freewheel diodes (between the collector and the emitter of the power transistors.) Don't forget to disconnect the PS and to discharge the filtercapacitors first! When you are done, restart at 12. - If if your lamp is dim or dark but your load does not work you may have defective or blocking power transistors. A fault on the secondary side of the transformer is another possibility. 13. Search for switching activity on the secondary coils of the transformer using an AC meter. - If you don't find AC-voltage you have to check the voltages on the power transistors. - If you find an AC-voltage you most likely have a defective rectifier, filter capacitor or regulator at the secundary side. Disconnect from de mains, discharge the filter capacitors and try to find the failing components with an ohmmeter. You will have to remove the rectifiers from the board prior to testing because of the secundary coils have only few windings of thick wire so they are the shortest shortcuts as far as your ohmmeter concerns. Another trick is to use a controllable power supply. Connect it to the point where the removed rectifier was connected to its filter capacitor. Beware of the polarity! Power on both PS's and rise the voltage of your controllable PS to the level of normal operating of your defective PS. The regulator that sucks to much current with respect to the light load will be the main suspect. You have to go deeper into the circuit of this regulator if you want to repair it. Another challenge. 14. Check the voltages on the power transistors. The collector of one of them should be at the primary power voltage, the emitter of the other should be at the common. The remaining collector and emitter are tied together and should be at half the primary power voltage. - If you can't find the primary power voltage at a collector you have an interruption. Maybe a bad soldering or the like. - If no emitter is connected to common you also have an interruption. - If the tied collector-emitter is not at half the primary power voltage you most likely have defective power transistors. (In my experience they always die together.) Disconnect, discharge and remove, check and replace the power transistors and their surrounding components. Restart at 12. - If the tied collector-emitter is at half the primary power voltage you can check the base-emitter voltage of the power transistors. If they are 0.6V you may be lucky and find only defective startresistor(s) and/or other base circuit components. But most of the times a defective base circuit will kill its transistor which in turn will kill its neighbour. So you will have to replace the whole bunch. Of course, this story does not cover all possible faults of PC-power supplies, but I only once failed to repair a PS using this scheme. end petrus bitbyter |
#4
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ATX power supply repairs
On Mon, 23 Nov 2009 16:42:30 -0600, bob wrote:
Of those brave souls who have ventured into trying to fix ATX supplies, what are you mainly finding wrong with them and how are you testing them? I know its hardly worth economically doing it. But its the challenge i guess. Most of the ones i have scrapped have not shown the typical cap failure. I have resoldered a few and replaced fans in some. Some cap failures. Recently one client had a batch of four identical 230V units die the same day. These were auto-sensing (supply voltage) types, and during brownouts they think they are on 110V rather than 230V, then the voltage rises and phattt! The pair of 1000V 1A ultrafast diodes went short on all four (same brownout!). |
#5
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ATX power supply repairs
On Tue, 24 Nov 2009 01:06:45 +0100, "petrus bitbyter"
put finger to keyboard and composed: Meanwhile I found that some Czech, Pavel Ruzicka, put a schematic of a 200W ATX supply on the net. You may like to know that DTK's original circuit diagram is he http://www.dtk.com.tw/ftp/circuit/power/2038.zip More DTK PSU circuits are he http://www.dtk.com.tw/tech/circuitp.html - Franc Zabkar -- Please remove one 'i' from my address when replying by email. |
#6
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ATX power supply repairs
On Tue, 24 Nov 2009 09:13:54 +0800, who where wrote:
On Mon, 23 Nov 2009 16:42:30 -0600, bob wrote: Of those brave souls who have ventured into trying to fix ATX supplies, what are you mainly finding wrong with them and how are you testing them? I know its hardly worth economically doing it. But its the challenge i guess. Most of the ones i have scrapped have not shown the typical cap failure. I have resoldered a few and replaced fans in some. Some cap failures. Recently one client had a batch of four identical 230V units die the same day. These were auto-sensing (supply voltage) types, and during brownouts they think they are on 110V rather than 230V, then the voltage rises and phattt! The pair of 1000V 1A ultrafast diodes went short on all four (same brownout!). Clarifying that, these were protection diodes. |
#7
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ATX power supply repairs
On Nov 24, 10:20*pm, Franc Zabkar wrote:
On Tue, 24 Nov 2009 01:06:45 +0100, "petrus bitbyter" put finger to keyboard and composed: Meanwhile I found that some Czech, Pavel Ruzicka, put a schematic of a 200W ATX supply on the net. You may like to know that DTK's original circuit diagram is hehttp://www.dtk.com.tw/ftp/circuit/power/2038.zip More DTK PSU circuits are hehttp://www.dtk.com.tw/tech/circuitp.html - Franc Zabkar -- Please remove one 'i' from my address when replying by email. neither URLs worked for me |
#8
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ATX power supply repairs
On Thu, 26 Nov 2009 08:07:30 -0800 (PST), Robert Macy
put finger to keyboard and composed: On Nov 24, 10:20*pm, Franc Zabkar wrote: On Tue, 24 Nov 2009 01:06:45 +0100, "petrus bitbyter" put finger to keyboard and composed: Meanwhile I found that some Czech, Pavel Ruzicka, put a schematic of a 200W ATX supply on the net. You may like to know that DTK's original circuit diagram is hehttp://www.dtk.com.tw/ftp/circuit/power/2038.zip More DTK PSU circuits are hehttp://www.dtk.com.tw/tech/circuitp.html neither URLs worked for me Sorry, it appears that the links are now dead. I found them at the Wayback machine, though: http://web.archive.org/web/200803130.../circuitp.html http://web.archive.org/web/200803130...power/2038.zip - Franc Zabkar -- Please remove one 'i' from my address when replying by email. |
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