Re: Task TK-940TX-DF Blowing fuse
Buzzing for single-transistor forward PSUs typically means over-load condition.
However, since you have a series light bulb, it could also be a false over-load condition, as some PSUs simply don't like high-impedance on their input.
So I think there are three possibilities for what you are seeing here:
1) There is a short-circuit or over-current condition on the main PS secondary side, causing the primary to over-load. (Could be a faulty rectifier on one of the rails, for example, or an open feedback path.)
2) There is a component fault on the primary side of the main PS, causing the PWM IC to over-drive main PS MOSFET until its protection kicks in (open opto-coupler, open opto-coupler feedback path, faulty snubber component, or sense resistor going high resistance.)
3) PSU's main PS design might be sensitive to high-impedance on the input, which means the series light bulb could be the reason why you're seeing this behavior. (On that note, Active PFC PSUs... or PSUs with APFC... are even more sensitive, and generally cannot be tested with a series bulb less than 200-300 Watts.) If that's the case, then using several 60-100 Watt incandescent bulbs in parallel... or one 200-300 Watt heating element... should provide sufficiently lower impedance on the output to make the PSU function.
I suggest you start by assuming case #1 above and check all secondary-side components. Start with the large power-handling components first, like large rectifiers and diodes for the 3.3V, 5V, 12V, and -12V rails. To do that, check resistance between each of these rails and ground. Note that there _may_ be minimum-load resistors on the output, which could possibly make you think that a rail is shorted to ground (common mistake when someone uses continuity test on their multimeter to check if a rail is shorted to ground, since most multimeters will beep at resistance under 100 Ohms, and load resistors on the 3.3V and 5V rails typically are 100 Ohms or less.) An alternative (and somewhat easier) method to checking the rails for short-circuit is to use a 20-40 Watt, 12V, incandescent/halogen bulb and backfeed an external voltage to each rail. If you have a spare ATX PSU, simply wire the output of that PSU to the rail you want to test. For example, if you want to test if the 12V rail is shorted, connected ground on both PSUs together, then connect the 12V rail of one PSU to the 12V rail of the other PSU _through_ the indancesence/halogen bulb (i.e. similar to how you use the series light bulb on the input.) Then, plug in only the working PSU and turn it On (the PSU under test should remain unplugged from wall.) If the bulb glows steadily, there is likely a short-circuit on that rail. If not, then rail is probably OK. Just make sure to always connect the right rails together - i.e. only use 12V rail to test 12V rail on the PSU in question, or 5V rail to test the 5V rail, and 3.3V rail to test the 3.3V rail. If you accidentally mix then up (particularly inserting a higher voltage on a lower-voltage rated rail), you could blow up some caps.
If you find that none of the rails are shorted with the above tests, then we could assume case #3 for a bit, and simply add a few more incandescent bulbs in parallel to your single bulb on the input, or simply replace that with a 200-300 Watt heating element. Then try powering up the PSU and see if it buzzes again. If it does, go back to assuming case #1, and check all secondary-side components again. After this, assume case #2 and check all primary side components related to the main PS.
The numbers on those caps indicates their capacitance and not their resistance.
caps with numbers xx5 = x.x uF
caps with numbers xx4 = 0.xx uF
caps with numbers xx3 = 0.0xx uF = xx nF
caps with numbers xx2 = 0.00xx uF = x.x nF
caps with numbers xx1 = 0.000xx uF = 0.xx nF
* where xx = 2-digit number
(Also, nF = nanoFarads, uF = microFarads)
So 472 = 4.7 nF and 221 = 0.22 nF
Buzzing for single-transistor forward PSUs typically means over-load condition.
However, since you have a series light bulb, it could also be a false over-load condition, as some PSUs simply don't like high-impedance on their input.
So I think there are three possibilities for what you are seeing here:
1) There is a short-circuit or over-current condition on the main PS secondary side, causing the primary to over-load. (Could be a faulty rectifier on one of the rails, for example, or an open feedback path.)
2) There is a component fault on the primary side of the main PS, causing the PWM IC to over-drive main PS MOSFET until its protection kicks in (open opto-coupler, open opto-coupler feedback path, faulty snubber component, or sense resistor going high resistance.)
3) PSU's main PS design might be sensitive to high-impedance on the input, which means the series light bulb could be the reason why you're seeing this behavior. (On that note, Active PFC PSUs... or PSUs with APFC... are even more sensitive, and generally cannot be tested with a series bulb less than 200-300 Watts.) If that's the case, then using several 60-100 Watt incandescent bulbs in parallel... or one 200-300 Watt heating element... should provide sufficiently lower impedance on the output to make the PSU function.
I suggest you start by assuming case #1 above and check all secondary-side components. Start with the large power-handling components first, like large rectifiers and diodes for the 3.3V, 5V, 12V, and -12V rails. To do that, check resistance between each of these rails and ground. Note that there _may_ be minimum-load resistors on the output, which could possibly make you think that a rail is shorted to ground (common mistake when someone uses continuity test on their multimeter to check if a rail is shorted to ground, since most multimeters will beep at resistance under 100 Ohms, and load resistors on the 3.3V and 5V rails typically are 100 Ohms or less.) An alternative (and somewhat easier) method to checking the rails for short-circuit is to use a 20-40 Watt, 12V, incandescent/halogen bulb and backfeed an external voltage to each rail. If you have a spare ATX PSU, simply wire the output of that PSU to the rail you want to test. For example, if you want to test if the 12V rail is shorted, connected ground on both PSUs together, then connect the 12V rail of one PSU to the 12V rail of the other PSU _through_ the indancesence/halogen bulb (i.e. similar to how you use the series light bulb on the input.) Then, plug in only the working PSU and turn it On (the PSU under test should remain unplugged from wall.) If the bulb glows steadily, there is likely a short-circuit on that rail. If not, then rail is probably OK. Just make sure to always connect the right rails together - i.e. only use 12V rail to test 12V rail on the PSU in question, or 5V rail to test the 5V rail, and 3.3V rail to test the 3.3V rail. If you accidentally mix then up (particularly inserting a higher voltage on a lower-voltage rated rail), you could blow up some caps.
If you find that none of the rails are shorted with the above tests, then we could assume case #3 for a bit, and simply add a few more incandescent bulbs in parallel to your single bulb on the input, or simply replace that with a 200-300 Watt heating element. Then try powering up the PSU and see if it buzzes again. If it does, go back to assuming case #1, and check all secondary-side components again. After this, assume case #2 and check all primary side components related to the main PS.
Originally posted by socketa
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caps with numbers xx5 = x.x uF
caps with numbers xx4 = 0.xx uF
caps with numbers xx3 = 0.0xx uF = xx nF
caps with numbers xx2 = 0.00xx uF = x.x nF
caps with numbers xx1 = 0.000xx uF = 0.xx nF
* where xx = 2-digit number
(Also, nF = nanoFarads, uF = microFarads)
So 472 = 4.7 nF and 221 = 0.22 nF
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