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Old 04-12-2018, 10:09 PM   #41
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Default Re: Trying to repair a cheap 12v power supply

Just replaced it and nothing happened.
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Old 04-12-2018, 11:09 PM   #42
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Default Re: Trying to repair a cheap 12v power supply

Something's weird. Most of these switchers the voltage on C8 needs to go above 12V or so (but that data sheet would be the end all to this question!) -- 6V is typically not enough.
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Old 04-13-2018, 09:42 AM   #43
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Default Re: Trying to repair a cheap 12v power supply

Quote:
Originally Posted by eccerr0r View Post
Something's weird. Most of these switchers the voltage on C8 needs to go above 12V or so (but that data sheet would be the end all to this question!) -- 6V is typically not enough.
That is what I am thinking too that the Voltage on C8 must HAVE gone up to more than 13V to get any output Voltage on the cold side.
I still like to know if he is getting steady 12V with light load or no load.
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Old 04-13-2018, 09:46 AM   #44
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Default Re: Trying to repair a cheap 12v power supply

I tested the power supply under a 1A load and no load after replacing the resistor and diode, even under no load this thing did not work.
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Old 04-13-2018, 09:56 AM   #45
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Default Re: Trying to repair a cheap 12v power supply

"After replacing the resistor, the power supply still does not work. but it's no longer pulsing, now it just shuts down after both the output and C8 go above 6v."

You are saying it does not work means there is NO output or the output is cycling up and down?
It is not making any sense that you were getting steady 12V with no load when R29 was showing over 6000 Ohms per your reports.
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Old 04-13-2018, 07:53 PM   #46
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Default Re: Trying to repair a cheap 12v power supply

Well it does sort of, up until it runs out of charge when running off of bootstrap. I suspect someone may have been mucking with the 6K resistor because it "seemed" to make it work better.

Now I'm wondering if the transformer's PWM power winding is shorted... this probably would explain the behavior somewhat.

Need to account for the purpose of all those amateurish bodges.
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Old 04-13-2018, 10:05 PM   #47
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Default Re: Trying to repair a cheap 12v power supply

https://www.badcaps.net/forum/attach...2&d=1523335356
That is the problem with this board, so far we know at least one resistor is wrong, then we still have the one shown in the picture, and what else were done wrong we don't have any ideas.
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Old 04-13-2018, 10:17 PM   #48
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Default Re: Trying to repair a cheap 12v power supply

Should i remove that bodge resistor then?
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Old 04-21-2018, 11:00 PM   #49
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Default Re: Trying to repair a cheap 12v power supply

My next guess is if you have a Q/ringtester is to remove the transformer and check if it has any shorts, I'm kind of surprised replacing the diodes didn't do anything (used a fast/schottky diode?).

So now, to be certain, the voltage across C8 never gets above 10V and you get 0 continuously on the output?
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Old 04-21-2018, 11:51 PM   #50
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Default Re: Trying to repair a cheap 12v power supply

I don't have a ringtester and the diode i put in is identical to the one that was on the board (same part number, same manufacturer, etc).

When i turn on the unit, C8 and the secondary capacitors both go up to 6v before the unit shuts down.
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Old 02-24-2022, 10:33 PM   #51
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Default Re: Trying to repair a cheap 12v power supply

I know it's been nearly 4 years since I asked this question, but I dug up the power supply today and managed to make progress on it, to the point where it's able to output a stable 12v under a light load, and apparently, the issue stems from R29, and as it turns out, a 6.6K resistor was too large for this, but the replacement 2 ohm resistor was too small, so I bodged a 5.1K resistor in place and the power supply somewhat works, but it's still finicky, as the power supply will not keep a steady output when connected to my load tester, unless I also hook up a LED strip to the output, but even then, I can only get it up to 2A max (including the LED strip) before the voltage drops below 11.5v, but maybe messing around with the resistor value a bit more can improve this, what do you think.
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Old 02-25-2022, 07:35 PM   #52
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Default Re: Trying to repair a cheap 12v power supply

I suspect R29 should be a low value around 2Ω, it is basically a fuse resistor. If a higher value seems to make the power supply operate, I suspect the problem is with the feedback, either the optocoupler is bad or the refernce ic on the secondary is bad.
What is the number on M3 and what is with the white smd near R22?

The feedback winding connects to R29 and then to the diode and C8, this supplies the run vcc for the ic, part of this voltage should also pass through M2 which controls the actual voltage the circuit outputs

The OB2273 seems to be the correct ic.
M3 could be a TL431/432

Last edited by R_J; 02-25-2022 at 08:24 PM..
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