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Old 09-19-2018, 09:52 AM   #1
wall
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Default AT2005B psu modification

Hi, I am trying to modify an old ATX power supply with an AT2005B PWM controller to output a higher voltage on the +12v rail.
I attached an image file I found on an old thread from another forum, and my question is if I need to lift pins 2, 3, 4, 5 and 6 before soldering those pots.
Will it also increase the +5v rail's voltage? And why?

Thanks for any help I can get.
Attached Images
File Type: jpg DSC_0003.JPG (158.2 KB, 3 views)
File Type: png atx.png (19.8 KB, 3 views)

Last edited by wall; 09-19-2018 at 09:53 AM..
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Old 09-19-2018, 10:03 AM   #2
wall
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Default Re: AT2005B psu modification


This was my attempt at tracing the connections on the AT2005B.


PWM chip.


Output caps, they're all 10v, except on the +12v rail it's a 16v cap.


Funny how the PSU doesn't mention -5v but on the previous attachment there is a white cable labeled -5V on the pcb.
Attached Images
File Type: jpg 20180919_122936.jpg (390.3 KB, 42 views)
File Type: jpg 20180919_122811.jpg (696.5 KB, 42 views)
File Type: jpg 20180919_122828.jpg (689.3 KB, 43 views)
File Type: jpg 20180919_122902.jpg (696.4 KB, 41 views)
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Old 09-20-2018, 11:03 AM   #3
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Default Re: AT2005B psu modification

Any help?
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Old 09-20-2018, 02:57 PM   #4
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Default Re: AT2005B psu modification

Follow the circuit going to pin 2, V_ADJ and draw it completely on your paper (looks like you're almost there). Then it should make more sense. Most likely, just adding a variable resistor to ground and slowly decreasing the resistance down should work for you. I suggest start with 47-100 kOhm variable resistor in series with a 2 kOhm resistor. Connect these two between pin 2 (V_ADJ) and ground. Then with the variable resistor turned all the way up for high resistance, slowly start turning the resistance down until you see the PSU voltage going up.

Quote:
Originally Posted by wall
Will it also increase the +5v rail's voltage? And why?
Hard to say without seeing a whole shot of the PSU... but it looks like a cheapo group-regulated power supply - in which case, yes, increasing the 12V rail will also increase the 5V rail (along with the -12V and -5V rails). 5VSB is going to stay the same. 3.3V rail, if it's mag-amp or MOSFET (linearly) regulated will still stay at 3.3V.

Note: PSU may start to oscillate if you try to increase the output voltage too much. So be careful. If you hear the PSU starts making funny noises, you might want to shut it down immediately to prevent something in it from blowing up.
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Old 09-21-2018, 01:38 AM   #5
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Default Re: AT2005B psu modification

Thank you for your help, I traced the circuit a bit more around the AT-2005B, hope it helps.
This psu has sense wires for 3.3, 5 and 12v, what do I do with them if I increase the voltage?
Attached Images
File Type: jpg 20180921_072625.jpg (347.7 KB, 26 views)
File Type: jpg 20180921_055326.jpg (722.0 KB, 3 views)
File Type: jpg 20180921_055228.jpg (685.0 KB, 3 views)
File Type: jpg 20180921_055222.jpg (729.7 KB, 4 views)
File Type: jpg 20180921_055210.jpg (697.6 KB, 2 views)
File Type: jpg 20180921_055130.jpg (813.8 KB, 2 views)

Last edited by wall; 09-21-2018 at 01:44 AM..
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Old 09-23-2018, 04:23 AM   #6
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Default Re: AT2005B psu modification

Looks like you are drawing that circuit by measuring values and not 100% tracing everything (for example, the 98 Ohm and 660 Ohm resistors, neither of which are standard values). This won't do. You need to trace the circuit only and read the resistor values from their color bands. Most importantly, just trace pin 2 of the AT2005b controller completely.

In any case, it looks like lowering the resistance between ground and pin 2 should give you higher voltages. If PSU OVP activates, then we might need to modify the resistor settings on the 3.3V, 5V, and 12V pins to move the OVP protection higher.
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