Adding voltage regulator to Abit AN4 R1 \ VOGONS

Adding voltage regulator to Abit AN4 R1

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First post, by Paar

Posted on 2024-02-21, 11:12

Paar Offline

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Hi. This board came to me pretty beat up - chips were missing, memory slots were broken, traces were burnt, leaked battery... But I managed to repair, at least to bootable condition. The last main thing to do is to add voltage regulator for 3.3V CPU support.

There is already a nice thread about this but the steps defined there are for a newer revision of the board. I have somewhat older and a little more limited revision, at least concerning voltage options. Mainly, there is only one spot for a resistor situated between GND and OUT pins of the LT1085 voltage regulator (I plan to used 3.3V fixed version as adjustable one is a waste on this board). There is a jumper (JP40, is overlapped by colored traces, sorry) which can connect GND pin to ground on the board for 3.3V, or ran through a resistor first which should allow 3.45V if a right value is used. Green traces are on the top side, blue are on the bottom. Middle pin, marked with yellow, is ground. Red arrow is a place for resistor.

I would be happy with only 3.3V support but I'd like to make it right. What resistor should I use for 3.45V? Or does someone here have this motherboard with regulator already integrated from factory? Thanks for any input!

Last edited by Paar on 2024-02-26, 06:19. Edited 2 times in total.

Reply 1 of 4, by Paar

Posted on 2024-02-21, 12:37

Paar Offline

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After some visual consultation with photos of AN4T R2, I guess the needed resistor value is 2.2kOhm 5%. There is a red-red-red-gold resistor between the GND and OUT pins, which should rise the voltage to 3.45V.
This board allows for voltages 3.3V, 3.45V, 3.6V and 4V. There four more resistors which help with that, the first one from the right being 0Ohm resistor, which probably results in 3.3V. A little suspicious. The next one is 27Ohm resistor, which results in 3.45V? As I don't have this revision personally I can only assume.

I'll probably start with 2.2kOhm value and see what happens.

Reply 2 of 4, by Paar

Posted on 2024-02-25, 17:08

Paar Offline

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A little update - though nobody seemingly cares 😁. Won't stop me anyway!
I shouldn't take AN4R2 photos as a reference because it uses different approach of adjusting voltage on the regulator. Upon closer inspection I realized that the jumper JP40 connects GND pin of the voltage regulator to ground or goes through a resistor first and then to the ground. So no connecting GND and OUT pins. By the way, probably have a AB-AN4 1.0 board, not AN4R2.
There is an example in the LT1085 datasheet of adjusting voltage on fixed LT1085:

Unfortunately it's for the 5V version of the regulator with 12V input. What's the correct value for 3.3V variation then? Who knows. Hopefully some tinkering with a potentiometer will answer this question.

Reply 3 of 4, by Horun

Posted on 2024-02-25, 18:07

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the LT1085 plain or 1085CT (none -5) can be adjusted from 1.1v to Vin-1v iirc. OK see page 12 datasheet here: https://www.analog.com/media/en/technical-doc … ts/108345fh.pdf
the "ground" you are measuring is for the ADJ pin. the LT1083 is the 7.5amp version of the 1085 3 amp....

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. Stuff: https://archive.org/details/@horun

Reply 4 of 4, by Paar

Posted on 2024-02-25, 18:59

Paar Offline

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That's for adjustable variation of the regulator where you need at least two resistors to adjust the output voltage to the desired level. It would be possible to adapt the adjustable regulator to my board, the work would be a little messy as I would need to solder another resistor probably on the bottom of the board. I like it clean.
That's why I want to stick to the plain LT1085-3.3 with fixed voltage output. It's possible to use only one resistor to get a little higher voltage than the one which the regulator is rated for.

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