Should be possible, but not by altering the jumper as I suspect the only thing it does is to short out the series transistor when set to 5 volts.

I think you will have to play with some resistors that bias the base of the TIP31C, or it could be something more complicated using a voltage reference. Please post a close-up photo of the transistor and the area surrounding it.

quicknick wrote on 2020-11-22, 20:36:

Should be possible, but not by altering the jumper as I suspect the only thing it does is to short out the series transistor when set to 5 volts.

I think you will have to play with some resistors that bias the base of the TIP31C, or it could be something more complicated using a voltage reference. Please post a close-up photo of the transistor and the area surrounding it.

Attached, thanks

Seems there's a LM431 involved, so I feel that's a bit over my head. But check its datasheet and try to trace out the schematic, find out the two resistors connected to the Ref pin - with a little effort you can replace one of them with a variable resistor and obtain a wide range of voltages...

Long shot, but does anyone know of a schematic for the board (or schematic library that might have it) online?

The best you can do is find a reference schematic for the LM431 IC. Finding a schematic for that motheboard is next to impossible.

computerguy08 wrote on 2020-11-23, 11:22:

The best you can do is find a reference schematic for the LM431 IC. Finding a schematic for that motheboard is next to impossible.

Thought it was a bit optimistic 😉

You can always try to reverse engineer it. It's a simple power delivery system, it shouldn't be too hard.

Simple is in the eye of the beholder! 😉

Okay, here's the relevant bit from the datasheet:

Vref is a fixed 2.5V. I think R5 (1.2Kohm) and R15 (3Kohm) from your board correspond to R1 and R2 on the above schematic. If I'm right, replacing R5 with a 1.5Kohm will yield a voltage of around 3.75V, or 4 volts when using 1.8Kohm. Or replace it with a 3.3Kohm multi-turn trimpot and dial-in any voltage from an useless 2.5V up to (almost) full 5V.
Of course there's a chance I'm wrong and your board will go up in flames, that's why I suggested you to study the datasheet and trace the connections around the LM431 😉

quicknick wrote on 2020-11-24, 00:22:

Okay, here's the relevant bit from the datasheet: LM431.jpg Vref is a fixed 2.5V. I think R5 (1.2Kohm) and R15 (3Kohm) from your […]
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Okay, here's the relevant bit from the datasheet:
LM431.jpg
Vref is a fixed 2.5V. I think R5 (1.2Kohm) and R15 (3Kohm) from your board correspond to R1 and R2 on the above schematic. If I'm right, replacing R5 with a 1.5Kohm will yield a voltage of around 3.75V, or 4 volts when using 1.8Kohm. Or replace it with a 3.3Kohm multi-turn trimpot and dial-in any voltage from an useless 2.5V up to (almost) full 5V.
Of course there's a chance I'm wrong and your board will go up in flames, that's why I suggested you to study the datasheet and trace the connections around the LM431 😉

Thanks - I was a bit confused about which circuit on the data sheet was the correct one to follow (No engineering training, as I am sure is obvious!!)

I was thinking R5 and R15 were the voltage dividers too.
There is 0 ohms between the Vref pin of the LM485 and one end of R5 and also of R15

Referenced to ground:
The ouput from the LM485 cathode is 4.1v and that is the same voltage as is fed to the TIP31C base (4.75v is on the collector == power supply output on the 5v line; 3.4v on the emitter)
Voltages on the Vref pin of the LM485 is 2.45v
on R5, on the side wired directly to the LM485 is 2.45v and the other side is 3.4v
R15 is 2.45v on the LM485 side and 0v on the other side

So that is all as it should be if those are the voltage dividers shown in the data sheet diagram I think?

Also, then if, as a test, I piggybacked a 15k resistor over the 3k resistor, that should give a resistance of 2.5k and a voltage output from the the TIP31C of 3.7v?

(so wire to vref pin and then a ground point - avoid soldering on the board)