It's alive!!! Muhahahahhahaha

Whenever I put something back together I'm always delighted if it still works, from that point everything else is gravy :)

Ok, in fixed mode I now have 47v output :naughty:

First attempt was across R12, I don't know what it is supposed to do but it doesn't affect the fixed mode voltage.

Second attempt I used 9k4 in parallel with R34 and the result is close enough to my calculations. Always a good sign. Why start with 9.4k? Simple, I found a bag of 4k7s.

Calculations:
Top divider resister is 9.1k
We are changing the value of the divider resistor, call this value x.
Desired output voltage is op
Ratio is the ratio of the divider
These calculations are based on k ohms

op = 4.29 * Ratio
Ratio = op / 4.29 : Required Ratio

Ratio = (9.1 + x) / x
x = 9.1 / (Ratio - 1) : Required resistance

As we are creating the value x by placing a resistor in parallel with the existing 1k resistor we need a bit more maths. Lets call the parallel resistor rp.

x = 1 / (1/rp + 1/1)
rp = x / (1 - x)

N.B. In the case of THIS power supply, the existing resistor is 1k. As x is the value resulting from paralleling a second resistor, x will always be less than 1.


Ok, now to work through for my measured 47 volts:

47 = 4.29 * Ratio
Ratio = 47 / 4.29 = 10.956

x = 9.1 / (10.956 - 1) = 0.914

rp = 0.914/(1 - 0.914) = 10.63

Hum, 10.63k is not the 9.4k we expected, lets work the calcs the other way:

rp = 9.4

x = 1 / (1/9.4 + 1/1) = 0.904

Ratio = (9.1 + 0.904) / 0.904 = 11.07

op = 4.29 * 11.07 = 47.49v

Less than 0.5v difference, maybe my calcs go wrong somewhere or my meter is worse than I thought, but I think the result is close enough.