exxos's Atari, Amiga & retro forum

Does anyone know which fixed resistor is used for 5V regulation feedback on a MegaST power supply?

From the schematic, I'm guessing this is R14/R13 since these form a divider across 5V/GND and there is a small stabilizing cap across R13...

If anyone knows differently, please say before I mess with it :)

Yeah probably them. I just normally slap like 100K across them and then see if the voltage goes up or down a bit..

exxos wrote: 04 Aug 2025 14:11 Yeah probably them. I just normally slap like 100K across them and then see if the voltage goes up or down a bit..

:thumbsup:

that was my thought exactly

@exxos is probably right, but looking at a datasheet for Q3, which is a voltage reference, it could also be R11 & R12 since the external components for the TL431 determine the voltage that it gives as it's reference, at least from what I understand.
Just a thought, probably needs somebody who knows electronics very well to determine.

Yes I think there are two parts to it - one (TL431) generates the fixed reference voltage and the second part is the measurement (the stabilised divider feeding back from the 5v rail). I think its the second part (measurement) which would be adjusted against the first (fixed reference).