I go on a bit long here, but eventually realise that some of the measurements are caused by a capacitor charging up. I'll leave what I originally wrote as it at least shows what I was thinking. The short version is that I still can't see anything wrong, and the regulator should be putting out around 3.4V ( 1.25*(1+91/160) ).
Try putting JP8 on 1-2, remove the 5V-3V jumpers. Then fit the spare regulator (maybe there is a problem with it) and check the connections (so pin 1&2 to +5, 3 to Ground, 4 to right-hand column of 5V-3V jumper block, 5 to bottom row of JP8. If those are all ok, turn on and see what the voltages on each pin are.
I've added a load resistor/capacitor to my earlier diagram, which hopefully makes it a bit clearer about some of the problems of measuring components in circuit.
Hoping wrote on 2021-11-30, 15:44:
Lifted 5 pin of the MIC,JP8 1-2;4-5 85k;3-5 69k
5 to far end of R20 85k but never stabilizes, oscillates up/down 0,3
5 to nearest end of R21-71k
So 3-5 should be the same as 5 to nearest R21 (pin 3 is Gnd, and the nearest ends of R21,R69,R68,R67 are all Gnd). I'd consider 69k and 71k to be close enough to look reasonable. Equally, 4-5 is the same as 5 to far end of R20 (4 is Vout, which goes to the far end of R20). You've got both of those as 85k. The oscillation you see might either be slightly noisy probe contacts.
I'm going to guess If we assume the existence of a ~100k resistor between Vout (pin 4) and Ground, then that more-or-less matches with your readings. Measuring from 3(ground)-5(adjust) is measuring across R21 (91k), but if current can flow from Vout to Ground then it's also measuring from Adjust to Vout (through R20, 160k) + Vout to Ground (through my guessed at resistor). So effectively it's measuring 91k in parallel with 160k+100k, which is always going to be less than 91k. If you were measuring with the 5V-3V jumpers in place then maybe that's that cause of the numbers not matching the resistor values.
On the spare regulator, is there a resistance between pins 3&4 (Ground-Vout)?
Lifted pins 3 and 5
5 to far end of R20-89K
So that's equivalent to measuring 4-5. Measured resistance has gone up a bit, but I think it still suggests there's a resistance between Vout and Ground.
Value of the resistors in circuit;
So they're as before.
This one's interesting because it breaks the circuit with that external resistance I think is confusing the results
R20-145k oscillating a bit.
R21-89k sometimes going of scale, tested a lot of times.
So these are not much closer to the actual resistor values. Going off scale could be interesting. It could either be a dirty probe contact, or maybe a dry solder joint. It might be worthwhile just reflowing the solder there.
I think that makes sense. R21 is now disconnected so reads it actual value. R20 is now in parallel with R69+my guessed Vout-Ground resistance. R69 is bigger than R21, so R20 appears to go up.
There's something odd with this board, I'm not very intelligent but, following your schematic those resistor readings are very w […]
There's something odd with this board, I'm not very intelligent but, following your schematic those resistor readings are very weird, without jumper on JP8, why R20 is not 160k, no more or less....
Measured resistance between the far ends of the resistors with JP8 off;
R20-R21=154K;R20-R69=198K;R20-R68=315K;R20-R67=191K;R21-R69=190K (but after a minute it still increases slowly).
So the nearest end of,R21,R69,R68 and R67 is connected, but also the far end of then and R20 is connected somehow.
Always taking the reference of the Mic as nearest or farthest.
If my guessed at resistor/load is real then I think these look reasonable, they're mostly adding the R21 to R67 resistors. So R21 to R69 is 91k+100k, or R68 to R69 is 110k+82k (ish). The interesting ones are the R20 ones. They have to go through my guessed resistor, then back up R21/69/68/67.
Which looking at those finally makes me realise that this guessed at resistor is actually probably a capacitor gradually charging up as you take your measurements.