IC502 on Pin 3 = 22 mV DCMarkOZLAD wrote:What do IC502 and IC503 voltages read on their output pins?
IC503 on Pin 3 = -001 mV DC
I'm still going at it. Trying to think about it from different angles. I've pretty much redrawn most of the SMPS section by hand in an attempt to mentally process it and also just trying to educate myself on how these work. Worst case, I'll have to learn it all and calculate the target voltages from the start of it to the exit of the isolated section. Just redid of the voltage tests, so some of that stuff I put down earlier should be wrong: Do appear to be getting 120v up until the bridge rectifier and then it's all about 57v from there.
Actually, I just realized those random numbers on the schematic seem to be target voltages.. duur. Which kind of align with stuff I'm seeing on the mains side except at the transformer. Hmm.. now I have some actual targets to test for.
Edit: IC103 (RESET IC) = 5v DC in and out. This runs to the power button where I am also getting 5v. When the power button is pressed, it shorts to ground and drops 5v off Pin 6 (Key 1) at the Micon. I have 5v at Micon Pin 27 (RESET) and 0v at Pin 29 (POWER) on the Micon.
It appears that 5v should exit that pin and trigger the STAND BY SW (Q505) & STAND BY SW (Q509) transistors, which would then switch on the +12V SW (Q508), +5V SW (Q510), and +B SW (Q503) transistors to feed the 5v (IC503) and 9v (IC502) regulators etc etc. The POWER pin of the Micon is also tied to P FAIL.. and I have no idea what that is. It's infuriating that I can't find a datasheet for the Micon OEC7074A. But I feel like the fact the transformer has nothing on one side, I shouldn't really worry about outside of that I guess. I sort of got distracted with the rest of the board and perhaps I should refocus on power switching. Pull that big switching transistor off to test.
Seems like a lot of what I did before wasn't quite right but I think I got it down now..
Current state of things in Photoshop with following traces and checking voltages: Photoshop Schematic Images
Edit 2: So I pulled out the main power switching MOSFET (Q502), touch the ground to Source, positive to Drain and get OL. Touch the positive to Gate and then back to Drain, still nothing. This is apparently not correct so.. maybe this is the problem. Will order replacement (and spares) and hopefully end this hellish nightmare.