Update: Thanks Armadillo for the reference. Unfortunately, I could not find points to test current in Main PCB. It's very tightly packed with covered traces. Also, I was not getting 7V at few places in main PCB, so I went to test DC/DC power board again.
There are 6 connections to DC-DC power board as below:
1) upper body - containing upper display, flash, on-off and other control buttons
2) DC driver board - Drives the sensor
3) Main PCB - Also, all other parts connect to main PCB for instructions and feedback.
4, 5 and 6) Connect to front body, Shutter and Mirror.
I am getting 7V output on 1, 2 and 3 now. However, negligible voltage on connectors 4,5 and 6 with voltage line shorted to GND. These connections share same line and in the path of two capacitors I think might be causing short. Pls refer to the attached pics. I have highlighted connections with 7V output in Green, Connections with no output in Red and doubtful capacitors in yellow. These two capacitors had all the corrosion on it. These are too small to be replaced easily.
My theory, since, main PCB is not getting voltage feedback from front body parts, it's causing the low battery error. Main PCB should be working as it's behaving as expected. Also, upper body parts getting power that's why upper display and on-off buttons working. When I insert battery, a low battery indicator symbol comes and stays on top left corner. Nothing else. When I try to switch on camera. It gets into a start loop. Low battery indicator starts flashing, green back-light of upper display flashes and everything switches off. Again, the low battery indicator comes, start flashing, green back-light and off. It continues till I switch the button off. Once off, stable symbol of low battery indicator comes in the corner and stays.
To this point, it looks like changing DC-DC board should solve the problem. Or at least I can't troubleshoot further till my replacement DC-DC power board arrives.
This camera had no power at all when I got it. At least it came till some power state from no power