Another project fixing an elusive issue of a power supply board. Photo of both sides attached (back side flipped).
What's known:
There are two sections of the circuity on the board, separated by a broad line on the frond side, which I believe are electrically isolated. AC mains comes in from the socket at the bottom right corner. There are three output sockets in the upper section of the board: P162 (green, 2 pin), P 161 (black, 2 pin), and P163 (red, 5 pin). The 4 pin green socket (P160) is input (into a couple of opto couplers), which we can ignore for now.
The output voltage of the three sockets are:
P162: 6V DC (pin 2)
P161: 28V DC (pin 1)
P163: 5V DC (pins 1/3), 28V DC (pin 4)
Symptom: The voltage of 6V DC from P162 presents at some time, not other times.
The circuit might be plain to the experienced who might be able to pinpoint the culprit at the first glance. All input is welcome, including the working of this board, topology of the transformers U146 (top left) and U145 (smaller, bottom right).
What's been done:
Caps on the affected rail C88(1000uF 50V) and on the one across the PWM controller IC (UC3844) VCC/GND (100uF 25V) are replaced with used similar ones (for test, no new replacement handy) but no cure.
[Worse than that, probably due to error while probing, further damages occurred beyond that. Detail to follow.]
Questions:
1. C86 (1000uF 50V) and C87(100uF, 35V) are in parallel, side by side, on the 28V DC rail. Why there need to be two caps, and of different voltage ratings (not simply one 1100uF, 50V) – for filtering different frequencies of ripples (even so, why different V ratings)?
2. From what I think is a fairly simple single layer board and the clear photos of both sided, side by side, is anyone able to tell the working principles of this SWPS? And likely weak spots probably common to this topology?
3. More to come.