krish2487 hi,
Yes, the transformer has 3 wires, so its a line interactive UPS.
It seemed strange to me the fact that there are no big smoothing capacitors in the circuit,
but then I looked at the output of the UPS...
With no load:
With a 15" LCD monitor connected:
Output Ripple with light loading...:
I am not sure if this output is any good for any of your PC components, even for 5 minutes...
I am starting to think that a pure sinewave UPS is a better idea.
4. Most often the stepdown and sense analog circuitry will have a couple of op amps, it is also possible that one of the op amps and /or associated circuitry is the cause (This seems more likely, since I dont see a sense tap on the transformer, meaning non isolated voltage sensing.) It is possible that during one of the brownout or spikes one or more of the components in the circuit path were affected.
In the first picture immediately right of the heatsink you can see a Cosmo 1010 optocoupler.
Two more are on the other side of USB control board. (The unit has RJ45 protection as well.)
So maybe its protected from such spikes.
I don't like the thought of replacing the opamps. They are heavily populated with tiny SMD devils
in a very small space....
What can I check on them without removing them?
I will replace the caps just to be sure and I have just removed a relay to check its switching speed.
PS: R182 and R67 SMD resistors in the previous pic , towards the top seem burnt. Is that the voltage divider section for AC??.
Sorry, this was not a good photo.
The red stuff is the glue that they put to hold in place the SMD parts.
A lot of the solder joints on the main board are not that good, there's a small darker band on the solder indicating the connection is bad, e.g. below C49.
Same here. Its the flux residue.