APC Back-UPS Pro 900 Output short

[arrgh]

Hi all,
I got this UPS from a friend that after a fault replaced it with a new one.
The UPS is an APC Back-UPS Pro 900 (BR900GI).
I've bought 2 new 12V 9Ah batteries to test it and when I connect the main 230V input, relays click to turn on the UPS, then I press the power on button, other relays clicks to turn on everything, the lcd flashes for a moment and then it immediately shutdowns (relays click to turn everything off).

So I've disassembled it and checked on the board for shorted components.
Attached 2 pics of the primary board and a secondary small board (I think this small board is the battery charger but I'm not sure).
On the primary board there are 4 IRF3205 mosfets, which 3 of them were shorted.
I've replaced all four IRF3205 with new ones.
Now the behavior is changed: when I connect the main 230V input, some relays click and the UPS goes into "standby" mode, and it seems to charge the battery normally (voltage on the battery goes up from 24.5V to 27V), but when I press the power button to turn it on, relays clicks another time to turn on the output and after around 5 seconds, the display shows F02 error (from the manual: Short on battery output). After around other 10 seconds, the display slowly goes black (it seems that cannot stay on...), the constant beep slowly goes low and after that, relays click another time to shutdown everything, I think for protect the UPS from the short. After that, relays click another time, I think for going into standby mode and continue to charge the battery.

I measured with multimeter the battery+surge output, and without the transformer connected there was no short apparently.

What can i check for trying to resolve this problem? Can it really be a short on the output?

Thanks in advance for any help and advice

[sam_sam_sam]

Re: APC Back-UPS Pro 900 Output short

Did you recheck the mosfets again to see if they are shorted out again if so then you have issue with what is causing this issue

Do you have any 24 volt incandescent light bulbs that you can connect to the transformer output instead of the transformer and see if they will light if so you might have an issue with the transformer when it is in the backup mode

If you have a ring tester you could test the the transformer for if it is shorted or not

[arrgh]

Re: APC Back-UPS Pro 900 Output short

Did you recheck the mosfets again to see if they are shorted out again if so then you have issue with what is causing this issue

Do you have any 24 volt incandescent light bulbs that you can connect to the transformer output instead of the transformer and see if they will light if so you might have an issue with the transformer when it is in the backup mode

If you have a ring tester you could test the the transformer for if it is shorted or not

Hi, sorry for the late answer.
I've rechecked mosfets with multimeter and they don't seem to be shorted.

I've 2 24V incandescent light bulbs, 3W each, can be ok for the test? If yes, where I need to connect it?
Attached there is a photo of transformer connections.

I don't have a ring tester (I didn't know it), but if it isn't too much expensive, I can buy it

[sam_sam_sam]

Re: APC Back-UPS Pro 900 Output short

Here is one place where you find one

https://www.ebay.com/itm/18423017789...3ABFBMvrfc3Jtf

[wyros]

HI
Could anybody explain or read for me WHAT is element D66 and Q49. My board is completly burn in this place


Thanks a lot