BACK-UPS PRO 420 MOSFET Burning Problem, please somebody help....

Anonymous user · ‎2021-06-29

Hi,
I`m new here and my english is not good but I`ll try to describe all details related to my problem at best possible way and I hope that somebody will help me in this problem...

I have old APC BACK-UPS PRO 420, and for some reason on this ups stop to work, after checking what is problem I see that main transformer is burned and one mosfet (q34 - schematic -> http://www.upsclub.org/schem/APC/218/218F5S5.pdf ), because I have one more same ups with some other problems I replaced this parts and when I start this ups to work on battery (unplugged from main power source) all working fine, without any problem, no heating no bad sounds, nothing, all fine...I leave this ups to work few minutes on battery and all was good...after this I plug it to 200v ups switch normal from battery to main power source and continue to work normal, no heating, no bad sounds...but after ~10 seconds ups stop to work, stop with beeps, fet q34 and q33 start to heat abnormal, from transformer I have some very bad sound like overloaded and I must unplug it from all power sources (now again don`t wont to work on battery, same sounds like on main power and after few seconds trying it switch to overload and stop to work)...

After checking mosfets again I see that Q34 is again burned (not q33), I replaced it again and again same test but again on same way same mosfet is burned!!!

Do you know why this mosfet burning after ~10seconds on main power source??? Without any load, without any heating of this mosfet (until it burned)...

All schematics you can see here: http://www.upsclub.org/schem/APC/218/

BillP · ‎2021-06-29

Unfortunately, We do not troubleshoot at this level on these units. Though it may be possible to replace components, we wouldn't be able to give you any advice as to how or any advice why a specific component isn't working as we don't trouble the individual components. If anyone outside of APC does have any experience with this, feel free to help xman123 out.

See Answer In Context

BillP · ‎2021-06-29

Unfortunately, We do not troubleshoot at this level on these units. Though it may be possible to replace components, we wouldn't be able to give you any advice as to how or any advice why a specific component isn't working as we don't trouble the individual components. If anyone outside of APC does have any experience with this, feel free to help xman123 out.