Smart UPS 1400 not switching to batts correctly

[Anonymous user]

Hi -

I picked up an older smart UPS: Model Number SU1400RMNET, main board part number 640-0732P, rev 16.

With no load, the unit power up correctly, goes through self test and charges batteries. I plug in some load (about 500W), cut the input power and the UPS shuts down completely. At very low load, it will try to switch, and eventually get on battery power before shutting down, btu the power to the load drops in the corssover. Sometimes it will switch correctly if there is no load. It will also power up correctly if there is a load attached prior to power up, but again, when cutting input power, it just shuts down.

The batteries are good - they measure good and I've also changed them.

There is no obvious damage to the board, I have checked all the electrolytics and none are shorted. What's the best way to test the relays? I'm an EE so I'm not gonna get myself killed doing this, and with a little info, shold be able to get this thing going again.

Any help appreciated..

Brent