I have a UPS about 11 years old. It is an IBM OfficePro 700 which is
actually a Tripp-Lite product. Inside on the circuit board it says
66-0546. The web says the Mfr model number is 10K1912.

Not too long ago its yellow light started flashing that says the
batteries are discharged. Because of its age, I assumed the batteries
were worn out so I bought new ones. Unfortunately, after installing
them the problem did not go away.

When plugged in to AC, the unit has its green LED lit which shows it
is getting AC power in. The yellow LED flashes. according to the
manual, "This yellow light will turn ON continuously to indicate that
the UPS's battery is less than fully charged. If it stays on
continuously, contact Tripp-Lite fo service. The light will flash
after you set the 'OFF/ON' Switch to the 'OFF' position to indicate
that the UPS will not provide battery backup during a blackout or
brownout." Regardless of whether I set the OFF/ON switch to OFF or ON,
the yellow light flashes.

There is a Mute/Test switch. If I run the test, the red LED briefly
lights and the alram briefly sounds, as if it tried to switch over to
battery power but there was no power there.

It kind of seems like the charging circuit is not charging the
batteries but I don't know how to test this theory. I left it sit
plugged in for a few days hoping it would charge but it did not.

There are several fuses on the circuit board, all soldered on, and
they all measure zero ohms so I don't think that is the problem. There
are no obviously leaky capacitors or any other components that look
burned up or damaged judging by visual inspection.

I like this UPS and would like to revive it, especially since I spent
money on the new batteries. If anybody could help me diagnose the
problem, tell me where to start, what to check, etc. I would
appreciate it.

All ideas welcome. Thanks in advance.