UPS BackUp-Time 525BT

A couple years ago, I swapped the original (7A2) battery for a car battery and had to be careful, because I measured some 90V or so, between the housing and battery terminals.
It all went good until the day I accidentally misplaced the setting on my multimeter; instead of the voltage measuring setting, I had it on Amp setting. Result: It shorted the circuit, the house fuses went off and now my 525bt only works via battery supply, running only about 5 minutes after switching on the main switch of the unit. The 5 fuses of the unit are all O.K.; nothing looks burned.
I'm a fourty year experienced hobby electronics fan and would need a circuit schematic to find and repair the damage.
In the worst case I thought of powering the unit via two 13V/4Amp transformers in (parallel phase)via a bridge rectifier, a 60oooMF/16V electrolytic capacitor and an automotive relay, switching over to battery PS in case power failure.
The only problem I then would have is: how to get rid of that timed 5 minutes automatic turn off cycle??? It must be in that IC or is there a timing cap responsible for it? What to do? I would greatly appreciate some advice on that.