SIEMENS UPS B4208 problem

[new_user]

Hello, dear experts. Maybe I create this topic in wrong conference, but I don't know where could I ask about it. So, we have problems with the following mature Siemens UPS (issued in 1993):

Data from enclosure label:
SIEMENS UPS B4208 / 16 puls rect.
TYPE: D400/115
Order No: 6SU4208-0AA00-1MA0
P = 80 KVA.

After switching off the UPS for battery maintenance the inverter doesn’t work. After switch on rectifier input switch-disconnector Q10 the red LED H4 lights up on the module A101 (see page 1 in the Attachement). After pressing the button Inverter On/Off and button Enter (Invertor On) a red error symbol lights up on the display. This error could be reset by the button S1 on the module A102. Simultaneously red LED H4 lights on the module A101 (see page 3 in the Attachement).

When we try to start the inverter again, red symbol of inverter’s error turns back on the display. After error reset (by the button S1 on module A102) red LEDs H1, H2, H3, H5, H6 light up on the module A101 instead of LED H4 (see page 2 in the Attachement).
When we try to switch on the UPS, situation is repeating cyclically on the unit A101.

Please help us in determining of possible causes of error: either fault of module A102 or fault in inverter power circuit. Or something else ...

Reserve module A102 is absent, so we can't replace the faulted one.

See pictures of LEDs states and display symbols in attachment:
http://www.filedropper.com/upspics

Thanks. Any your help will be appreciated.

Best regards, Alexander