I dunno about a spike but the 90 VAC is low for what it should be delivering. Typically in the 115 - 120 VAC range.
Upon disconnecting the unit from the household current - my voltage drop across one battery outlet was 90vac and another was 95vac. I'm of the opinion that the unit is shot and may've contributed to the motherboard failure by allowing a spike to get thru.
You don't mention if the UPS was under any load? If you are getting the 90 VAC off an unloaded UPS that is not good. Have you checked the battery voltage when the unit is running and unplugged?
This is a typical specification for a basic ATX PSU:
3.1. AC Input
Table 1 lists AC input voltage and frequency requirements for continuous operation. The
power supply shall be capable of supplying full-rated output power over two input voltage
ranges rated 100-127 VAC and 200-240 VAC RMS nominal. The correct input range for
use in a given environment may be either switch-selectable or auto-ranging. The power
supply shall automatically recover from AC power loss. The power supply must be able to
start up under peak loading at 90 VAC.
Table 1. AC Input Line Requirements
Parameter Minimum Nominal+ Maximum Unit
Vin (115 VAC) 90 115 135 VAC rms
Vin (230 VAC) 180 230 265 VAC rms
Vin Frequency 47 -- 63 Hz
+Note: Nominal voltages for test purposes are considered to be within Â±1.0 V of nominal.
If the UPS in question was only delivering 90 VAC that is bottom end. If the 90 VAC you measured was unloaded UPS voltage that sucker is scrap! I would look at the battery voltage with the UPS under a load. Not a computer either but maybe a few 100 watt household lamps.