Hi,
Is this the UPS you have? 
http://www.apc.com/resource/include/techspec_index.cfm?base_sku=br800i&tab=models
Does this look like the correct manual for it? 
http://www.apcmedia.com/salestools/ASTE-6Z7V5B/ASTE-6Z7V5B_R0_EN.pdf
I'm just going through what I would check. I'm not sure of your level of 
experience and I hope you don't feel I'm insulting you.
On 07/03/13 18:26, Martin N wrote:
>
>
> Not really convinced yet that it is the battery.
>
Some times, batteries do go short and prevent gear from powering up. I 
recommend you disconnect the UPS then unplugging the battery. When 
you've done that, power the UPS up again. The manual says that if it is 
powered up with no battery, it should chirp and show the green (power) 
and red (replace battery) LEDs. If it doesn't do so with no battery 
connected, that eliminates a present battery short as the culprit.
Next, I would eliminate all the silly "D'Oh!" problems. Check things 
like there are volts on the end of the IEC cable and that you're 
pressing the power button for the right length or time. (You did mention 
holding the button down. The manual seems to suggest that a short press 
is on and a >10 second hold is programming mode)
When you press the power button, do you feel the switch behind it click? 
The switch is subject to mechanical force. Have the solder joints that 
fix the switch onto the board broken?
Next I would look for fuses inside and bad connections (also do a 
nose-test for "brown smell"). I always hope things like this are 
something simple like loose/corroded connections, broken wires fractured 
solder joints on things like the back of the IEC input. I know fault 
finding why a switched-mode inverter isn't oscillating is beyond me.
Cheers,
Paul.
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