The unit is an APC BR1500MS "Back-UPS Pro 1500 S".

I recently purchased this UPS off of facebook market for a decent price, but I now believe I see why he was selling it! The UPS makes a faint whine. I've got the thing disassembled on my floor and used some silicone bendy straws in my ears (funny picture I know) to "search" for the noise origin. Initially I thought it to be a coil, but, it turned out to be the buzzer! This makes me think t hat there's something fishy with the circuit that drives the buzzer, or perhaps the DC power rail that it runs off of. I've also noticed the fan doesn't spin when I test the thing with a large load and the inverter transformer gets a bit warm. I don't see any small wire leads going into the transformer either so I don't believe it to be temperature activated. I ran my SMD rework station off of it to see if it was triggered by power draw, but that didn't do it either. I've also noticed that only 2 of the 4 Silicon power chips attached to the inverter transformer get warm, well actually, quite warm. The other 2 are arctic comparatively.

I've got a Rigol DS1054Z Oscilloscope (No current probes, sadly ) and a Fluke 87V DMM to diagnose this with.

But in writing this out and thinking about it, I wonder if the power transistor shorted out and is pulling a bunch of current from both the battery & the dc power rail which is why the buzzer is so noisly and fan doesn't come on. I'll have to check the output of it to see if I'm getting 120V sine wave & do a continuity test across the leads of the silicon power chips.

Will post some pics in a bit, but before then, anybody familiar with UPS design have any ideas?

 

hello there.
Troubleshooting Problem Possible Cause Corrective Action The Back-UPS is not connected to AC power. Ensure that the Back-UPS is securely connected to an AC outlet. The circuit breaker has been tripped. Disconnect non-essential equipment Back-UPS will not switch on. from the Back-UPS. Reset the circuit breaker. Re-connect equipment one item at a time. If the circuit breaker is tripped again, disconnect the device that caused the trip. The internal battery is not connected. Connect the battery. The AC input voltage is out of range. Adjust the transfer voltage and The Back-UPS does not provide power during a AC power outage. Ensure that essential equipment is not plugged into a SURGE ONLY outlet. sensitivity range. Disconnect equipment from the SURGE ONLY outlet and re-connect to a Battery Backup outlet. The plug has partially pulled out of The Back-UPS is operating on battery the wall outlet, the wall outlet is no longer receiving AC power, or the circuit breaker has been tripped. Ensure that the plug is fully inserted into the wall outlet. Ensure that the wall outlet is receiving AC power by checking it with another device. power, while connected to AC power. The Back-UPS is performing an automatic self test. No action is necessary. The AC input voltage is out of range, the frequency is out of range, or the waveform is distorted. Battery Backup outlets may be fully or improperly loaded. Adjust the transfer voltage and sensitivity range. Disconnect non-essential equipment from the Battery Backup outlets and connect the equipment to SURGE The Back-UPS does not provide the expected amount of backup time. outlets. The battery was recently discharged due to a power outage and has not fully recharged. Charge the battery cartridge for 16 hours. The battery has reached the end of its useful life. Replace the battery. The REPLACE BATTERY indicator is illuminated. The battery has reached the end of its useful life. Replace the battery as early as possible. The OVERLOAD indicator is illuminated. The equipment connected to the Back-UPS is drawing more power than the Back-UPS can provide. Disconnect non-essential equipment from the Battery Backup outlets and connect the equipment to SURGE outlets. The SYSTEM ERROR indicator is illuminated, all the front panel indicators are flashing. There is an internal error. Determine which internal error message is displayed by matching the number displayed on the LCD with the corresponding Error Message (see "System Errors") and contact SEIT Technical Support.

 

Thanks for copy / pasting the user manual at me
So the fets (IRFB7440 N-Channel Enhancement MOSFET) seem fine. High resistance across all pins. Now to check the driver circuitry...