APC SMT1500RM2U UPS only blinks and beeps when plugged into mains

[mattrey]

Good Morning,

Its my first thread here in the forum, so Hello to everyone

I recently found a defective UPS for free and am trying to repair it.

What i´ve got so far:

- When plugged into mains, it only blinks (all LEDs) and beeps frequently
- opened the chassis and found a modular Power Supply PCB (soldered out the big 22uF Caps -> both had >20 uF capacity)
  The purpose of this PS seems to be supplying voltage for the logic circuits on the main PCB


- disconnected the little Power Supply PCB and hooked up the Battery Terminals to my Bench Power Supply -> can switch on the UPS and navigate the menus
  Can even activate the outputs, the output 230V AC

- Plugging Mains back in results in switching to Mains Supply, the Unit then tries to load the Battery (Amps on Bench PC drop to 0 and voltage is increased)
- When pulling Mains Cable, the unit goes into standby, it does not use the Inverter then, so i assume the little PS PCB does provide the voltage for that part of the circuit

As i don´t have schematics for the Unit, i´m struggling to troubleshoot the PS PCB. does anyone know, which voltages have to be output there (8 PIN connector to the right on my image)?

i found some DC Voltage but also AC 800 mV across some Pins, not sure which ones got together...
does not make any sense to my right now.

Thanks in advance!

Greetings
-Matthias

[asis]

Hi,

Welcome to the forum.
There are many talented people here.
I assure you it won't be boring.

So.

This is the charger module.(SMT1500RMI2U PCB 640-3081J-Z-001 (BAT-24V))
In order not to repeat - look at this thread:

https://www.eevblog.com/forum/repair/apc-smt3000rmi2u-battery-not-charging/

If you have any questions, I'm ready to answe

[mattrey]

Hi and thank you, asis,

i measured Pin 1-8 and got 21,5 V, as from 1-3 (or 1-4 ?), both with disconnected J906.
I decided to measure resistance between Pins 8 and 3 (or 4?) but it was o.L.

Powered the Board again and it sounded more silent (there was a high tone surring).
I thought i fried the board and measured only ~11V between 1 and 8 on J905.

But - now its whether blinking nor beeping.
Of course, the battery is junk and it does not detect it, but one time i even managed to switch back and forth between mains and "battery" (a.k.a. Bench Supply)
and it did well.

I´ll look for new Batteries and try running it again.

Regards,
Matthias

[asis]

Of course, you need a set of good batteries.

I advise you to check all ECAPs, especially C118 (33x35V); C717 (10x50V); C906 (330x50V).
The presence of ripple can adversely affect the operation of the entire circuit.

I prefer to do this with an ordinary arrow tester, because the arrow is like a living organism and can show a lot of interesting things.

Connector pins J905 pin3 and pin5 are charge voltage/current control.
Pin3 control DOWN (increment U), Pin5 control UP (decrease U) - control is performed by MPU IC11 89C51RC (pin24, pin6).

The contact of the J905 pin7 connector is the control voltage, which is generated through the Zener diode D717 (MMBZ5237) with a voltage threshold of 8.2V.

If the charger is working properly, the voltage is supplied to the Q43 base and creates the conditions for starting the UPS.

[Koray]

Without a good battery my ups acts like dead. Maybe before repair you should try the unit with a new set of batteries.
K.

[mattrey]

Hi,

i installed new batteries and the Unit works like a charm

Thank you again for your help

Regards
Matthias

[asis]

Good luck!