apc smart ups 1250

[BillP]

My 1994  Smart UPS 1250 has been working since then with maybe three battery replacements.

Last week the batteries started to show exhaustion so I replaced them again. Two new 12V, 17Ah Yuasa NP17-12I went in.

Everything back together, turn the unit on , the fan runs a few seconds, then stops, the transformer makes the usual hum noise, but the front panel has no LED lights on and the two buttons have no effect whatsoever.

I am not sure if I hooked up the primary wires of the transformer correctly (White Neutral, Black Hot, Yellow Tap).

The board looks like new, no burns, no blown capacitors, no bad diodes.

Voltage between Hot and Neutral is 240Volt, between hot and Tap it is 27 Volt, transformer output is 15 Volt.

I suspect that maybe I connected the primary wires incorrectly, but do not want to try switching them without looking at a wiring schematic.

Any hint about what the problem might be? Or where I can get a schematic for this old UPS ?

[BillP]

hello,

Out of curiousity, why did you disconnect transformer wires? The batteries can be swapped without disconnecting any of those wires.

Transformer wirings going to PCB:
Black - XFMR-HOT
White - XFMR-NEU
Yellow - XFMR-TAP

Transformer wirings going to the heatsinks:
Black - first heatsink viewed from front of the unit
White - last heatsink viewed from front of the unit

(see attached picture of my unit)

Idr2NmBrrwE9CAMenhoZ%2BA%3D%3D.jpeg

[BillP]

thanks Wolf.

since the old batteries measured both  12 Volts,  first I thought something was wrong with the MB. I disconnected the wires in order to remove the main board to check for any bad solder or damaged components.

Then I rechecked the batteries and found out they were no good when trying to recharge them with an automatic car battery charger. They did not take any charge.

I have the connections correct as I can see, but still getting no signal on the front panel. Some other component is probably gone, but hard to find without a schematic. I am working the main board 2 days now and thinking that maybe its time to trash this UPS and to get a new one.

[BillP]

Check the front panel ribbon cable, it is very rigid and can be easily broken or damaged while servicing the unit. Might explain why there is no reply when you press the buttons. The next step after this would be to check the presence of +5V supply voltage of the controller logic. I could send a useful pdf to you but first I need to add you as a friend on the forums. Please approve friend request.

[BillP]

right, when removing the MB the flat cable broke two connections, which I soldered back again and checked continuity once installed back in place. That worked correctly.

[BillP · Accepted solution]

I sent a private message to you with a link to the download, see if it helps to repair 🙂