Repairing an APC UPS 550

Youve seen these APC battery backup units for sale.. or on marketplace type websites... these units are either ES-350 or ES550.

Finding a schematic or Block Diagram was a challenge that I could not find. They must have made these by the 1000's. I've seen websites from all over the world about issues with these units, from constant beeping, to chronic battery replacement, to electrical crackling noise from the inside.

This article is to address these issues. All of these issues require you to be very handy with a soldering iron, and some component replacement experience.

All this is based on my hands on with a few of these units, from getting a freebie, to one found at a donation store, and one from a marketplace website.

What components you need is based on the issue.

Soldering iron,,, solder wick or solder sucker.

If you have a good junk parts collection, then you may have the Capacitors, Diodes, Automotive Fuses, etc.etc to get some issues resolved.

You need to take your time, be patient, be careful. work with the unit disconnected from AC and battery. Many of the PCB traces are very narrow, and some holes are feed thru. This is not a beginner repair. Best of luck to you.

The unit just beeps constantly when installing a new battery.

This issue has been a loss of battery going to the ups circuitry.... I've found the automotive fuses to be open in most cases. You can have a bad stake-on terminal going to the battery, cold solder joint on the negative side of the battery to the PCB.

With the battery disconnected, check the following:

Ohming out the battery cable to the pcb is a first start.

Ohming out the Automotive fuses. If you see more than one, check them all.

I've had one with Dual 30 amp fuses blown... tho the green automotive fuses do not show the open condition.

The picture shows the back side of the PCB where I replaced both 30 amp Auto fuses with miniature 20 amp fuses. They are smaller and were easier to solder to the back side without taking the whole PCB out to remove.

Their is clearance if you break off a plastic tab on the case when you put the PCB back into the slots of the front case.

Crackling noises coming from inside the unit while the unit is running.

I have found the main transformer in the unit to be at fault. Some units were wound with copper coated aluminum wire. If you want to repair the breakage, you need special electrical connectors made for Aluminum wire. You cannot Solder this as the copper clad is no longer bonded to the aluminum. King Innovation makes an alumicon wire connector for multiple wires to a small form factor purple block.. ( picture shown )

Go a google search for this or an inline splice for aluminum wire, to transistion to copper wire. the pictures show the transformer sitting in the front case with the PCB in its slot.

I'm not showing the transformer procedure, as their is a youtube from Mr Carlsons lab showing a transformer failure discovery having aluminum windings... tho no attempt was made to repair it. If you can find the small screw down lugs, you can make that transition from the windings to copper

This issue is the UPS Battery dies after replacement. The UPS seems to work fine at first, but after a while... aka 2 weeks to a month or two, the UPS will alarm.

Basically, the UPS is designed to Float charge (13.2v) the battery, while its installed. This feature has stopped working. This repair has been related to a failure of some of the Electrolytic capacitors failing in the unit.

This problem was described on a website from another country, which had to be translated, but from other sites... i.e. badcaps, and found a pattern of people finding the issue.

It's good to note that their are not very many stand up style Electrolytics in these units. From posts I've read from the net, start replacing caps.

The ups units that i have, were found caps that are out of spec (dropping capacitance over time. I will assume it is due to the internal heat of the unit and runtime. Since they run 24x7 they are allways warm internally.

The unit with this issue is using a SMPS circuit to supply the voltages needed to run the circuitry in the unit. The battery is only needed to supply power for the conversion from 12 volt to 120 volt. The circuitry monitors the battery state and reports with a buzzer and led state change if their is a low battery condition, or failure, or unit is on battery mode.

I've supplied pictures of the PCB with the replaced Capacitors.

The 9 capacitors replaced are shown removed in a picture

2 - 4.7uF/50 volt C52 - C18

5- 22uF/25 volt ... C1 -C3 - C27 - C30 - C51

1- 100uF/16 volt C14

1- 47uV/63 volt C43

The 22uf caps were replaced with 25uF/25 volt Tantalums (Orange Drop)

The two 4.7uF caps were replaced with 10uF/50 volt caps

The 47uF/63 was replaced with a 100uF/63 Volt cap

The 100uF/16 was replaced with a 12uF/50 Volt cap.

The only caps I did not replace were the 2 high voltage caps in the SMPS (4.7uF/450 Volt) C13 - C36. If you look at a full view of the PCB, the SMPS circuit is located in the lower left corner. IC1 is the SMPS device.