Once more unto the breach, dear friend, once more ... APC Back-UPS RS1000 Repair

 

APC Back-UPS RS1000

I first encountered it 13 years ago, in 2008. Even then it was past its prime, bought at a private auction for peanuts. It started up, then indicated 'overload' and shut down. Back then I needed to learn about UPS and the RS1000 was a nice roomy design and a large PCB which seemed easy to work with.

Main PCB: top of picture is front of UPS. Charging section is on the bottom left corner

You took out the screws and lifted the front cover from the middle in 2 sections. The bottom half covered the battery compartment. The top front flap had to be pried off together with the right side cover, and comes off with a nasty crack. Once exposed it was easy to reconnect the sections for testing.

Important: Do Not Attempt this Repair unless you are qualified. Not only there are hazardous voltages inside, it will continue to be hazardous if the UPS is disconnected from the mains. In additional since there are switch-mode power modules inside, connecting an oscilloscope to it will result in damage, fire or death.

It sat in my study for 2 months before I found the fault: there were two shorted MOSFETS, Q1 and Q2, IRF740. Replaced that plus a blown fuse and it worked again. I had a good guess at the root cause: there were signs a gecko used to live there. The nice roomy UPS must have been a good place for a gecko to keep warm.

Typical Malaysian house gecko 

It was some 9 years, in 2015 before it was brought in for repair again.  It started up, indicated 'overload' and shut down. Call me simple, but I went straight to the battery charging circuit. This time a deceased and mummified gecko was still in there. 

Bad Neighborhood: An IoT Power Extension used for lamps has a lot going for a gecko: lots of insects attracted to the light. It is warm, dry and safe from kingfishers, rats and other predators.  There is just one problem ... electrocution

Again two shorted IRF740s, this time Q1 and Q3. The PCB was getting a little beat up with the second rework, but it started up nicely.

It was a little quick to switch over to battery operation, but you could decrease the sensitivity by turning it off, then holding the button down until all 3 lights lit up. Then you pressed the button again to select: one light for minimum sensitivity. 

And back it goes into service. It came back 5 years later, about 4 months ago this year. This time there was nothing wrong, and everything wrong: the owner did not need it anymore. Desktop computers are now notebooks with 3 hour or more battery life and there is little use for a UPS with a backup time of 15 minutes.

For four months it backed up my npm server, a laptop. During one long power outtage, it got quite hot and right after that would not go back to 'online' mode even after the power came back. There is AC power in its non-battery-backed power sockets, and it charged the batteries when it is shut down. But when it is started up, it tests the batteries, then the mains voltage, and switches to battery operation. 

Perhaps after a few years its 'Line Sense' circuit had deteriorated. Maybe a gecko had set up shop in it again. My first thought was to throw it out: there was little use for it. But then a half-hearted google search turned up the full schematics for it!

Well maybe one last hurrah. If I moved my npm server to a raspberry pi 3, it should be able to run on battery for 2 hours or more.

So off with the cover. This time there was no gecko. Just the 'Line Sense' problem. The components are surface-mount but just about large enough to handle without an airgun.

The first opamp in IC8 (LM358) is a difference amplifier. It reads the incoming mains AC directly, using 2M series resistors to step down 325V (230Vac nominal is 325 Vpeak) to around 3V.

Line Sense (IC8). CPU is at top center.

The second opamp is (I think) a precision half-wave rectifier which is read directly by the CPU analog input. The 'Sync Circuit' output is connected to the CPU digital input, probably used to sense zero crossings for a smooth switchover.

When I measured the resistance of the resistors, diodes and opamps (with power off and circuit discharged!) they seems OK, nothing short or open-circuited. Next would be a power test. Plugging in the battery alone did not produce power at the LM358.

But plugging in the AC mains (with the UPS still shut down) did. 12V supply came up. But the negative rail '-8V' seemed a little low at -4.2V. Now the AC mains Live is connected to the opamp inverting input, and it can only go as negative as its voltage rail. The precision rectifier will rectify it to the correct polarity for the CPU analog input. If the negative rail is not low enough, the Live voltage sensed will correspondingly drop. This seems to square with the observed fault.

So we go on to the -8V power schematic.

Maybe the designer is old-school, but this is a flying-capacitor charge pump, straight out of the textbooks. Most of the kids would have dropped in a cheap surface-mount DC-DC IC. The CPU digital output line 'pumps'  CHRG_PUMP_OSC at a fixed frequency. This provides 12V on an off to the 'flying' capacitor C40 which charges up to 12V via Q25.

C20 is prevented from discharging by the diodes D21. Their polarities ensure that the output is negative, which allowing for diode forward drops and Q25's Vce, -8V seems a reasonable final output.

But what could be wrong?  There is some output, just a little low at -4.2V. Q25 or Q26 could be defective. It could be D19 shorting or D21 leaking. But the most likely is C41 and C40 has lost most of their charge. Electrolytic capacitors contain electrolyte, and is likely to dry out. They have a shelf life of 6 months before they are out of spec. And it has been, what, 12 years?

Again, with power off and battery disconnected and the PCB allowed to discharge, diode-tested the diodes and transistors (a bipolar transistor is simply 2 diodes stacked up). They seemed OK. This leaves the capacitors.

220uF test capacitor soldered across C41. 

 

To test the capacitors C40 and C41 it seemed easiest to simply parallel a new capacitor across the old one. In fact just adding one capacitor should improve the resulting output power. If possible, over-size the rating (I used 220uF) a little as the old one will act more as a load. I simply soldered it across C41.

On connecting the battery and mains voltage the voltage went up to -5.8V. And even better, when the UPS was started up, it tested the battery, then switched to 'online' mode!

For final repair, I ordered some surface-mount 22uF capacitors. There is nothing wrong with my 'test' capacitor, but the capacitor is heavy and lies horizontally when the RS1000 is upright and as a result may work itself loose after a few years.

[2020-11-10 update]: With new (Panasonic EEEHD1C220AR) SMD 16V 22uF electrolytic capacitors for C40 and C41, the negative rail "-8V" now measures -9.2V. Element14 is not the cheapest, but they hold local stock and I really did not want to wait 4 weeks with the UPS innards scattered over the work table.

So, it is "once more into the breach, dear friend". It's better to soldier on. Better the line of fire than the storage shelf.

Happy Trails. 

"How dull it is to pause, to make an end, To rust unburnish'd, not to shine in use!" - Tennyson, 'Ulysses'