NRG GUARD 2000A UPS, NOT STARTING REPAIR
This UPS came in my hands after an unsuccessful repair attempt from another technician. The symptom it showed right after powering it was no reaction at all. The front and back views of this UPS are shown below:
Well, as I almost always work without any schematics, I removed the main PCB from the unit and as a first step I watched carefully what the previous technician did on it.
The unit was generally in very good condition. It suffered an overvoltage at its input and blew a 3A power diode and a power transistor in the battery charging circuit along with its input fuse of course. The main board is shown below:
The previous repairer had replaced the power transistor shown at the right side of the electrolytic capacitor located at the middle of the PCB, behind the blue cable, along with the 3A power diode nearby (all of them shown better in the below photo) and also repaired a part of foil trace of the PCB which was lifted and melted due to the excessive fault current that passed through it.
In the above photo you can see the power transistor which replaced the original (for which I don’t know what type it was). It is the Q13, TIP 31C located in front of the electrolytic cap at the middle of the PCB. Also the new 3A diode (shown in front of the right side of the first relay, or at the right side of Q13) is a 1N 5404 replacement of the original one (for which, again, I don’t know what type it was).
Below is a better view of the location of the power diode. It’s the one between the blue cable and the current transformer, at its left side. He also replaced the small signal transistor (Q26) shown with the yellow sleeve in one of its terminals (Base), at the right side of the heatsink shown, around its middle.
Well, this transistor caused me enormous trouble for the reason I will give you further explanation later on…For the moment keep in mind that the one shown below is my own replacement and not his…
Well, as usually, I checked all the semiconductors of the circuit along with all the electrolytic capacitors and found another small signal transistor (Q25, 2SC945) in the relay area to be defective. In fact it was a relay driver. I was then sure that I had finished the repair, but this was not the case. You can see the replacement transistor in the photo below, having reverse orientation from the original as marked on the PCB. It is the one located just above the single one at the middle of the PCB, downwards. I put a yellow sleeve in one of its terminals (Base):
After a new test, I had only the improvement that when pressing the power switch I could hear a click of a relay, a long beep sound and then nothing at all.
Believe it or not, I “swept” the entire PCB again and again. I checked everything on it. I used magnifier lenses to search for “non typical failures” like cold joints or broken PCB foil traces. All went in vain. No result at all. Everything was in perfect condition and yet the device refused to start. So, I left it for a few days sitting on my bench and smiling ironically at me…
After that (indeed very useful for me) interval, I decided to go on with it. Only this time by trying to get any useful electrical measurements of dynamic conditions, neglecting the static component measurements which one way or another were perfect and all they brought me was an equally perfect confusion…
During the troubleshooting procedure, I noticed that the device when trying to start it was giving at the battery cables an output (charging) voltage of the order of 14V D.C which was not enough for a normal charging of the 2x12V batteries connected in series in its circuit. Half of the needed voltage was missing. Then I thought that the processor “sees” this inadequate voltage output and stops the entire system.
With this in mind as a good starting point, I removed the main board again, located the components involved in battery charging and rechecked them again one by one. It was that time when I noticed that the transistor for which I said above I owe you a further explanation was giving me the right indications about its B-E and B-C forward voltage drops, but the physical arrangement of its terminals was different from the other, same type, transistors used in the circuit.
What happened with this transistor was: correct replacement in terms of electrical equivalence and at the same time wrong replacement because the previous technician took care only for the electrical parameters of the equivalent transistor he put there without paying any attention to its terminals’ arrangement which was different from the original transistor. The original had an E-C-B arrangement and the replacement had an E-B-C arrangement. That’s why you see this transistor in the photo above with different orientation and a plastic sleeve in one of its terminals (Base). I had to interchange the C-B terminals end therefore to insulate the Base terminal with a plastic sleeve as I had to bend it and place it in the collector’s place. The collector’s terminal was also bent to reach the place of the base, but now it was in front of the base terminal. The sleeve was put there therefore to prevent any likely short circuit of the B-C terminals by accidental bending of the entire body of the transistor. I did exactly the same when I replaced the original relay driver (small signal transistor Q25, 2SC945). The replacement transistors I used were of BC546 type in both cases.
Well, after the rearrangement of the Q26 transistor’s terminals the device started normally after the self tests, giving an output of 27 Volts in the battery terminals. After a full charging (overnight test) of its batteries and when being in stand by status thereafter this voltage was absolutely stable.
The problem was finally eliminated and the UPS was ready for normal use again.
This article was prepared for you by Paris Azis from Athens-Greece. He is 59 years old and has more than 30 years’ experience in electronics repairs, both in consumer and industrial electronics. He started as a hobbyist at the age of 12 years and ended his professional carrier as a senior electronics technician. He has been a specialist in the entire range of consumer electronics repairs (: valve radio and BW TV receivers, transistorized color CRT TV, audio amps, reel and cassette tape recorders, telephone answering and telefax devices, electric irons, MW cooking devices e.t.c) working in his early stages at the official service departments of National-Panasonic first and JVC afterwards, at their premises in Athens.
Then he joined the telecoms industry, working for 20 years as field supporting technician in the sector of DMRs (: Digital Microwave Radio transmission stations), ending his carrier with this subject. Now he is a hobbyist again!
Please give a support by clicking on the social buttons below. Your feedback on the post is welcome. Please leave it in the comments.
P.S- If you enjoyed reading this, click here to subscribe to my blog (free subscription). That way, you’ll never miss a post. You can also forward this website link to your friends and colleagues-thanks!
Note: You can check out his previous repair article below: