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After first look inside I found two resistors that were apparently broken R and R I think that these resistor pass in fire if your CRT is starting just in the moment of battery test or change of power type.
But it did not help, so I started to analyze the switching power supply and considered that the oscillator based on UCB is not working properly.
Symptoms You can easily verify that your device is broken in the same way: Measure voltage on R If you get more than 11V, the oscillator is probably OK. Less than 10V means breakage mentioned here my had about 8V. Warning: There is a dangerous voltage on R! Use an insulated voltmeter! What is happening here? Oscillator failure causes permanent discharge resulting in a battery fault.
If you connect a new battery to a broken UPS without a fix, your battery will permanently fail in a few days or weeks and you will have to purchase new one again! Some UPSes use high voltage electrolytic C22 capacitor. There can be a dangerous voltage several seconds after disconnecting it from mains.
In case of blown resistor R, dangerous voltage may remain there for hours or days! Easy fix Far the simplest fix of this problem: Replace C7 by a capacitor with a higher value, e. It should start to work.
If the oscillator works properly, there should be about If it does not, the voltage is zero or very low. Note that if charging was broken for more that few days, your battery is very probably dead and needs to be replaced as well.
You may see Replace battery signal. It describes easy fix of the same problem in more detailed steps. Remove the parallel 3. C7 also provides filtering for a smoother DC for U2 so a bigger capacitor would be better for that too. Winding on T1 actually provides the charging current for the battery.
Technical description of the modification Original design used a very hard divisor R28 and R that divides the plug voltage and can supply about 10mA. But the the resistor divisor should be used just to charge capacitor C7 and then use its energy to start oscillation. Once the oscillator runs, it is powered from the transformer which gets its energy through 2SK power FET and the divisor is not needed for the regular work. You can verify this fact by disconnecting the large divisor resistor, touch, and remove.
Oscillator runs until you disconnect the power. The UCB data sheet says, that if there is not enough voltage to run oscillator, the IC eats less than 0.
It means, that the divisor could be designed to provide only 0. Additionally, low current of the divisor allow to omit resistor to the ground completely: The IC has a protection Zener diode well, it should be never activated with the design used in this UPS — once the voltage reaches about 14V, regulation feedback using opto-couplers will prevent further raise of the voltage. In this new design, capacitor must be larger than in the simple fix — e.
The complete start of the oscillator is powered nearly exclusively from it. It must have enough juice to keep voltage above the minimal voltage until the transformer starts to give regular power. Computing the resistor R28 value: Any resistor of that size that survives V with a sufficient reserve i. Any resistor of the same size or a bit larger as the original should be OK. Credits Thanks to George Helas for sending a fixes and providing a photo of broken resistor.
APC Back-UPS RS 500VA, 300 Watts (BR500CI-AS)