4 tube Fluorescent desktop lamp with electronic ballast repair
Lamp refused to start, no flicker. Opened it and found electronic ballast with the following markings:
Landlite Electronic Ballast for Fluorescent Lamp
PL 27W Lamp
Output Voltage: 600V
After a few minutes of eye inspection (and nose), I couldn’t find anything with excessive burn marks or any bad smell. The fuse had continuity along with the connecting wires, which made me think the problem was the bulb. The glass was fairly dark near the filaments. That could be from age and constant use. Bulb is 27W and 6500 Kelvin color temperature. I got a new bulb but when I received it a few days later it didn’t light.
So back to the ballast. I first studied the solder side of the board looking for dry joints. You can see a few I found looking at the picture.(top left)
But these were not the problem. Having a schematic greatly simplified troubleshooting. I went through the circuit, testing the likely causes in the high voltage section: open or shorted capacitors, diodes or transistors. Finding none, the mystery deepened. I cycled the power on and then carefully (if operating there is 600 volts present!!) went around poking the voltages to find that the mains rectifying circuit was working fine but when prodding with the multimeter probes next to the diac suddenly the bulb flashes! Maybe the diac is defective since it’s used to start the circuit, somewhat replacing the old mechanical/thermal starter that pairs with reactive type ballasts.
Testing a diac is tricky with just a multimeter so I checked the circuit around it, finding to my surprise an open 510kΩ resistor (R1 in the circuit) just to the right of ecap C6 in picture, with no burn or overheating marks. This resistor charges capacitor C2. When C2 reaches a high enough voltage to overcome the diac breakdown voltage, turns on transistor Q2 to provide the initial kick in the oscillator circuit. Replacing this resistor brought the circuit back to life and now the lamp was working again.
Please click on the schematic to enlarge
Analysis: As you can see from the darkened marks on the board the circuit operates at high temperature in an enclosed space. I believe this heating and cooling places thermal stress on the board components, and in this case causing the 510K resister to develop an internal crack. To combat this I mounted the new resistor ¼ inch above the board on its leads to allow cooling.
This article was prepared for you by George Persico from USA. He is 66 years old and has more than 40 years’ experience in TV repair both Tube and Flat Screen. He also currently repairs small appliances, battery operated tools and computers.
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