A 21” GoldStar CRT TV set was brought to my shop with the problem of a dead set and the stand by LED was continually flashing on the set.
As always, checking for burnt and loose connections joints on the PC board and found none. There were some voltages coming out of the power supply to the HOT in the high tension section but dropping fast which indicated a power supply problem. I traced the problem backwards to the secondary power supply output and it showed the same problem. This assured me that there is a problem in the power supply section.
Upon fast scanning of the components in the power section, I noticed the power supply IC STRS5707 was getting hot. The fuse was intact and all the components looked fine to me on my first scan. There was power reaching the STRS5707 but the output voltage was very little and this prompted me to conclude there was something wrong with the STRS5707 IC. I used the LOPT/FBT meter to check on the primary winding and it showed only 3 rings (LEDs on). This was another indication of a bad IC in the power section.
I removed the IC out from the PC board and compared its reading with the new one and sure enough, the IC was shorted but to my surprise, it did not blow the fuse on this set. From my experience when an IC like this gets shorted, it will usually blow up something in its path. So I started checking the component around this IC and found that there was a transistor blown (Q810) and there was a Zener diode (ZD810) shorted as well. When both components were removed, I checked on the transformer primary winding again with the LOPT/FBT meter and this time it gave me full rings. I was very happy to see such a result on the meter.
I replaced the IC, transistor and zener diode with exact part number. I did not turn the TV for one reason and that was because I was thinking there might be another shorted component in the set and by turning the set on, I might blow up the new components that I have just replaced. So, the light bulb trick was used. I put a 100 watt light bulb on the fuse connections and turn on the TV and guess what? The light bulb came on very bright then it became dim and that put a smile on my face because I knew once I replace the light bulb with the actual fuse, the set will work. Lastly the bulb was replaced by the original fuse and with power on the TV set came back to life!
There was 129 volts reaching the HOT and I was very concerned about it. It was a 21” TV set and that voltage is too high comparing with the size of the set. I kept the TV on for more than two hours and kept a close eye on the HOT. I kept touching the transistor once in a while to make sure that it will not go too hot. You can only touch the HOT when the power is off. The result was the HOT ran a little bit warm and this means it was a good sign. After done a research on this GoldStar TV model, 129 volts was a normal voltage.