A dead CRT was brought to my shop by a friend of mine. He said that this TV suddenly stopped working. I could see a stand by light but there is no high tension sound, no audio and no picture.
I took the cover off and scanned the area for bad capacitors and found no bad ones. I plugged it in and started testing voltages in the power section area. There was 300 volts reaching the main capacitor and there was 24, 110, 10 volts coming out on the secondary diodes in the secondary power section.
A friend of mine brought an electronic welding machine to my shop. The main problem was, it does not weld. The moment I plugged it in and connected all the welding cables, it did not weld, not even a small spark. The guy told me that he sent this machine to someone else to use it and over loaded the circuitry.
The moment I opened the cover of the welder, I could see a black burn near one of the positive legs of bridge rectifier. This is a high amp bridge rectifier (25 amps) and it is a big part. I soldered out (it was not an easy thing to do due to a lot of soldering joints on both sides of the board). I checked it out with a multi meter and it checked ok. One of the legs that have the burn marks did not have solid connection on the board. I re soldered that part and put the bridge rectifier back on board. I made sure it was soldered in a solid way on four legs and on both side of the board.
I plugged it in and tried the machine again but to my surprise it did not work. Checked the voltages in the machine and they all checked ok. I have the schematic for it and it really helped to have it. I worked on that machine for about four hours and could not find anything wrong with it.
I was really getting frustrated with it. I had to take a break. So I went for lunch break. Played with me son for about an hour or so, then I went back to the shop.
The only way to make it weld is to have the negative side and the positive side working fine. So I checked the connection from the positive side to the coil inside the machine with the LOPT/FBT meter and it gave me a full bar but when I checked the negative terminal, it did not give me a full bar indicating a bad coil or bad connection. I took the coil out and checked it again and this time it gave me a full bar. I checked the wire connection between the coil to the negative terminal and it did not give me a full bar indicating a problem in the wire it self. I traced this wire back and found out that this was the main computer chip feed back to the main computer of the machine. If the machine does not get the feed back to the main chip, it does not work.
I changed the wire with a brand new one and the moment I tried the machine it worked nicely. I was so happy with it.