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Twenty years ago, when I was taking electronics in high school, one of the teachers brought in a mid '70's 9" B&W Sears portable TV for repair that had intermittent audio. The audio would pop in and out and the problem was vibration sensitive; so, I pulled out the soldering iron and resoldered everything in the audio circuit and the problem was still there. I re-resoldred and checked as best I could for cracked foil and couldn't find anything. My instructor came over and after we put our heads together, we removed the audio driver transistor. When the transistor was connected to the ohms scale on a Simpson 260 meter and when the transistor was tapped, the meter needle jumped all over the place. A new transistor cured the problem.
A few years ago, someone gave me a 19" Curtis-Mathes color TV with intermittent video and sync. It had already been in a shop and they bailed on it after they resoldered a bunch of connections. I resoldered more connections and the problem still existed. What made matters worse, the set would sometimes work for hours without problems and no amount of vibration would cause it to act up. One day, I was working on it and happened to wiggle one of the IC's. The set went crazy; so, I cleaned the pins of the IC and it's socket. I thought I had it fixed until it started acting up again a few days later. At that point, I pulled every socketed IC from the chassis, removed the sockets, and hard soldered the IC's the the PC board. Doing what I did made it harder to replace the IC's; but, that set was to the age that the chances of it ever seeing a repair shop again were slim-to-none. After that, the set never gave problems.
More recently, I repaired a Silvertone solid state console stereo that had an intermittent left channel. When it worked, all was fine. When it didn't work, the audio was full of noise and was popping in and out. I just happened to make it act up on the bench and I was able to make some quick voltage checks at one of the driver transistors; which, revealed a suspect transistor. After I replaced the transistor, it never acted up again.
These experiences show that not all intermittents are due to bad soldering or a cracked PC board trace.
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