I replaced my first transistor few hours ago: the end of the worst intermittent ever

[Electronic M]

I've been trying to isolate an intermittent in a Zenith 12B13C52 hybrid chassis set for several years now and for the past few months I had convinced myself that it was the socket of the blue video out transistor that was the problem. Voltage tests, and the way re-seating the transistor greatly reduced the problem had me convinced of the area of the problem. The problem being that the blue gun would randomly go most of the way into cutoff. It was almost impossible to localize until a few months back (when it was bad almost all the time) because the slightest stimulus could potentially make the symptom go away(and always did when I tried to find it). Well I tried two strategies to cure what I thought was a bad socket: contact cleaner, and tinning the brass leads with solder. When that failed I got a wicked clever idea...Swap the identical socketed color video output transistors, and see if it would go intermittent...And if so would it be in the same color primary circuit as the suspect socket or transistor. Well I swapped the red transistor for the blue, and after some observation it finally went intermittent this time in the red channel! Proving it was the transistor and not the socket causing the troubles.

I almost could not believe it(yet at the same time I was feeling damn clever for finally localizing the EXACT part causing the problem)! I've never seen a transistor go bad in a hybrid set before. I literally observed this happen over the course of probably 2-4 of the 5.5 years this set has been a daily driver for me.

The next step was to locate a replacement for the Zenith 121-743 transistor. After looking in the wrong places in my parts bins I remembered a service case full of Zenith modules and parts, and I actually found a decent sub! Sam's also lists 121-776 and 121-777 on the parts sheet, and I had a NOS Zenith 121-777 in the case. It don't look as nicely made as the original, but hey at least I don't have to pay for one and then wait for it to arrive. After a test run to make sure it was in fact a good sub I trimmed the leads to match the original, installed the retainer, buttoned her up, and she has run bug free for over two hours since.

I'll need to put some more hours on the clock to wash away the last of my doubt and skepticism, but it looks like I FINALLY fixed an intermittent that has been perhaps the most annoying if not the worst white whale issue I've ever had to trouble shoot in a TV.