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.

[zeno]

A lot of people dont realize this so just in case...
Zeniths can & do get bad connections. The ends of the strips
are the grounds & sometimes break loose. There are also sometimes
grounds along the strips. Not common, the first one drove me nuts !
Finally the boss saw it & told me. Seemed to happen mostly up
under the jug on the left. Most common on 18" tin cans & other
sets of the late 60's.

73 Zeno

[Electronic M]

Yeah I've heard of that before....In fact before I was able to localize the problem I heated and re-flowed like half the above chassis terminals in hopes that would fix it.

[radiotvnut]

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.

[walterbeers]

I used to change a lot of those color output transistors in Zenith hybrids years ago. Also resoldered a lot of the grounds at the ends of the connection strips. I beleive the transistors replace with an ECG 154 / NTE154, has a round case with a heat sink pushed on the transistors. I would always put a little heat sink compound on it to make sure it made good thermal contact with the push on heat sink.

[technicolor]

Originally Posted by 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.

I tested my first tube at home with my new/old sencore, do i get a prize?

[DaveWM]

good fix on the transistor, intermittent problems are the worst, and I know SS stuff is often assumed to be good. poking and prodding and coolant spray can help but sometimes it can be a real bear to narrow it down on a tight pcb.

[Electronic M]

Originally Posted by walterbeers

I used to change a lot of those color output transistors in Zenith hybrids years ago. Also resoldered a lot of the grounds at the ends of the connection strips. I beleive the transistors replace with an ECG 154 / NTE154, has a round case with a heat sink pushed on the transistors. I would always put a little heat sink compound on it to make sure it made good thermal contact with the push on heat sink.

Good memory. I don't know about the NTE part number, but the ECG and the rest is spot on. I almost went and put some heat sink compound on, but they did not do that at the factory, and I figured it would be a mess.

The intermittent was really weird as it would respond to both electrical and mechanical stimulus. Sometimes it would respond to a good slug on the cabinet other times not, and if I changed channels(off channel and back) on the turret tuner it would often clear up. Moving the cabinet could affect it, and sometimes checking voltages could get it to behave it's self...Much to my annoyance because usually I would wait for it to act up for a while, and as soon as I would probe for the cause of the symptom the symptom would clear up(thus thwarting all attempts to localize it).

Technicolor: If it helps you get your set working then you win it's contribution to that effort, and the satisfaction of having accomplished that....Ooh! And a banana...
Upon diagnosing and repairing the issue in my set all I got was a better working set, and the immense satisfaction of FINALLY overcoming an issue that was driving me nuts for YEARS. It was so annoying that if the cabinet was not as sturdy as it is(not a scratch from hand injuring punches) I would have destroyed it already trying to fix the issue percussively. It would do it between once an hour and every three seconds. You have no clue how annoying it is to get up to clear the intermittent go back and the INSTANT you sit down have it go back to a lousy picture...Utterly maddening(and it had a habit of this)!

But I've put many more hours on the clock and she has not lost blue for an instant. It is getting to the point that if the trouble comes back I'd be VERY surprised.

I have over 150$ in this set as I got in an ebay bidding war. This was my first tube color TV. It worked great in the beginning as it was one of Doug's (aka DRH4683) restorations(he even has stuff on You Tube from when he worked on it), but with all the hours I've put on it, so much work has went in to keeping it going that it is probably more my repair work in there than his now. I've probably put more hours on her than the smaller screen living room BPC CRT set has gotten from the whole family over the 5 years I've owned her.

[DavGoodlin]

Electronic M, great work and percussion probably rattled the transistors innards just like a tube. We KNOW its solid state then think there is No WAY this can happen!

I just slapped my forehead because, 10 years ago I tearfully junked my 1977 Magnavox touch-tune remote due to a suspected bad connection or solder joint causing it to lose video.
I was using chopsticks, touching every possible thing and getting varied results- SHORT of replacing a transistor or IC. It was my daily driver for 18 YEARS and probably just needed a luminance module....ARGHHH

[Electronic M]

That happens to everyone at some point. I've sold sets(radio and TV) which I've given up on fixing numerous times, only to have the new owner tell me it was something simple I had overlooked. Sucks when it happes, but it does happen.

I sort of thought transistors were infallible(unless ran at or beyond their specked limit) until I found that problem...