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 Post subject: Interesting symptom - 1950's Admiral B&W
PostPosted: Jun Sun 19, 2022 7:45 pm 
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Location: Monte Vista, CO
This is a very unimportant post -- this television is long gone. But I've never solved this problem in my mind, and would love some ideas on how it is even possible.

Every once in a while the picture would suddenly and instantly turn negative (all whites were black and all blacks were white), then slowly fade back to normal (several seconds). From what I remember there was no warning, it just did it every once in a while. The sound was unaffected.

Has anyone else ever witnessed anything like this? What kind of a circuit anomaly could even cause such a behavior?
-Phil


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 Post subject: Re: Interesting symptom - 1950's Admiral B&W
PostPosted: Jun Sun 19, 2022 8:09 pm 
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Could be an H-K short or similar...Those can come and go.


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 Post subject: Re: Interesting symptom - 1950's Admiral B&W
PostPosted: Jun Sun 19, 2022 9:02 pm 
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Electronic Memory wrote:
Could be an H-K short or similar...Those can come and go.
Agreed...

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 Post subject: Re: Interesting symptom - 1950's Admiral B&W
PostPosted: Jun Sun 19, 2022 9:54 pm 
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I looked in an early '60's TV repair book I've used, and the symptom could be caused by any of the following; video detector, video amp, video output, IF strip or the picture tube itself.

That should narrow it down... :wink:

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 Post subject: Re: Interesting symptom - 1950's Admiral B&W
PostPosted: Jun Sun 19, 2022 10:00 pm 
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Even though I was just a kid at the time (maybe 12), I bet I replaced all of those (the tubes at least, except for the CRT itself) before replacing that set.

I'm still trying to wrap my head around how a H-K short would cause an inverse picture.


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 Post subject: Re: Interesting symptom - 1950's Admiral B&W
PostPosted: Jun Sun 19, 2022 10:04 pm 
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It could have been an intermittent CRT, or a component in the IF strip, so I wouldn't lean on a heater to cathode short all that much. BTW, the video detector could have been a Diode, not a vacuum tube.

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 Post subject: Re: Interesting symptom - 1950's Admiral B&W
PostPosted: Jun Mon 20, 2022 2:05 am 
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Location: Middletown NJ
I would first suspect the detector diode or tube. But yeah could be anywhere from the IF strip to the jug.


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 Post subject: Re: Interesting symptom - 1950's Admiral B&W
PostPosted: Jun Mon 20, 2022 8:30 pm 
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My money would be on the video det diode. Other thing not yet mentioned
an AGC problem. A good problem to use an anylist on.


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 Post subject: Re: Interesting symptom - 1950's Admiral B&W
PostPosted: Jun Sat 25, 2022 1:18 pm 
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This problem would unlikely be anything to do with the IF, AGC or video detector diode. The reason being that if the video became inverted at the output of the video detector for any reason, the synchronization would likely fail, and presumably from what you described that remained normal.

So likely the fault was in the video stages close to or at/in the CRT, after the signal was picked off for the sync pulses and sound. It would have been obvious with the scope to see where it was happening.


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 Post subject: Re: Interesting symptom - 1950's Admiral B&W
PostPosted: Jun Sat 25, 2022 1:30 pm 
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Joined: Jan Thu 01, 1970 12:00 am
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Location: Monte Vista, CO
That is correct, the sync was unaffected.

Oh how I wish I had owned a scope when I was 12! My school had a nice DuMont scope with a bright blue trace that didn't work right. I was told that if I could fix it I could have it. I took it home and trouble shot it down to a tube (a 6Q7G I think). This was way before computers and the Internet, so the only source I could find for the tube had it priced at $60 USD (in 1970 dollars). My parents said no.

Sometimes I wonder how my electronics career would have been different if I had been able to obtain that scope at such an early age.


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 Post subject: Re: Interesting symptom - 1950's Admiral B&W
PostPosted: Jun Sat 25, 2022 9:26 pm 
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Location: AUSTRALIA
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PhilF wrote:
That is correct, the sync was unaffected.

Sometimes I wonder how my electronics career would have been different if I had been able to obtain that scope at such an early age.


Yes, it would have been good to have one as it is a window into the world of the behavior of circuitry of all types and shows the mind things that are invisible otherwise.

I managed to borrow a scope when I was about 14 and loved it. I had to wait nearly a decade to get my first one. And now I am so crazy about them I must have at least 15 of different types including three Tek 2465B's (400MHz) that can actually see and sync lock a 900MHz wave and revolutionized my ability to design repair and fault find VHF & UHF circuitry as well as many other uses. One very useful scope is the Tek 222ps because its inputs are total isolated and it is great for working on line powered devices like SMPS's. The 466 is a great storage scope. I have bought some digital scopes but have generally been disappointed with them.

There are so many great scopes, I still prefer the Analog scope for most work, including computer repairs, than Digital ones. One excellent feature of some (not all) analog scopes is the independent delayed timebase. This has great utility in video & digital repairs, but many are unaware of this unless they have owned an analog scope with this feature...and learned how to use it. It allows examination of individual downstream pulses in a pulse train. Especially if they are very narrow, say 50 to 100uS and come in groups where they are widely spaced say by 20mS, because at that timebase setting they are invisible on a normal scope display. This is one reason I often suggest the compact Hitachi V-509 as a good 50MHz starting analog scope on forums, as it has this function. But it gets overlooked mostly.


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 Post subject: Re: Interesting symptom - 1950's Admiral B&W
PostPosted: Jun Sun 26, 2022 2:16 am 
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Location: Lincoln City, OR. 97367
Greetings to the Forum:

I hope this isn't considered hijacking the thread, but I too had a problem back around 1968 or so that I couldn't solve. It was in a Zenith 110 degree deflection B&W metal cabinet table model set. That's all I can remember; sorry no model or chassis number. The set had no horizontal sync; the horizontal oscillator was free running.

I traced the problem down to the circuitry associated with the selenium diode in the AFC circuit. All of that circuit was contained on one of those Zenith upside-down conical tie point strips where the component leads are stuffed in and the chassis turned upside down and wave soldered. All of you who have worked on Zenith sets of the 1960's vintage know what I am talking about.

Anyway, I replaced every part on that tie point strip and it didn't fix the problem. I got lucky that an identical set came in for service. Once we fixed it, I set the two sets up side by side on the bench. I could take the sync signal that was input to the AFC circuit from the bad set and run it into the good set (AFC circuit input) and the good set locked up just fine. I took the output of the AFC circuit from the good set and ran it into the bad set and it locked up just fine then, also. I took the AFC circuit output from the failed set and ran it into the good set and the good set developed the problem.

It had to be in that circuit.... but I had already replaced every part in it at least twice. I gave up and fudged a brute force sync circuit that caused the picture to tear on the edges with any interference but it got the set out the door.

Years later, for some reason, I got to thinking about that set and a remark from an old and wise ham I knew popped into my head. What is the most important component in any circuit yet never appears on the schematic? The answer is the INSULATION. The one thing I never replaced (or even inspected closely because it was under the bell of the CRT) was the doggone tie point strip. It could have had a carbon track or even a loose screw lying on the top side and it never occurred to me to replace it. Looking back, I bet that was the problem.

Regards,

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