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Old 01-18-2011, 09:59 PM
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A hard learned lesson

I am making this post in hopes that it may save someone a lot of time and un-needed agravation looking for a phantom problem without an apparent solution....

Last summer I finished the re-cap of my ctc5 Wingate chassis. When it was installed into the cabinet for the initial power up we encountered a problem. So we put the chassis back onto the bench to trouble shoot it.

The problem was that the Wingate cabinet and crt were up in the living room and the chassis was on the bench in my basement lab without a crt and associated components to allow me to run the chassis on the bench for trouble shooting purposes. So I got a bright idea to build a crt test stand. I had another CTC5 that was a parts set with a rather sad masonite cabinet. So I proceeded to cannibalize the cabinet and associated crt components and I built a crt test stand for my bench utilizing a 21AXP22 that still had some usable hours left in it.

So we hooked up my newly cobbled together crt test stand to the Wingate chassis and started trouble shooting. Before long we located the source of the problem (a bare ground wire that was cracked at the point it was soldered to a lug on a peaking coil. You couldn’t see the break but it revealed itself when I tugged on it)

Then we fired the chassis up and expected to proceed on to set up adjustments and ultimately a nice picture. Alas, that was not to be. The picture on the screen was terrible. It had the poorest fidelity that I can remember on any set I have worked on. So over the next months I worked off and on, trying to locate the source of the poor picture fidelity.

As a test source I have been using my Leader NTSC pattern generator. I like to use a single cross hatch patern with one vertical and one horizontal line. If I can produce a good, well defined vertical line of the same intensity and clarity as the horizontal line, I feel I have good fidelity. Well, the vertical line really SUCKED big time.

I put my scope on the input signal to the grid of the first stage video amp, and the signal was nice and clean (see first photo). Theoretically I should also see the same waveform with much greater amplitude at the plate of the second stage video amplifier. However the waveform looked like crap. (see second photo) The waveform was ringing like a church bell on Sunday morning. The resultant vertical line on the 21AXP22 was fuzzy, and had 4 or 5 additional traces of succeeding reduced intensity just to the right of the primary vertical line. It was pretty much as you would expect to see, considering the ringing waveform at the output of the video amp.

I spent countless hours trying to locate the source of the ringing. I bypassed the tuner and IF sections, removed the last stage IF tube and fed a video signal direct from the output of my Leader generator to the grid of the first stage video amp. Still the same results. As a last resort I obtained an entire video board from John Folsom and rebuilt it from the ground up. Then I swapped the newly rebuilt board into the chassis. Still the ringing persisted.

At this point I was getting pretty frustrated and defeated. What could be the issue? I started trying to dampen the ringing by tuning with small value capacitors. I had very little success with that. Anything that damped the ringing also reduced the amplitude of the video signal to the crt. I was about at the end of the road.

As often happens I laid awake in bed pondering the problem. I had tried just about everything I could think of. I had eliminated everything except the CRT, which was the only thing left in the video chain after the output of the video amp. So I got up this morning figuring I would give it one last try and see if the CRT had anything to do with this ringing issue.

And here is where I found the problem………… AS it turns out the ringing was not caused by the CRT, but rather the additional capacitance in the additional length of wires in the home made crt extension cable. As soon as I unplugged the extension cable from the crt connector on the chassis, the ringing disappeared. I unplugged the cable from the crt base and reconnected the cable to the chassis socket and the ringing came right back. So the cable was definitely at fault. I spent the afternoon fashioning another shorter extension cable, but even lengths as short as 24 inches, caused some objectionable ringing. I finally settled on a short 16” length. There is still a small amount of ringing, but so small that I can live with it for testing purposes.

In an ideal world, keep your crt cable length as short as possible. Any additional length will cause a loss of fidelity at the cathodes of the crt.
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Old 01-18-2011, 10:51 PM
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The phrase "as short as 24 inches" begs the question: how long was the extension cable to begin with?
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Old 01-18-2011, 11:06 PM
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The original cable was around 4 feet. However, the 24 in cable caused almost an equal amount of ringing.
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Old 01-19-2011, 08:55 AM
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I'm glad you got that worked out, you were pulling out your hair the last time I saw you. How's she looking now? I already know how pretty the cabinet looks, now I wanna see it with a picture.
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Old 01-19-2011, 06:27 PM
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Been There; Done That

Quote:
Originally Posted by ohohyodafarted View Post
The original cable was around 4 feet. However, the 24 in cable caused almost an equal amount of ringing.
Bob,
It's moments like this one that can be quite humbling. I'm qualified to say that because I've been there, too.
Two of the hardest lessons learned, at least for me, regarding DIY extender cables were:
1. Twist the filament leads.
2. Shield the grid leads.
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Old 01-20-2011, 10:20 AM
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I'm working on a Whitby, found I can hook it up like this, needing only an octal yoke extension and coax tuner/IF extension. The important CRT & unique rectangular convergence socket reach OK. A yoke extension can be made from junk B&W yoke lead/socket.
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Old 01-20-2011, 10:54 AM
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Hi Colin, How's it going?

YES, I am aware that can be done. The problem is that you are on one side of the set, and the face of the crt is on the other side where you can't see it unless you use a mirror. I like to be looking directly at the face of the tube so I can see all the detail up close and personal. When I was a young fellow, I could hang over the top of the set and look down at the crt while adjusting the chassis in the rear. But now I am too old, fat and stiff to be doing those kinds of contortions. Some day,regretably, like it or not, most of us end up old and less able to do the things we did when we were younger. Let me tell you, it sucks to be getting old.

Fortunately the short 16" extender I made, seems to work pretty good, and it allows me to place the chassis on edge (like your photo) right along side of the crt test jig I made; so I can work on the chassis and see the crt at the same time.

There is a possibility that not all sets are affected by the long extension cables. I used the same cable that gave me problems on this Wingate chassis, when I set up my 21CT55 and I don't recall having any ringing problems in the picture on the CT55. I guess only time will tell as I use the cable on other sets. In any event, I know what to look out for, on future restorations, if I see ringing wave forms in the video .
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Old 01-20-2011, 10:58 AM
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Just remember what I always told my techs. "Every problem is simple until proven otherwise". I saw a lot of techs chasing their tails on problems that sometimes were staring them right in the face. In fact I had a problem similar with a Sherwood receiver with distortion from the output transformer. Come to find out someone had switched the secondary leads on the transformer causing a phase shift which turned into a distortion problem.
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Old 01-20-2011, 01:58 PM
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Yeah, there are disadvantages to my setup like poor access to top of PC boards - and this chassis has problems: no raster/sound after recap. Still haven't made the octal extender which will allow me to proceed to troubleshooting.
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