A hard learned lesson

[ohohyodafarted]

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.