Robert Grant
12-08-2009, 11:12 PM
Lately, I've again been working on a Zenith C2004 / 14B38 chassis 19" B&W TV.
This was the first TV I took on as a "save" (a distant relative was having an estate sale, I had briefly played with the set, saw it was weak [narrow raster, dull pic] and left it - hours later she called me back, and told me it had no buyer, and that in no uncertain terms that it would be thrown out if I did not take it for free).
I hunted down a replacement 21HB5A, and that got the raster wider and the picture brighter. I still had problems with imperfect focus, horizontal foldover, and (a problem I think EVERY black-and-white TV has), buzz in the video whenever anything in the picture is max white.
More recently, I got the photofact for the set (set 1156, folder 3), and started troubleshooting the set.
I noticed that two of the tubes did not match the photofact data. There was a 17BE3 in in the damper (Sams said 22BW3), and a 2BU2 in the HV (Sams said 1BY2). Furthermore, the 17BE3 looks grayish on the inside and is actually leaning on its base by several degrees (did it get hot enough for the glass to melt?).
Out of blind luck, I happened to have a good 22BW3 and an older 1BY2 in my decades-old junk box. Replacing the damper got the horizontal a little wider yet, and greatly reduced (but did not eliminate) the foldover (I can hide the foldover by using the centering ring to move the picture off-center and thus the foldover region off to the left). I got better focus by switching the focus lead from the ground pin to the B+ pin, but bright areas bleed over the focus. When I replaced the 2BU2 with the 1BY2 I got NO RASTER! I then put the 2BU2 back in place, and it works "OK".
When I grabbed the back to put the set back together, I saw the tube chart on the inside. It differs from the photofact, showing the 17BE3 as the original damper and the 2BU2 as the HV rectifier. Thus it seems there was a production change in the set after Sams 1156/3 was published. The old 17BE3 is a Zenith brand tube marked "72-09", which I presume is a date code, as marks like "72-13" and "72-10" are found on Zenith branded tubes throughout the set.
That being said, should I keep the 22BW3 in the damper, or hunt down a new 17BE3?
As for the other nagging problem - buzz. I was able to use the "BUZZ" control to reduce, but not eliminate, the buzz. The BUZZ control seems to have a greater effect on constant buzz than buzz caused by white in the video.
A very confusing direction was found in the Sams. Adjust a trap coil between the tuned and the first IF for MINIMUM response at 41.25 MHz. This means provide as LITTLE desired channel audio signal to the IF strip, which sounds like a recipe for getting a picture but no sound! Should I realign to allow a little more audio carrier to pass through the IFs?
Any ideas are welcome on how to keep this abandoned Zenith going. I can't bear to take the name off, so I have to put the quality back in.
This was the first TV I took on as a "save" (a distant relative was having an estate sale, I had briefly played with the set, saw it was weak [narrow raster, dull pic] and left it - hours later she called me back, and told me it had no buyer, and that in no uncertain terms that it would be thrown out if I did not take it for free).
I hunted down a replacement 21HB5A, and that got the raster wider and the picture brighter. I still had problems with imperfect focus, horizontal foldover, and (a problem I think EVERY black-and-white TV has), buzz in the video whenever anything in the picture is max white.
More recently, I got the photofact for the set (set 1156, folder 3), and started troubleshooting the set.
I noticed that two of the tubes did not match the photofact data. There was a 17BE3 in in the damper (Sams said 22BW3), and a 2BU2 in the HV (Sams said 1BY2). Furthermore, the 17BE3 looks grayish on the inside and is actually leaning on its base by several degrees (did it get hot enough for the glass to melt?).
Out of blind luck, I happened to have a good 22BW3 and an older 1BY2 in my decades-old junk box. Replacing the damper got the horizontal a little wider yet, and greatly reduced (but did not eliminate) the foldover (I can hide the foldover by using the centering ring to move the picture off-center and thus the foldover region off to the left). I got better focus by switching the focus lead from the ground pin to the B+ pin, but bright areas bleed over the focus. When I replaced the 2BU2 with the 1BY2 I got NO RASTER! I then put the 2BU2 back in place, and it works "OK".
When I grabbed the back to put the set back together, I saw the tube chart on the inside. It differs from the photofact, showing the 17BE3 as the original damper and the 2BU2 as the HV rectifier. Thus it seems there was a production change in the set after Sams 1156/3 was published. The old 17BE3 is a Zenith brand tube marked "72-09", which I presume is a date code, as marks like "72-13" and "72-10" are found on Zenith branded tubes throughout the set.
That being said, should I keep the 22BW3 in the damper, or hunt down a new 17BE3?
As for the other nagging problem - buzz. I was able to use the "BUZZ" control to reduce, but not eliminate, the buzz. The BUZZ control seems to have a greater effect on constant buzz than buzz caused by white in the video.
A very confusing direction was found in the Sams. Adjust a trap coil between the tuned and the first IF for MINIMUM response at 41.25 MHz. This means provide as LITTLE desired channel audio signal to the IF strip, which sounds like a recipe for getting a picture but no sound! Should I realign to allow a little more audio carrier to pass through the IFs?
Any ideas are welcome on how to keep this abandoned Zenith going. I can't bear to take the name off, so I have to put the quality back in.