Magnavox T931

[old_tv_nut]

See post #33 here:
http://videokarma.org/showthread.php...et#post3203336

where I compare chassis voltages and 6GH8 spec limits

[kvflyer]

Originally Posted by old_tv_nut

See post #33 here:
http://videokarma.org/showthread.php...et#post3203336

where I compare chassis voltages and 6GH8 spec limits

That would explain it. Interesting how RCA put Sylvania tubes in their set on the line yet the tubes were labeled "RCA".

[AlanInSitges]

Originally Posted by Electronic M

"Oops all 5/6GH8!" Chassis

LOL a couple of times

[LukeSimon]

I am almost embarrassed to ask, but I don’t want to damage anything… how do I remove the shield on this tube? Twist and pull? Just pull hard?

[Tom9589]

Just push DOWN on the top portion of the tube shield. It will telescope into the bottom half, allowing you to grasp the upper half of the tube and remove it.

You won’t get anywhere trying to pull on the tube shield!

[zeno]

There is usually a hole or two at the base also. You can slip a thin
screwdriver in & pop it out.

[LukeSimon]

I am trying to adjust horizontal linearity. There is a faint vertical bar a little less than halfway across the screen, where the horizontal scanning is slightly compressed.


In the book “Basic Television” by Grob, he shows a picture of a much more pronounced vertical bar in the same region of the screen, and says it is excessive horizontal drive.


I know this is the region where the damper diode stops scanning the left hand side of the screen and the horizontal output tube starts scanning, so I replaced the damper tube since it looked like the original Magnavox branded tube and looked very baked and worn. However, replacing the damper tube didn’t help. The previous owner, who last used the TV in the 1970s, had replaced the horizontal output tube 6JE6A, with the much cathode higher current rated GE branded 6MJ6, and it still looks new. So I don’t think it needs to be replaced.

The Sams only describes adjusting the horizontal efficiency coil. It never mentions adjusting the “horiz waveform” coil. Which coil should I adjust to fix this horizontal compression issue? There is no horizontal drive control in this TV.

I think the right thing to adjust is the horizontal efficiency coil, but I wanted to confirm, and I also wanted to know what the “horiz waveform” coil does. The horizontal efficiency coil used to be called the horizontal linearity coil. It adjusts the current of the damper and the horizontal output tubes in the region where they both are partially conducting, and I guess you could call this distortion a type of “foldover”, though it is a very minor form of foldover that is barely visible.




The “Horizontal Sweep Servicing Handbook” by Darr describes two possible adjustments: horizontal linearity coil and horizontal drive. If horizontal drive is “overdriven”, then grid clipping causes excessive grid-leak bias, which means the horizontal output tube doesn’t turn on soon enough. The “drive line” is very pronounced and white. This doesn’t seem to match the subtle bar that my TV is manifesting.

[ARC Tech-109]

Waveform coil only pre-distorts the actual sawtooth to compensate for the non-linearities of the yoke and flyback similar to the vertical linearity adjustment, the efficiency coil tunes the horizontal section and should be adjusted for the minimum cathode current. I strongly recommend installing a 0.1uf 50V or greater bypass capacitor between the cathode terminal at the socket and the chassis before placing a meter on the measurement lead. This will bypass the high frequency from the meter and give you the real DC current without detuning the flyback circuit. I burnt up a meter without a bypass capacitor.

[LukeSimon]

Originally Posted by ARC Tech-109

Waveform coil only pre-distorts the actual sawtooth to compensate for the non-linearities of the yoke and flyback similar to the vertical linearity adjustment, the efficiency coil tunes the horizontal section and should be adjusted for the minimum cathode current. I strongly recommend installing a 0.1uf 50V or greater bypass capacitor between the cathode terminal at the socket and the chassis before placing a meter on the measurement lead. This will bypass the high frequency from the meter and give you the real DC current without detuning the flyback circuit. I burnt up a meter without a bypass capacitor.

I have a Seco HC-8 for testing the horizontal output current in circuit.

[Electronic M]

The 6ME6 is listed as a valid replacement in the sub books, but I'd be tempted to try the correct 6JE6 specified.

Early in my TV repair days I ended up selling a RCA 9T246 because I couldn't get rid of bad horizontal non-linearity.... Somehow I never noticed that the original 6BG6 had been replaced with a 6DQ6 or similar later H output, and apparently according to the next owner that was basically all that was wrong with it.

[LukeSimon]

Originally Posted by Electronic M

The 6ME6 is listed as a valid replacement in the sub books, but I'd be tempted to try the correct 6JE6 specified.

Early in my TV repair days I ended up selling a RCA 9T246 because I couldn't get rid of bad horizontal non-linearity.... Somehow I never noticed that the original 6BG6 had been replaced with a 6DQ6 or similar later H output, and apparently according to the next owner that was basically all that was wrong with it.

I have NOS 6JE6 tubes of different versions (A, B, C), as well as the 6LQ6, and 6MJ6. Each subsequent tube model has a higher cathode current rating. I will try to adjust the efficiency coil first and then drop down to the next lower tube revision only if it doesn’t fix the non-linearity. Even the 6LQ6 is obviously less robust than the 6MJ6, which has not only a black plate for better heat emmisivity, it has two plate to top anode cap connections that are much thicker (more current , more heat), the top anode cap is also black, and the tube is taller (more heat tolerance again). The cathodes themselves must be bulkier to have a higher current rating, and so all of these differences possibly do change the tube characteristics enough to cause resonance issues required for the output stage to have the linearity the chassis designers intended.

[ARC Tech-109]

I'd be inclined to look at C126 & C127 along with the screen bypass C118 for leakage or other age related issues. C126 & C127 make up the tuned LC circuit and a low cathode current doesn't always indicate correct tuning if one of the caps are bad. I had a bad C118 on my T940 and it loaded the screen down enough to lower the cathode current.
There's a small R/C snubber behind the deflection yoke cover that I've had trouble with, I don't recall the values but it's a 2W 5% carbon. Failure caused a noticeable "ripple" in the blue horizontal line during convergence and headaches trying to figure that out. Unrelated but something to stuff under your hat.

[LukeSimon]

Originally Posted by zeno

BTW on the conv PCB there is a red square. Thats a diode pkg and they
do go bad.

73 Zeno
LFOD !

I desoldered the diode pack X9 and one of the diodes (X9b) tests bad as open in both directions. I will solder two 1N4005 diodes as replacement for the X9 diode pack.

[ARC Tech-109]

Do keep in mind the red diode pack is selenium and going with the later silicon diodes is going to make the convergance very touchy, been there done that.

[LukeSimon]

I replaced the selenium diode pack with two 1N4005 silicon diodes. I also used a different convergence process than the one mentioned in the Sams Photofacts. The Sams only uses 12 of the 13 dials on the convergence board. Here is the Sams:



Note that the Sams does not make use of the “Blue Shaping (Blue Center)” coil adjustment. The “RCA Color TV Servicing Handbook Volume 2” lists a dozen different convergence procedures. Most of them are 12 dial procedures, but there are three 13 dial procedures. “Procedure C” matches the Magnavox convergence board the closest. So I tried it and it works better than the Sams convergence guide. Here is the RCA Handbook guide’s diagram. It has pages of narrative procedure description, which I won’t include here:


I will take a few more attempts at getting the convergence perfect. It is still has some convergence issues in the upper left corner and the right side of the picture.