Magnavox T931

[ARC Tech-109]

Looks great as it is now, honestly I don't know how much better the convergence can get, maybe spend another hour or so tweaking everything but a delta triad tube is never going to be 100% all the time. Thermal expansion of the shadow mask and other influences stack the variables against it and from what I can see in the pictures you have already done an excellent job on the set.
Well done!

[damen]

RCAs first generation ctc17 chassis had that 13th adjustment. They later replaced that adjustable coil with a fixed choke.

[Yamamaya42]

Originally Posted by damen

RCAs first generation ctc17 chassis had that 13th adjustment. They later replaced that adjustable coil with a fixed choke.

http://suzaku.live-evil.org/hof-blu.jpg


no mention of distortion or damage on my setup for my set :O ( ctc17 clone)

[LukeSimon]

Tuning the horizontal efficiency coil according to the Sams procedure first resulted in a minimum cathode current of 195mA, but the horizontal linearity wasn’t good in the transition area between the damper diode and the horizontal output amplifier.



Here is the 240p Test Suite vertical stripes test pattern, where the stripes in the transition region are closer together. The white stripes in the transition region are slightly brighter because more cathode ray current hits the same phosphors due to a slower horizontal deflection in this region. Sams calls this “foldover”, and says to turn the adjustable core of the efficiency coil clockwise to eliminate the non-linearity, but not to the point where cathode current exceeds 230mA.


Here is the best linearity that I could achieve, and it is at 230mA cathode current. A lower cathode current results in more non-linearity.

[ARC Tech-109]

Have you replaced the caps in the efficiency network by chance?

[LukeSimon]

Originally Posted by ARC Tech-109

Have you replaced the caps in the efficiency network by chance?

I haven’t checked them yet. I will take a look. It makes sense that minimum cathode current results in a nonlinearity in the transition area because for cathode current to be minimized, the horizontal output tube must stay shutoff longer. Having it turn on earlier/more in the transition area increases cathode current, but also eliminates the nonlinearity.


Not sure how I didn’t notice this until now, but looking at older pictures I took of the TV immediately after having received it, I can see that the TV’s conversion assembly was over tightened at some point, causing it to crack. This has caused a very minor misalignment of the assembly and probably explains why dynamic convergence is still a bit off. Any suggestions on how I can fix this? I can’t find any Magnavox convergence assemblies for sale online.

If anyone knows where I can buy a spare convergence assembly, please let me know. I’d also like recommendations on vintage plastic repair. JB PlasticWeld epoxy seems to have the tensile strength needed for this.

[ARC Tech-109]

The cathode current is a function of the overall tuning of the "flywheel" or ringing of the horizontal circuit as a whole, the efficiency network is the tuning mechanism and a bad capacitor is going to throw things off.

[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.

This was great advice. I tried multiple horizontal output tubes, and while they all had the same minimum cathode current, 195mA, each tube had a different cathode current where it achieved its best horizontal linearity.


I tried two 6MJ6 tubes, which are the highest wattage rated tube that is compatible with the 6JE6A, and one achieved good horizontal linearity at 230mA. The other 6MJ6 topped out at 220mA and was not able to achieve good linearity.



I then tried a 6LQ6/6JE6C, and it was able to achieve great linearity at 215mA. It’s achieved horizontal linearity is even better than what the 6MJ6 was able to achieve at 230mA!



The Sams Photofacts and Darr’s “Horizontal Sweep Servicing Handbook” both say that color TV’s should first adjust the efficiency coil to minimum current, and then turn the core clockwise until best linearity, but without exceeding the TV’s max current spec.

I also made sure to calibrate my SECO HC8 in-circuit current meter using an accurate digital current meter.

[LukeSimon]

Another observation, replacing V19 with a NOS tube dramatically improved brightness.

[LukeSimon]

Another observation when swapping in NOS tubes, I replaced tube V5, which is used as an amplifier that also separates luma, chroma, and sync signals. The tube has a grounded shield. The chassis originally uses a 6LM8, but I replaced it with a 6LM8A. As you can see from the picture, the 6LM8A is the tube on the right. Its plate has twice the surface area of the tube on the left, and it has a black plate (for a higher max wattage rating).

The 6LM8A tube works as a substitute in the TV, but the colors streak badly. This is likely due to a much higher parasitic capacitance between the tube’s plate and the tube shield.

[damen]

Back in the day we replaced those 6GH8 blanker tubes in just about every Magnavox being serviced due to the brightness being slow to come up when first turned on. RCAs blanker circuit was almost identical to Magnavox, but RCA used a 6GU7 that had a much larger plate area, they held up better. Most of those Sylvania video amp tubes that called themselves "silver something?" never seemed to last, we sent a lot back in warranty, 6LM8, 6LF8 were the worst.

[zeno]

That conv yoke was a common one. Should be all kinds of ##'s
on it to ID it. Maggy p#, OEM p#, EIA ##, date code, run ## etc.
And to save time just change the broken part.
And YES that blanker tube was trouble. Try a new one whenever
you get a maggy.

Zeno

[old_coot88]

Originally Posted by LukeSimon

...The Sams Photofacts and Darr’s “Horizontal Sweep Servicing Handbook” both say that color TV’s should first adjust the efficiency coil to minimum current, and then turn the core clockwise until best linearity, but without exceeding the TV’s max current spec.

Back in the day, in the sales & service arena at least, striving for perfect H linearity was never even a consideration. Even factory field reps (in our experience at least) never alluded to it. Just dip the cathode current for maximum life of the output tube, period. When dipped, there was no noticible non-linearity, even on the crosshatch pattern during setup.

[LukeSimon]

Originally Posted by old_coot88

Back in the day, in the sales & service arena at least, striving for perfect H linearity was never even a consideration. Even factory field reps (in our experience at least) never alluded to it. Just dip the cathode current for maximum life of the output tube, period. When dipped, there was no noticible non-linearity, even on the crosshatch pattern during setup.

The difference in horizontal linearity between 195 mA (greatest horizontal efficiency) and 215 mA (best horizontal linearity) is hard to see with a crosshatch test pattern. It is easier to see with a test pattern consisting of 256 alternating vertical white and black stripes. The white vertical stripes in the transition region are slightly thinner, slightly brighter, and slightly closer together when 195 mA is used, whereas when 215 mA is used, all vertical white stripes are the same brightest, thickness, and spacing. Also, I am referring to the 215 mA setting that is clockwise of the minimum setting of 195 mA.

Here is an excerpt from the manual that comes with the Seco HC-8, a device for measuring horizontal output tube cathode current during TV operation. It also implies that the optimal setting for color TVs is not absolute minimum cathode current. The best cathode current is “near minimum”.