Magnavox T931

[old_coot88]

Originally Posted by LukeSimon

If I turn the “color” saturation knob on the front of the TV fully counterclockwise, as expected, the picture becomes black and white...

If I turn the “color” saturation knob on the front of the TV to about 25% away from fully counterclockwise, I get green colors and a little purple.

The 3.58 oscillator is not running. What you're seeing being displayed is the chroma signal minus the reference signal.

[zeno]

Originally Posted by old_coot88

The 3.58 oscillator is not running. What you're seeing being displayed is the chroma signal minus the reference signal.

Ditto
If this were an RCA I would go right to the color osc. They got the same look
but a little more pinkish.

Zeno

[LukeSimon]

Originally Posted by zeno

Ditto
If this were an RCA I would go right to the color osc. They got the same look
but a little more pinkish.

Zeno

I will see if replacing tube V23 with a new 6GH8A fixes things.

[Electronic M]

If not adjusting A16 or A15 with color bar pattern should fix it.

My process is typically make sure there's a good tube, adjust 10 turns in either direction looking for barberpoling color then adjust for fewer bars until sync, then max out color level control and detune fine tuning till you get the weakest discernable color and check for barberpoling...If you get it readjust A's till it syncs with weak color signal, then return fine tuning and color level to normal settings and check your work by tuning to blank channel and back to test pattern and see if it barberpoles for a noticable period. If not it should be fine...This is a pretty common thing with RCAs.

[Jeffhs]

Originally Posted by zeno

There was a cap, IIRC in the burst that killed the color
Boost usually feeds G-2 controls, audio & vert size. Looks
bright enuf so probably OK.
OK on HV probe, focus 5-6 KV apx & control near center of
range for best focus
Push & turn for VHF FT.
There is an auto color switch on the back. Best to leave it off.
It causes overall brownish color & was known as the "brownalater". Yuk !
Ticking, look inside the cage for arcing with room lights OFF.
Also do a complete clean up from HV rect to the CRT. It may also
be your focus problem.
BTW the pix looks about normal brightness. It will NOT be as bright as a newer Chromacolor jug or newer in line sets.

73 Zeno
LFOD !

You are so right about the so-called "brownalater" switch on these TVs, and also the "auto color" switch. These automatic color schemes actually did (and still do) more harm to the picture than good, so if you don't care for the effect on the picture (I never did), the best advice is to slide the switch to "off" and forget it. My Insignia flat screen TV has a menu adjustment which will switch the automatic color system on and off; I long ago turned it off and didn't look back. This TV has a picture many, many times better than any CRT television I have ever owned in my life (thanks to the DTV system now used world wide) and I would not dream of messing up that picture by activating the automatic color system.

BTW, as to "ticking" around the HV system, I would be absolutely certain all connections carrying HV are as clean as you can get them; this should eliminate the noise. I personally cannot see why an HDTV would produce this sound anyway, since I don't believe there are voltages anywhere near high enough (in the kilovolt range) in a flat screen TV which could arc to ground and cause interference in the picture, let alone make ticking sounds of any kind.

[LukeSimon]

Before trying to tune dynamic convergence, I tested the electrolytic capacitors on the dynamic convergence board. Electrolytic capacitors C7 and C8 (85 celsius blue axial Nichicon brand) tested bad with high ESR and low capacitance. So I replaced them with new 105 celsius Sprague brand capacitors.


This is my first time tuning dynamic convergence. I used the Sams and RCA’s Color TV Service Handbook Volume 2 from 1968. After hours of tuning the calibration pots and coil slugs, I am incredibly frustrated, and feel slightly loopy.


Convergence is improved, but still has issues. Left is before, right is after.




The upper left and the far right parts of the picture have convergence issues. Three adjustable slug coils do not seem to make any difference regardless of how the slug core is adjusted: “R-G Horiz Lines (Right)”, “R-G Vert Lines (Right)”, and “Blue Shaping (Blue Center)”.. Is something broken?


Here is the right side of the test pattern, where you can see red, green, and blue diverge. I can get blue to straighten, but is causes blue convergence to get worse in the center of the screen. “R-G Horiz Lines (Right)” is supposed to be able to converge red and blue horizontal lines near the center right of the screen, but the adjustable core coil doesn’t seem to have any impact on picture! What is going on?


Why do some of the coils not seem to make any difference? Should I be able to get better results than this, or do I need to start adding convergence strip magnets to the backside of the picture tube?

[jbattles]

because its a delta gun tube. you get it close as you can and live with it. that's the way it was. remember its not a modern Sony kv27130.

[kvflyer]

You have come a long way. Possibly, that is the best that can be achieved? I have to wonder if the set came from the factory with perfect convergence?

Please, don't beat yourself up! Step back, rejoice in what you have already accomplished. The, back at it!!

[zeno]

You can not make a rectangle delta CRT perfect. The L & R blue
is hardest. So here are some hints.
Warm se up a half hour.
Turn OFF the color
Turn down the brite & contrast & adj focus for sharp dots & grids. The idea in
not to drive the CRT hard.
Do a DGS then purity then lock down the yoke.
Do center only looking at that.
Move on to the instructions but recheck the center as you go.
Using a mirror dont work for me. I would lay across the set & look
directly at the screen. Much faster.

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

73 Zeno
LFOD !

[LukeSimon]

Originally Posted by Electronic M

If not adjusting A16 or A15 with color bar pattern should fix it.

My process is typically make sure there's a good tube, adjust 10 turns in either direction looking for barberpoling color then adjust for fewer bars until sync, then max out color level control and detune fine tuning till you get the weakest discernable color and check for barberpoling...If you get it readjust A's till it syncs with weak color signal, then return fine tuning and color level to normal settings and check your work by tuning to blank channel and back to test pattern and see if it barberpoles for a noticable period. If not it should be fine...This is a pretty common thing with RCAs.

As recommended, I replaced V23, the 3.58mhz oscillator tube. Removing the grounded shield was difficult. I had to use an old fashioned pencil with a big rubber eraser on the end to push the tube out of the shield.



Immediately after powering it on, with zero calibration, I got color. The hue and saturation need calibration, but it is impressive how well the advice provided in this forum worked. Thanks!



RF reception for video and audio takes a few minutes to warm up, and AGC is also maxed out and yet no overload distortion, which suggests a weak IF amplifier. So I will try replacing RF and IF tubes to see if that helps. The vertical deflection and pincushion tubes seem worn out too, with the vertical size pot at max setting to fill the screen and vertical linearity is off, pincushion correction is also a bit wrong. So I will try replacing the vertical output tube and pincushion tube too.

Then I will revisit dynamic convergence, by exhaustively testing every component, replacing any that are out of spec, and I will try the procedures listed in the RCA handbook for dynamic convergence for TVs with 13 dials, as opposed to the more typical 12 dial procedure. The Magnavox has 13 dials, where the 13th dial controls blue “center shape”.

[kvflyer]

Excellent! that looks so good. Congratulations.

Note that the 6GH8A tube that you have is a Sylvania. RCA ran those tubes very hard and they (RCA manufactured) had a high failure rate. RCA actually purchased 6GH8A tubes from Sylvania which were labeled "RCA". You can tell which ones were RCA vs Sylvania easily by looking at the tube number font. I have both types and when I get into my CTC-28, all will be replaced with NOS Sylvania.

[LukeSimon]

Originally Posted by kvflyer

Excellent! that looks so good. Congratulations.

Note that the 6GH8A tube that you have is a Sylvania. RCA ran those tubes very hard and they (RCA manufactured) had a high failure rate. RCA actually purchased 6GH8A tubes from Sylvania which were labeled "RCA". You can tell which ones were RCA vs Sylvania easily by looking at the tube number font. I have both types and when I get into my CTC-28, all will be replaced with NOS Sylvania.

What caused the RCA manufactured 6GH8A tubes to fail? The Sylvania branded tube that I removed from the TV had a different internal construction than the NOS RCA tube.

[kvflyer]

Originally Posted by LukeSimon

What caused the RCA manufactured 6GH8A tubes to fail? The Sylvania branded tube that I removed from the TV had a different internal construction than the NOS RCA tube.

I will see if I can find the documentation about the failure. RCA worked them on the ragged edge in some of its color sets and they were not as "robust" (my words) as those manufactured by Sylvania. I have 6GH8 tubes which are labeled "RCA" but the type font clearly indicates that it was manufactured by Sylvania.

Keep in mind also that manufacturers were often buying from each other and having them labeled as their own brand. Sometimes, you can tell by the EIA code one the item who really was the manufacturer.

[Electronic M]

It probably came down to cathode thickness and or material quality.
The more rugged the cathode the longer a tube can pull strong emissions. Some projection CRTs that used common consumer gun designs like the 5TP4 had beefed up cathodes to support sustained higher emissions.

IIRC in the CTC-19 and later "Oops all 5/6GH8!" Chassis the 2 that got the worst flogging were usually RCA badged Sylvania from the factory and the rest were RCA made....RCA apparently knew their GH8s weren't up to the tasks they were giving them.

[nasadowsk]

Was the 6GH8 an RCA tube? Why would they run it so hard, outside of the obvious reasons?