1967 Magnavox T931

http://www.videokarma.org/attachment.php?attachmentid=205197&stc=1&d=1672195351

I need help: this TV has AFT, which can be disabled via a switch on the back, but I can’t find the fine tuning dial. How is fine tuning done on this TV? The outer dial changes the VHF channel number and the inner dial changes UHF channels number. The Sams schematic is cryptic. It indicates a manual fine tuning dial exists, but it is not clear what they are indicating.
http://www.videokarma.org/attachment.php?attachmentid=205202&stc=1&d=1672194892http://www.videokarma.org/attachment.php?attachmentid=205204&stc=1&d=1672196392


The picture tube tests good.
http://www.videokarma.org/attachment.php?attachmentid=205199&stc=1&d=1672194892http://www.videokarma.org/attachment.php?attachmentid=205200&stc=1&d=1672194892


I powered up the TV gradually using a variac, dim bulb, kill-a-watt, and then removed the dim bulb. At full voltage, the TV uses 290 watts. The Sams lists 310 watts, so not too far off. The problems: audio volume is weak even at max volume, brightness must be turned all the way up to display a picture, color is not working, reception is weak, focus is bad, and I can hear a clicking or ticking sound every few seconds from the HV cage. After 15 minutes of warm up, reception got better, which suggests some of the tubes need to be replaced.
http://www.videokarma.org/attachment.php?attachmentid=205203&stc=1&d=1672195319

I will measure B+, boosted B+, and ultor voltages tomorrow to track down the brightness and focus issues. Again, the picture tube tests good, so the chassis must be applying the wrong voltages to picture tube electrodes.

Please help me answer the following questions:

How is manual VHF fine tuning performed? Is the UHF dial used for fine tuning of VHF?
Can I use a 40kv rated probe for measuring focus voltage?
How should I fix the clicking / ticking sound coming from the HV cage?

Long shot, but will the inner knob push in?
I assume that it has a strip type vhf tuner and each strip has a fine tuning slug at the front of the strip.

Looks like my T940 Astro Sonic chassis the only difference being the focus rect, mine uses the tube. Nevertheless what you're dealing with is low B+. Powering up the set and trying to "reform" a 50+ year old capacitor is a gamble at best. The best advice I can give before replacing any tubes or making any assumptions is to replace the power supply caps and check the dropping resistors on the side board. C105 is located near the power transformer, C107 and C130 are behind the HV cage, see the attached pics. My Astro Sonic has been in the family since early 1970 and in my care since 1984 when dad replaced it with a Zenith S-3... the Magnavox survives to this day. I'm using my T940 as an academic example here.

The only tube that has been a problem with these is V204 the 6MU8, the other issues you describe point to low B+ caused by bad/leaky capacitors behind R312, it's a resistor divider chain that supplies most everything in the set. Poor focus and low HV, limited brightness and what I can see in your pic is a lack of color caused by reduced voltage to the horiz section. This also provides the burst gate keying signal and when low will not open the color killer to allow the colorburst into the chroma bandpass amp. Low B+ will also cause weak audio and washed out contrast, the convergance & pincushion on the left reinforces my conclusion. Measuring the focus with a 40KV probe might make the meter wiggle some but the numbers are meaningless. HV ticking is leakage in the cage caused by dust that needs to be cleaned with some Windex and small towels after connecting a cleap lead to ground then the center contact of the 6BK4 plate cap, learned this the hard way at 14! Let things dry for an hour before power up.
Looking at your pix tube readings they are cathode currents and still strong. The voltages going to the pix are arbitrary, its the differentials between K&G1 that are important. A weak power supply is going to throw them all off. My T940 uses a 6MD8 triple triode for the R-Y G-Y B-Y drive and G-1 bias, I don't know what your T931 uses but it should have roughly 195 volts difference between cathode and grid-1. The picture tube section of the RCA Receiving Tube Manual details this better.


The T940 SAMS is Set 1109 Folder-1 and you mentioned having a Sams as well. I have over 40 years of experience with mine and it's posted here, look toward the bottom of the page http://videokarma.org/showthread.php?t=273555 and if you look at Image34 it details how I addressed mounting the replacement capacitors.

What you have there is an EXCELLENT Magnavox console, a frosted front CRT without a cataract? WOW! Really complements your PortaColor.

John is right, push in the middle knob to fine tune.

Thanks for the help.

I will measure B+, boosted B+, and ultor (aka high voltage cap) today and report back to this thread. I ordered newly made in 2022 Mallory capacitors so that I can keep the chassis looking as close to original as possible.

http://www.videokarma.org/attachment.php?attachmentid=205211&stc=1&d=1672244759http://www.videokarma.org/attachment.php?attachmentid=205213&stc=1&d=1672245233

I have a theory about the no cataract. The TV has sliding doors that cover the screen, and this TV was a bedroom TV, not the main room TV. So it got light usage and was closed up and off for most of its existence.

http://www.videokarma.org/attachment.php?attachmentid=205212&stc=1&d=1672245298

Inside looks to be in good condition too. Definitely a secondary, back bedroom TV, and wasn’t the main TV of the house.
http://www.videokarma.org/attachment.php?attachmentid=205215&stc=1&d=1672245658

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:smoke:
LFOD !

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.


Opening the HV cage unplugs the HV rectifier (red circled cover). How am I supposed to open the HV cage without unplugging the HV rectifier? Am I supposed to observe the arcing with the rectifier unplugged?

http://www.videokarma.org/attachment.php?attachmentid=205216&stc=1&d=1672256702

It looks like there's a top lid that is held by a lone 1/4" bolt above the red red circled area.

Some sets (cough RCA CTC-16 and up) weren't designed to allow the door to be open and the rect to be plugged in.

IIRC the lid swings out. Arcing can happen to both the HV & focus including
both fillament windings.
You can also pry the cover to the HV cup ( red circle ) & look in there too.

Zeno

My wall outlet measures at 125VAC, so I used a variac to step down the voltage to 117VAC to match the Sams. According to my Kill-A-Watt, at 117VAC, the TV uses 302 watts and 2.92 amps, which is within 5% of the Sams spec. Stepping down the wall outlet from 125VAC to 117VAC made the arcing clicking and ticking sound from the HV cage completely stop!
http://www.videokarma.org/attachment.php?attachmentid=205221&stc=1&d=1672283176

The most readily accessible areas to probe B+ and Boosted B+ are the blue circle and red circle in the schematic, that is, the two leads of the spark gap, which are right above the blue screen pot. The B+ measurement should be accurate at this point, but the boosted B+ should be lowered by the resistor network in the screen grids to 1012V.
http://www.videokarma.org/attachment.php?attachmentid=205223&stc=1&d=1672283176

http://www.videokarma.org/attachment.php?attachmentid=205222&stc=1&d=1672283176

B+ measures 383.12V, which is within 5% of the Sams spec of 370V.
http://www.videokarma.org/attachment.php?attachmentid=205224&stc=1&d=1672283370

Boosted B+ measures at 960.92V, which is within 5% of the Sams spec of 1012V (the theoretical voltage if each pot and resistor in the screen resistor network measures exactly to spec).
http://www.videokarma.org/attachment.php?attachmentid=205225&stc=1&d=1672283370

So B+ and Boosted B+ appear to be good. I will measure ultor voltage and focus voltage tomorrow. I bet focus voltage is low, which would explain the blurry picture.

I exhaustively measured all B+ sources and summarized them in the table below. The B+ 220V source is the farthest from the schematic at 270V, which is 23% higher than specification. Since color is not working, this may be a symptom of the root cause of the picture being black and white.

http://www.videokarma.org/attachment.php?attachmentid=205230&stc=1&d=1672363216

http://www.videokarma.org/attachment.php?attachmentid=205231&stc=1&d=1672363457

If you haven't already tried it, see if switching the AC input to the 128 volt tap of the transformer helps with the high voltage issue without the need of the variac. ARC Tech-109s diagram of his set shows a switch, your set should have the black-white primary wire connected to a terminal on the PC board. They use a slip on connector with a blue wire to select the 120 or 128 volt tap.

simple things first, check the color killer control? Turn it to the left on a blank station and the screen should look like it has colored confetti all over. No change from the control? Check the phase det diodes CR701 and chroma sync diodes CR702 (T940 schematic) the nominal voltage should be about -2.0V on the grid of the triode section of the killer/burst amp 6MQ8/6KE8. The diodes are germanium 1N34's and are known to die without warning. I pulled mine and wired in a pair of 6AL5 twin diodes on wafer sockets. Same with the horiz AFC diode pack. If the burst amp isn't getting a signal it won't feed the color killer and that will keep the bandpass amp grid-1 cutoff so make sure it's opening up the bandpass.

Check for chroma subcarrier coming into the chroma take-off coil, it's fed from the 1st video amp plate via 18pf ceramic cap (C742). Look at the the output of the bandpass amp pin-6 of the 6HG8 V707a. Signal present then check the output of the 3.58 osc, you can pick up the output on the X-Z demod 6MK8 at pin-9 and it should have greater than 15V P-P. Signal present? Check to see if the auto color circuit or the switch is cutting off the chroma subcarrier. In my T940 its broken away to feed the TAC just before the color control and the total auto color circuit is switched in & out by a slide switch on the back, regardless of the setting the subcarrier passes thru the board then down to the main color control R402 before feeding G-1 pin-7 of the 6MK8 demod. I completely bypassed my TAC auto color board as it never completely shut off. That board was too far advanced for the time and proved to be a big pain in the back side, germanium diodes and transistors are not heat tolerant.

The best advice I can give here is to follow the signals on the schematic, the big challenge I had with my T940 was drifty carbon comp resistors. Cold they were fine but would drift up with heat causing all kinds of weird problems later in life. At some point I will be replacing them all on the chroma/luma and IF boards, the sweep board got all new 5% films in 2005 to address horiz & vert stability problems.

There are a number of those mylar type capacitors that I've had trouble with over the years. They are as follows in my T940:
C715 0.1uf 400V DC filter on leg of 6DT6 audio det from boost B+ (low distorted audio)
C721 0.033uf 100V AGC line to tuner
C722 0.1uf 100V IF AGC drive (washed out video)
C732 0.047uf 200V killer output bypass cap.
C749 0.1uf 100V 3.58MHz osc control R/C in series with R764
C773 0.047uf 200V. XZ demod screen bypass
Normally Mylar is like a lifetime capacitor but being baked over half a century they will die. I replaced them all with orange drops from AES.

Seeing the glow in your set makes me want to pull mine out of storage, unfortunaly it would take up a large chunk of my apartment being a lowboy console. Someday I will get my sanity in check and dump my plastic fantastic sets for the AstroSonic.

Thanks for the advice. Turning off the color killer has brought in color, but it is lots of purple and green colors.
http://www.videokarma.org/attachment.php?attachmentid=205246&stc=1&d=1672539913

When the color saturation knob on the front of the TV is turned 50% away from fully counter clockwise, I get lots of purple colors.
http://www.videokarma.org/attachment.php?attachmentid=205247&stc=1&d=1672539913

If I turn the “color” saturation knob on the front of the TV fully counterclockwise, as expected, the picture becomes black and white (aside from dynamic convergence issues).
http://www.videokarma.org/attachment.php?attachmentid=205248&stc=1&d=1672540407

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.
http://www.videokarma.org/attachment.php?attachmentid=205257&stc=1&d=1672540886

One successful thing I accomplished is I got focus working! The slug in the ajustable focus buck boost transformer was not unscrewing as far as it should. I didn’t realize this a few days ago, but after playing with it for an embarrassingly long time, I was able to get it to screw in all the way and unscrew almost all the way. Maybe it was jammed with dust or gunk and constantly screwing it back snd forth unjammed it. Regardless, the cathode rays are focused sharp enough to show gaps between the scanlines when displaying 240p pictures.
http://www.videokarma.org/attachment.php?attachmentid=205243&stc=1&d=1672541176


http://www.videokarma.org/attachment.php?attachmentid=205245&stc=1&d=1672541176

Tomorrow I will see if I can get dynamic convergence calibrated, and if I get that working, then I will take an oscilloscope to the color circuit.

Excellent progress on the set, you will prevail and when you're done I have a nice Panasonic CT34WX54 widescreen CRT just for your collection :D

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.

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:smoke:

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:smoke:

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

http://www.videokarma.org/attachment.php?attachmentid=205271&stc=1&d=1672689494

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.

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:smoke:
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.

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.
http://www.videokarma.org/attachment.php?attachmentid=205278&stc=1&d=1672724091

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.
http://www.videokarma.org/attachment.php?attachmentid=205279&stc=1&d=1672724091

Convergence is improved, but still has issues. Left is before, right is after.
http://www.videokarma.org/attachment.php?attachmentid=205280&stc=1&d=1672724091

http://www.videokarma.org/attachment.php?attachmentid=205281&stc=1&d=1672724091

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?
http://www.videokarma.org/attachment.php?attachmentid=205282&stc=1&d=1672724091

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?
http://www.videokarma.org/attachment.php?attachmentid=205283&stc=1&d=1672725614

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?

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.

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!!

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:smoke:
LFOD !

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.
http://www.videokarma.org/attachment.php?attachmentid=205285&stc=1&d=1672806128http://www.videokarma.org/attachment.php?attachmentid=205286&stc=1&d=1672806128
http://www.videokarma.org/attachment.php?attachmentid=205289&stc=1&d=1672806128

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!
http://www.videokarma.org/attachment.php?attachmentid=205287&stc=1&d=1672806128http://www.videokarma.org/attachment.php?attachmentid=205288&stc=1&d=1672806128
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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”.

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.

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.

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.

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.

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

See post #33 here:
http://videokarma.org/showthread.php?p=3203336&highlight=6gh8+spec+sheet#post3203336

where I compare chassis voltages and 6GH8 spec limits

See post #33 here:
http://videokarma.org/showthread.php?p=3203336&highlight=6gh8+spec+sheet#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".

"Oops all 5/6GH8!" Chassis

LOL a couple of times

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?

http://www.videokarma.org/attachment.php?attachmentid=205303&stc=1&d=1673066236

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!

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

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.
http://www.videokarma.org/attachment.php?attachmentid=205306&stc=1&d=1673109416

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.
http://www.videokarma.org/attachment.php?attachmentid=205307&stc=1&d=1673109801

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.
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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.
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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.

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.
http://www.videokarma.org/attachment.php?attachmentid=205311&stc=1&d=1673157652

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.

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.
http://www.videokarma.org/attachment.php?attachmentid=205316&stc=1&d=1673197948

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.

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

73 Zeno:smoke:
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.

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.

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:
http://www.videokarma.org/attachment.php?attachmentid=205332&stc=1&d=1673579206
http://www.videokarma.org/attachment.php?attachmentid=205333&stc=1&d=1673579206

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:
http://www.videokarma.org/attachment.php?attachmentid=205334&stc=1&d=1673579206

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.
http://www.videokarma.org/attachment.php?attachmentid=205335&d=1673579202

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!

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

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)

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.
http://www.videokarma.org/attachment.php?attachmentid=205341&stc=1&d=1673679118
http://www.videokarma.org/attachment.php?attachmentid=205342&stc=1&d=1673679118

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.
http://www.videokarma.org/attachment.php?attachmentid=205339&stc=1&d=1673679118

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.
http://www.videokarma.org/attachment.php?attachmentid=205340&stc=1&d=1673679118

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

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.

http://www.videokarma.org/attachment.php?attachmentid=205350&stc=1&d=1673752843
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.

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.

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.
http://www.videokarma.org/attachment.php?attachmentid=205356&stc=1&d=1673839611

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.

http://www.videokarma.org/attachment.php?attachmentid=205361&stc=1&d=1673839727

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!

http://www.videokarma.org/attachment.php?attachmentid=205360&stc=1&d=1673839727

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.

Another observation, replacing V19 with a NOS tube dramatically improved brightness.http://www.videokarma.org/attachment.php?attachmentid=205362&stc=1&d=1673840791

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.
http://www.videokarma.org/attachment.php?attachmentid=205366&stc=1&d=1674000115

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.

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 :smoke:

...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.

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”.
http://www.videokarma.org/attachment.php?attachmentid=205413&stc=1&d=1674952923