This is a Magnavox T933. There is a 0.5pF to 3pF trimmer capacitor called “overall osc adjust” in the schematic and it is referred to in the VHF alignment instructions in the Sams, but the trimmer is not marked anywhere in the photos. Where is this trimmer?

https://www.videokarma.org/attachment.php?attachmentid=206014&stc=1&d=1693027343

https://www.videokarma.org/attachment.php?attachmentid=206015&stc=1&d=1693027343

https://www.videokarma.org/attachment.php?attachmentid=206016&stc=1&d=1693027343

https://www.videokarma.org/attachment.php?attachmentid=206017&stc=1&d=1693027343

The dashed lines mean not all have this in every model. My T940 Astro Sonic does not have the trimmer however I have seen them in the past and learned the hard way they are best left alone.

Since we both seem to have the same Maggie chassis TVs, and even same copies of the SAMS for it with the red writing on it, any idea what the omission of the AFT switch is about? It has the “Chroma Tone” as listed on the SAMS, but where SAMS has AFT disable on the back, mine has “Auto Color off/on” and can find no info about it on the SAMS. :/

AFT was often an upgrade you paid for.

When you work on a set both the AFT & any auto color switches should be off.
Tuners have AFT off switches to turn it off for a second at channel change.
Those are on the side or rear of the TNR assy. Also some had a switch up
front of TNR to defeat it when fine tuning is engaged. Generally you can
defeat the AFT by grounding the TNR aft pin.
Keep in mind both the AFT AND auto color may have AFT on/off on them.
When done you can put the AFT back on, it does work good.

Chroma Tone was Maggys auto color scheme. In the day we called it the
" brownilator". When on it gives an overall puky brown cast to the pix.
Leave it off for best pix.

73 Zeno:smoke:
LFOD !

Searching the schematic (SAMS), I can see what this “Chroma Tone” does, (horrid idea) and what AFT seems to be, using a 6BA6 as an PLL on the tuner somehow, radical concept for way back then :O

But I have looked over every page of the SAMS, and can't find what this silly “color switch” does or goes to.

And yes, I have thought of putting in a AFT defeat switch.

Since we both seem to have the same Maggie chassis TVs, and even same copies of the SAMS for it with the red writing on it, any idea what the omission of the AFT switch is about? It has the “Chroma Tone” as listed on the SAMS, but where SAMS has AFT disable on the back, mine has “Auto Color off/on” and can find no info about it on the SAMS. :/

Your T933 doesn’t have an AFT switch on the back? Mine has the switch. It is labeled “auto color” on the TV, but it is really just the AFT defeat switch.


The issue I am struggling with is chroma bleeding, smearing, or streaking. I have a T931 too, which is almost identical. So I know how the tuner should behave: turning fine tuning counterclockwise increases sharpness and turning it clockwise increases smearing. That is how my T931 fine tuning works, and quite frankly all of my 1960s work that way.

My T933’s fine tuning has increased smear when turning clockwise, as expected, but counterclockwise doesn’t decrease the color smear to a sufficient level. When I turn the chroma gain to minimum, there is no smear in the black and white.

Also, the fine tuning doesn’t “wrap around”. So if I continue turning it counterclockwise it seems to rotate to one extreme and not circle back to the other extreme. Any ideas as to what is wrong?

On newer tuners the FT has a set range. Usually if you say
put it on ch 6 you can tune UP to FM radio or DWN to ch 5.
So yours is normal.

Old tuners had 12 screwdriver holes on the front of the TNR and
one FT coil. To adj center coil then adj the channel for best pix.

Zeno:smoke:

On newer tuners the FT has a set range. Usually if you say
put it on ch 6 you can tune UP to FM radio or DWN to ch 5.
So yours is normal.

Old tuners had 12 screwdriver holes on the front of the TNR and
one FT coil. To adj center coil then adj the channel for best pix.

Zeno:smoke:

So it is normal that the FT doesn’t wrap around, but the fact it can’t FT fully away from smearing color is still a real issue. Any suggestions on what to try for fixing this?

I wish they would have labeled accurately, as it really has little or nothing to do with the color, the switch on mine was doing nothing, until i cleaned it aggressively a 2nd time, NOW it does something, the annoying change in picture quality when you push in the FT knob is gone, which is what I wanted.

A picture of this smearing color you have would be helpful.
As mentioned with mine, there is a slight color shadow to the right of things, that is not affected by fine tuning that I suspect has to do with the chroma bandpass filter being slightly out of tune.

I wish they would have labeled accurately, as it really has little or nothing to do with the color, the switch on mine was doing nothing, until i cleaned it aggressively a 2nd time, NOW it does something, the annoying change in picture quality when you push in the FT knob is gone, which is what I wanted.

A picture of this smearing color you have would be helpful.
As mentioned with mine, there is a slight color shadow to the right of things, that is not affected by fine tuning that I suspect has to do with the chroma bandpass filter being slightly out of tune.

My T931 has a minor shadow like your T933, but my T933 has very bad color smearing. Fine tuning can make it a bit better, but not as good as my T931. I can also fine tune to the color smearing being very bad. I will post pictures later today. I have a B&K 415, but have not learned how to use it. So worse case, I will do a full RF, IF, and chroma IF alignment.

The other issue I have is with pincushion correction at the top of the picture, which sags down in the middle. I tubed L49 as the Sams says and it helped a bit. I am going to try tuning L48 next, even though the Sams doesn’t state that it should be adjusted. It is behind the flyback cage, so Magnavox made it hard to adjust without pulling the chassis out.

First step is treat it like a B&W ! Turn color off, hows the pix ?
If still streaks are still there turn off the room lights, set the brite & contrast
down low. Do they go away or get better ? Step by step......
DO NOT try an IF alignment !!!

By convention FT goes CW up in freq, CCW down. Effect is at CCW pix with no color & weak SND. As you go up the color starts to show & SND improves.
Beyond that point the pix starts to tear & go away, audio starts buzzing & pix gets wormy. At that point you are nearing the next channel ( up ) or
the FM band ( 88mhz + )if you are on ch 6.

Color edges are usually convergence. With color OFF they will still be there.

REMEMBER you need a good B&W TV to have a good color TV.

Zeno

First step is treat it like a B&W ! Turn color off, hows the pix ?
If still streaks are still there turn off the room lights, set the brite & contrast
down low. Do they go away or get better ? Step by step......
DO NOT try an IF alignment !!!

By convention FT goes CW up in freq, CCW down. Effect is at CCW pix with no color & weak SND. As you go up the color starts to show & SND improves.
Beyond that point the pix starts to tear & go away, audio starts buzzing & pix gets wormy. At that point you are nearing the next channel ( up ) or
the FM band ( 88mhz + )if you are on ch 6.

Color edges are usually convergence. With color OFF they will still be there.

REMEMBER you need a good B&W TV to have a good color TV.

Zeno

When color gain is minimized, the black and white picture looks great and sharp. When color gain is high enough for the picture to no longer be black and white, the smearing is visible.

Sight uneen go to color demod area first.

Zeno

I tried fine tuning again. Turned color gain off, so picture is black and white. Then rotate fine tuning clockwise until I start to see wormy sound in the picture, then I slowly rotate counterclockwise just a tiny bit until the picture stops being wormy. The black and white picture looks sharp, but when I turn color gain up to a normal level, the color smearing becomes visible. Fine tuning counterclockwise makes the color smear much worse. The tint control has very little range, and color gain almost has to be set to its lowest setting, otherwise the picture is too saturated.

All tubes have been replaced with tested NOS tubes, except the 25XP22 picture tube which is used but tests very strong cutoff, very strong emissions, and no shorts. Also, I did the Sams chroma AFC alignment, which tunes the chroma oscillator, so it locks in the color signal even on a weak signal. I have not performed the chroma bandpass alignment or the 4.5mhz trap alignment.

What should I try next? A chroma bandpass alignment? I have a B&K 415 and an oscilloscope, but I have never used the B&K 415 before.

http://www.videokarma.org/attachment.php?attachmentid=206043&stc=1&d=1693860185

http://www.videokarma.org/attachment.php?attachmentid=206044&stc=1&d=1693860185

Jug looks good, just some touch up needed.
That pattern is very nice. Never seen it before. Anyhows try the delay line.
The video gets to the CRT faster than the color. So if the video DONT
get delayed the Y & C dont match up. No real test for one unless its opened
( not ). Usually there are 3 legs. In, out & GND. Try to find an OEM
Maggy format is ######-#. you can try different brands but there are
slight differences between them.

73 Zeno:smoke:
LFOD !

If I fine tune into a mildly wormy picture, the color smearing is reduced a lot. It is still there, but reduced. Here is the test pattern when fine tuned to be a bit wormy.http://www.videokarma.org/attachment.php?attachmentid=206047&stc=1&d=1693879814

I wonder if I can adjust the 4.5mc trap to reduce the wormy picture.

It DOES say to de-tune FT until a beat pattern is seen (herringbone pattern ) , then adjust for minimum, so it can't hurt to try, I'm more curious about the slight mirroring seen to the right of the characters/ letters seen on the BW screen, mine also has the same to a lesser degree, no idea of the cause however.

What is the source of the test pattern?
NTSC transmitters had a phase pre-distortion filter meant to compensate the IF phase/delay response of early color sets. The effect was to delay the luminance a bit compared to the chroma. Sets soon began using different IF responses, so there was a mismatch to the standard FCC NTSC predistortion, corrected by picking the correct delay for the luminance delay line in the receiver. BUT, consumer video sources did not/do not have the predistortion filter, so the chroma may appear positioned a bit to the right of the luma. How much of this effect occurs depends on the particular receiver's IF design.

I tried fine tuning again. Turned color gain off, so picture is black and white. Then rotate fine tuning clockwise until I start to see wormy sound in the picture, then I slowly rotate counterclockwise just a tiny bit until the picture stops being wormy. The black and white picture looks sharp, but when I turn color gain up to a normal level, the color smearing becomes visible. Fine tuning counterclockwise makes the color smear much worse. The tint control has very little range, and color gain almost has to be set to its lowest setting, otherwise the picture is too saturated.

All tubes have been replaced with tested NOS tubes, except the 25XP22 picture tube which is used but tests very strong cutoff, very strong emissions, and no shorts. Also, I did the Sams chroma AFC alignment, which tunes the chroma oscillator, so it locks in the color signal even on a weak signal. I have not performed the chroma bandpass alignment or the 4.5mhz trap alignment.

What should I try next? A chroma bandpass alignment? I have a B&K 415 and an oscilloscope, but I have never used the B&K 415 before.

http://www.videokarma.org/attachment.php?attachmentid=206043&stc=1&d=1693860185

http://www.videokarma.org/attachment.php?attachmentid=206044&stc=1&d=1693860185

I think this is either ghosting in the signal path between the generator and antenna input, or an IF alignment / video peaking problem. The picture above on the top numbers you can see that the number has synchronized luma and chroma, but then next to the numbers there's (EDIT lumina and) chroma ghost numbers where it should be dark. This is some kind of ghosting/ringing effect somewhere.

It DOES say to de-tune FT until a beat pattern is seen (herringbone pattern ) , then adjust for minimum, so it can't hurt to try, I'm more curious about the slight mirroring seen to the right of the characters/ letters seen on the BW screen, mine also has the same to a lesser degree, no idea of the cause however.

Remember, I have two Magnavox’s: a 1967 T931 and this 1968 T933. The differences between the two chassis versions are very minor. The T931 doesn’t have this issue, and when I look at how the core of the 4.5mc trap is screwed down into the adjustable transformer in the T931, and how it is poking up outside of the T933, I am starting to wonder if I should fine tune into a very wormy picture and then screw the core into the 4.5mc trap so that the picture isn’t wormy. The color smearing goes away if I fine tune to a very obnoxiously wormy picture.

Smeary T933’s 4.5mc trap core is poking out of the coil.
http://www.videokarma.org/attachment.php?attachmentid=206051&stc=1&d=1693943703

No smear T931’s core is screwed down into the center of the coil. According to the Photofacts, this part number is identical between both chassis versions. In fact, this part of the circuit (video detector) is identical in both chassis versions.
http://www.videokarma.org/attachment.php?attachmentid=206048&stc=1&d=1693943703

T933’s color smear goes away if picture is tuned to be wormy.
http://www.videokarma.org/attachment.php?attachmentid=206049&stc=1&d=1693943703

Sams says that 4.5mc trap should be aligned by fine tuning to a very wormy picture, and then adjusting the core util the wormy interference is minimized. So once that is done, I may be able to fine tune to zero worminess and near zero smear.
http://www.videokarma.org/attachment.php?attachmentid=206052&stc=1&d=1693944443

Since I was fine tuning CW to a wormy picture and then fine tuning CCW until not wormy, I may simply be not using the proper fine tuning because the misaligned 4.5mc trap makes the worminess excessive too early in the fine tuning CW rotation.

I would check the resistors around the delay line. Perhaps a series resistor at the input (or not) and usually a resistor to ground at the output. If these aren't presenting the delay line with the proper impedance the delay will not be correct. I had that problem in an old RCA.

I would check the resistors around the delay line. Perhaps a series resistor at the input (or not) and usually a resistor to ground at the output. If these aren't presenting the delay line with the proper impedance the delay will not be correct. I had that problem in an old RCA.

Ditto. Coils around the delay line also.

Ditto. Coils around the delay line also.

Wouldn’t that cause smearing in the black and white picture, and/or the color and luma signal not being aligned. I don’t see smearing in the BW picture, and the color signal is not shifted to the right of the BW signal. It seems aligned with luma but is heavily smearing.

I think another clue is that the color burst has smearing, and the color burst doesn’t pass through the chroma bandpass. The smearing of color burst is causing the tint control to have a small range, but after aligning the chroma AFC, the color oscillator is still able to lock instantly and without issue.

These clues seem to suggest it is not delay line and not chroma bandpass. It is RF or IF that is not peaking enough at 3.58mhz.

I think that they are referring to the shadowing / echo seen on the BW picture.
http://videokarma.org/attachment.php?attachmentid=206043&d=1693860184

this should be cleaner , not sure of the cause :o

...
Sams says that 4.5mc trap should be aligned by fine tuning to a very wormy picture, and then adjusting the core util the wormy interference is minimized. So once that is done, I may be able to fine tune to zero worminess and near zero smear.
http://www.videokarma.org/attachment.php?attachmentid=206052&stc=1&d=1693944443

Since I was fine tuning CW to a wormy picture and then fine tuning CCW until not wormy, I may simply be not using the proper fine tuning because the misaligned 4.5mc trap makes the worminess excessive too early in the fine tuning CW rotation.

Sorry I didn't see this before.

Yes, with the core so obviously misadjusted, it makes sense to try adjusting it. But if you can;t improve it by adjusting the core, you will need to look for something else.

Adjusting the 4.5mc trap and then fine tuning again didn’t help much. Another symptom that may hint at the issue is the sharpness potentiometer has no effect. This may be unrelated to the smearing, or it may indicate a peaking coil or the tube is not correctly peaking at 3.58mhz. The chroma signal is output from the tube’s plate and luma is output from the tube’s cathode. There appears to be peaking coils on both cathode and plate.
http://www.videokarma.org/attachment.php?attachmentid=206056&stc=1&d=1694057538

It should be easy to check the coils for opens and also check that the connection to the sharpness control is not open.

BTW, I'm scratching my head about how that sharpness control is supposed to work. Where does the luminance signal go next (which tube and what electrode of that tube)?

It should be easy to check the coils for opens and also check that the connection to the sharpness control is not open.

BTW, I'm scratching my head about how that sharpness control is supposed to work. Where does the luminance signal go next (which tube and what electrode of that tube)?

I find it a bit confusing too, but it DOES do something on my set.
http://suzaku.live-evil.org/maggie-video.jpg
however the control WAS very dirty at first, and needed 2 cleanings before it would work on my set.

you could try to short the top (input) point to ground to see it that makes any change.

You don't have to believe me, but I suspected the next stage was a grounded-grid amp.
With the sharpness turned to minimum, current from V5A cathode is divided between the 39 ohm resistor and the cathode impedance of V5B at all video frequencies, so the circuit is actually lossy, current-wise, while R66 provides voltage gain. When sharpness is set to max, there is 139 ohms impedance at the resonant frequency of C48 and L15 (maybe tuned to about 2 MHz), which causes more of the current to go into the cathode of V5B. The bandwidth of the peak depends on the Q of the circuit, which is determined by the ratio of R4 at max (100 ohms) to the inductance L15.

I just find it strange that R4 would have no effect at all, unless it was bad/dirty and not working correctly, if there was a fault with L15, which is only about ½ ohm, the cathodes of V5 A/B would lose ground point and drift way above 1v, and video would be way more messed up then he seems to be getting, I would assume.

R4 should be easy enough to check, it's on the back of the set on the top.

It should be easy to check the coils for opens and also check that the connection to the sharpness control is not open.

BTW, I'm scratching my head about how that sharpness control is supposed to work. Where does the luminance signal go next (which tube and what electrode of that tube)?

The 6LM8 acts as a cathode follower for the luma signal. So luma is output from its cathode and that is input to another luma amplifier. Dialing up the sharpness pot decreases the load resistance of the cathode follower amplifier which reduces its gain but increases its bandwidth.

I will clean the pot. While I am at it, I will try a few alternative tubes. Even though my NOS 6LM8 tests good… in the chassis performance can still vary.

Turning the sharpness pot does not change the DC bias by any significant amount, because it is in parallel with the 0.5 ohms of L15.

The sharpness control being open and having no effect is probably what's causing the ringing (echoes) to the right of the sharp letters. L15 and C48 will resonate with a higher, narrower peak in frequency without the 100 ohms in parallel, becoming underdamped and ringing in the time domain.

According to the handy-dandy LC Tank Circuit Resonance Calculator, L15 & cap are tuned to 2.131MHz, (8.2 mh coil & 680pF cap).

I don't have the SAMS. Is that where the 8.2 mHy is stated?

Anyway, I assumed 2 Mhz and calculated 9.3 mHy. Using that and using the tube characteristics to calculate the cathode impedances, I calulate that the 39 ohm and the cathode resistances result in an equivalent resistance of 128 ohms in parallel with the sharpness control 100 ohms (total 56 ohms), resulting in a net damping factor of 1.04 (assuming a 9.3 mHY coil). For a 8.2 mHy coil, the damping factor is 0.98, so still essentially 1.0

For either inductance, the peak current gain at the resonant frequency is 0.4426 at resonance, compared to 0.1822 at DC, for a gain increase of 2.43 times or 7.7 dB at resonance compared to DC.

By the way, if the sharpness control is open, the damping factor is 0.45, meaning underdamped and ringing.

https://upload.wikimedia.org/wikipedia/en/thumb/8/8e/RLC_transient_plot.svg/350px-RLC_transient_plot.svg.png

I don't have the SAMS. Is that where the 8.2 mHy is stated?

Anyway, I assumed 2 Mhz and calculated 9.3 mHy. Using that and using the tube characteristics to calculate the cathode impedances, I calulate that the 39 ohm and the cathode resistances result in an equivalent resistance of 128 ohms in parallel with the sharpness control 100 ohms (total 56 ohms), resulting in a net damping factor of 1.04 (assuming a 9.3 mHY coil). For a 8.2 mHy coil, the damping factor is 0.98, so still essentially 1.0

For either inductance, the peak current gain at the resonant frequency is 0.4426 at resonance, compared to 0.1822 at DC, for a gain increase of 2.43 times or 7.7 dB at resonance compared to DC.

Yes, I looked up the replacement specs in the sams, and it said that L15 was 8.2, (rf choke) along with a list of subs.

I have never seen a tube amp used in this way. It acts as both a common cathode amplifier for chroma output from the plate and a cathode follower amplifier for luma output from the cathode. Wouldn’t the different sharpness setting also change the amplifier characteristics for the chroma signal output from the plate?

I have never seen a tube amp used in this way. It acts as both a common cathode amplifier for chroma output from the plate and a cathode follower amplifier for luma output from the cathode. Wouldn’t the different sharpness setting also change the amplifier characteristics for the chroma signal output from the plate?

I have the same question in my mind. Maybe the peaking frequencies are mostly outside the chroma bandpass - need to see more of the service info and schematic to make a guess.

The equivalent cathode load of V5A including its internal impedance varies from 214 ohms far away from the resonance to 259 ohms at resonance, so 17% max, probably much less over the chroma band. I'd need to crank up a spreadsheet to plot some curves.

Result from a spreadsheet:

https://live.staticflickr.com/65535/53174084753_597bc5b0fb_o.jpg

At minimum sharpness, the curve is flat at 1.0 for all frequencies.

Any updates/ progress with your maggie?
To quote Spock from Star Trek II : The Wrath of Khan.
https://getyarn.io/yarn-clip/79475c41-dd80-49fa-a04f-a2b3598a16ab

I did try to adjust the 4.5 MHz (A11) on mine again, to see if it had any impact on the slight color shadow, it did not, it DID however have a point at where it started to interfere with the color and vertical lock, but really no noticeable improvement in adjusting it other than very slight reduction of herringbone patterns when FT is off tuned.

I did however have one annoying embarrassing incident that made me had to remove the chassis yet again, after I had it all bolted back in :(, while tuning A11 from extreme to extreme, I went a wee bit too far on the lower, and the damn core fell out the bottom into IF cage, I had to open it to get it out, I did not realize it would do that, I thought it would hit bottom and stop, oh well, no harm done other than a hour or so lost! :D :yes:

Any updates/ progress with your maggie?
To quote Spock from Star Trek II : The Wrath of Khan.
https://getyarn.io/yarn-clip/79475c41-dd80-49fa-a04f-a2b3598a16ab

I did try to adjust the 4.5 MHz (A11) on mine again, to see if it had any impact on the slight color shadow, it did not, it DID however have a point at where it started to interfere with the color and vertical lock, but really no noticeable improvement in adjusting it other than very slight reduction of herringbone patterns when FT is off tuned.

I did however have one annoying embarrassing incident that made me had to remove the chassis yet again, after I had it all bolted back in :(, while tuning A11 from extreme to extreme, I went a wee bit too far on the lower, and the damn core fell out the bottom into IF cage, I had to open it to get it out, I did not realize it would do that, I thought it would hit bottom and stop, oh well, no harm done other than a hour or so lost! :D :yes:

I have been reading the manual for my B&K 415. It has a chapter at the end that explains RF, IF, and chroma alignment for a couple models of TVs. Luckily, the first example explains the steps for the Magnavox T933 by showing the alignment steps called for by Magnavox in their OEM service manual and comparing how it is done using the B&K 415.

Both OEM alignment steps and B&K alignment steps are equally thorough. The B&K steps just involve less test equipment. It involves tuning coils and pots on the RF tuner, the IF board, and the chroma board. There is no way this can be done without a sweep and mark generator. Also, the steps in the Sams Photofacts are not as thorough as the OEM and B&K steps. The quality of the alignment wouldn’t be nearly as good if the Sams guide was used.

For example, the sweep generator sends a 400hz waveform through the RF signal and minimizing the amplitude of that in various parts of the IF circuit is how the sound traps are tuned. Much more scientific than the Sams guide. The bode plots provided also differ a bit from those in the Sams.

I noticed the same thing for Sams dynamic convergence steps, which are also inferior compared to the OEM steps. Ditto for the chroma AFC alignment which is very bad in the Sams. The hand written notes in the scanned copy of the Sams are a big improvement. Definitely makes me lose respect for Sams. OEM and B&K guides are superior.

The thing that worries me is the fact that some of the IF coils use two cores: a top core and a bottom core. Each must be separately adjusted. I have many alignment tools, but I worry that I don’t know how to “feel” my way between the top and bottom coil.

http://videokarma.org/attachment.php?attachmentid=206091&stc=1&d=1695874707
http://videokarma.org/attachment.php?attachmentid=206092&stc=1&d=1695874931

http://videokarma.org/attachment.php?attachmentid=206152&stc=1&d=1698042032

Let’s just say that the IF sweep response clearly shows why chroma was smearing. My oscilloscope has the bode plot mirrored horizontally. So the chroma marker is on the right hand side.

http://videokarma.org/attachment.php?attachmentid=206149&stc=1&d=1698040996

Below is the result of my first IF alignment. Note that the achieved gain is 10x more than the curve above. This took hours, and was very frustrating, just like my first time converging a delta gun.
http://videokarma.org/attachment.php?attachmentid=206150&stc=1&d=1698040996

http://videokarma.org/attachment.php?attachmentid=206151&stc=1&d=1698040996

I am going to take another pass at it on another day after I decompress from the stress. It is pretty close to matching the reference curve, but I am sure I can dial in the sound traps a bit better and improve the response shape. Then I will align the chroma bandpass, and finally I will align the RF end to end.

From what I have seen of the videos posted to u-tube about this, there is no such thing as a “perfect” IF or chroma bandpass alignment, just varying degrees of ALMOST there! :yes::D

From what I have seen of the videos posted to u-tube about this, there is no such thing as a “perfect” IF or chroma bandpass alignment, just varying degrees of ALMOST there! :yes::D

I struggled for a while because the TV was so badly out of alignment. There are so many adjustable slug coils and pots that need to be adjusted, I amost ran out of alignment tools. If one of the slugs is far from correct position, the gain is terrible and adjustment doesn’t seem to help. Every slug I turned would make the bode plot look worse. So I figured, I try large changes in slug positions and by trial and error “explore” for a combination of slug positions that simply gave better gain. Ignoring shape and focusing on gain at first allowed me to get to a slug combination where all slugs and pots started to behave like their description in Magnavox’s manual.

The markers 42.17mc and 45.75mc are supposed to be at 50% +/- 5% down the curve and they are currently at 49%. 42.75mc and 45mc are supposed to be 85% +/- 15% and they are currently at 78%. The 47.25mc trap is just slightly off from being perfect too.

I can only imagine what negative gain was doing to the chroma signal. It is impressive that the TV was able to draw a picture with how badly it was misaligned.

Alignment would absolutely be impossible to achieve without the proper tools, and even with the proper tools, it takes lots of trial and error.

I readjusted the audio traps and the tilt and this is the closest that I have gotten the IF response to the Magnavox service manual. As in the Magnavox service manual, the bode plot is inverted on both X and Y axis. So frequency increases from right to left, and amplification increases from top to bottom.

Frequency: 41.65mhz (chroma lower)
Target: 20% +/- 5%
Actual: 17%

Frequency: 42.17mhz (chroma mid)
Target: 50% +/- 5%
Actual: 51%

Frequency: 42.75mhz (chroma upper)
Target: 80% +/- 15%
Actual: 81%

Frequency: 45.00mhz
Target: 80% +/- 15%
Actual: 81%

Frequency: 45.75mhz (luma mid)
Target: 50% +/- 5%
Actual: 51%

http://videokarma.org/attachment.php?attachmentid=206153&stc=1&d=1698125168

Chroma alignment was so bad in the beginning. This chroma response took me hours of tuning coils. The double slug coil was especially challenging because the two slugs were touching each other. Every marker is spot on except the gain for 3.58mhz is too low by 5%. I will touch it up after some rest.

Note the marker for the 4.5mhz trap on the far right. The Sams procedure for aligning the 4.5mhz trap is useless. It is easiest to align it when aligning the chroma bandpass.

http://videokarma.org/attachment.php?attachmentid=206157&stc=1&d=1698304933

http://videokarma.org/attachment.php?attachmentid=206160&stc=1&d=1698457597

http://videokarma.org/attachment.php?attachmentid=206161&stc=1&d=1698457597

The chroma bandpass is now within spec for 3.08mhz through 4.08mhz. I will do RF alignment tomorrow, and then post before vs after pictures of test patterns.

After aligning IF and RF, the color is much sharper and there is no more smearing/ bleeding of colors. However, the hue (tint) is wrong now (approximately 90 degrees out of phase), and just as before, the tint control has little effect: maybe 5 degrees phase shift up or down.

I have two hypotheses: first is that the tint control is not shifting the phase of the color oscillator the full range of plus/minus 30 degrees. I will oscope the oscillator to test that hypothesis. The second hypothesis is that the IF and/or chroma bandpass alignment is what is causing the tint control to not work. Remember, before I did this alignment, the tint control didn’t change the hue. But the alignment was very bad before too.

If the alignment is not symmetric on both 0.5mhz side bands of the 3.58mhz chroma signal carrier, then during demodulation, the average of the side bands is what becomes the Pb and Pr signals. I think that reduces the effectiveness of the tint control’s range. I will test this hypothesis after ruling out tint control not phase shifting the oscillator.

...the tint control has little effect: maybe 5 degrees phase shift up or down.

Can you post the part of the schematic that includes the burst amp area?

Can you post the part of the schematic that includes the burst amp area?

http://suzaku.live-evil.org/maggie-tint.jpg
:D

Can you post the part of the schematic that includes the burst amp area?

The Sams states it should be able to shift the phase of the oscillator plus or minus 30 degrees. I have two of this TV, and one is easily able to do that, but it also had no alignment issues. This TV seems to have had every coil “peaked”, that is, tubed to maximize the measured DC offset. Supposedly that was a poor man’s alignment back in the day, done by techs that didn’t do the job right.

If the alignment is not symmetric on both 0.5mhz side bands of the 3.58mhz chroma signal carrier, then during demodulation, the average of the side bands is what becomes the Pb and Pr signals. I think that reduces the effectiveness of the tint control’s range. I will test this hypothesis after ruling out tint control not phase shifting the oscillator.

If the overall alignment is not symmetric, it is likely to introduced bad color transients on edges of brightly colored objects, so for example, the transitions between color bars would be bad. This could be seen particularly in the transition between the green and magenta bars as a noticeable color rather than going from green through gray to magenta. The tint control should still work, and should change the tint of both the large areas and the bad transients.
So, I think your problem likely is in the tint control area itself.

The way it's done in this set seems rather odd to start with, I have never seen a air capacitor (variable) used for tint before, especially on the front of a set on a long wire like this, just seems like extra trouble.
As mentioned with my set, it did not like some NOS tubes in various places, V22 being one of them, I had to try a few new 6KE8s till it worked well, loss of color (killer problem) and reduced tint range was the result when it harfed on a tube it did not like.

Assuming the voltages are correct on the 6KE8/burst amp, adjusting A17/L30 should be able to grab the correct tint range. Seems like, anyhow.

Edit. Forgot, some Maggies did have a 'distant' variable cap for tint. Thanks, Y'42

The distant variable air capacitor does seem problematic. It is connected by a grounded cable from behind the tint dial on the front of the TV. So that grounded cable is itself a capacitor between the cable and the grounding around it. I bet the total capacitance of the grounded cable is a sizable amount of the capacitance of the variable air capacitor.

I will redo chroma AFC alignment, oscope the range of phase shift of the oscillator and then hopefully that sheds light on why tint control lacks range.

I am pretty sure I know what is going wrong. The Sams instructions for aligning the color AFC are easily misunderstood. It talks about adjusting the oscillator transformer for max VTVM deflection, and then adjusting the burst transformer for maximum deflection. I was interpreting that to mean maximum negative DC bias at the test point.

But I found an RCA book that explains that oscillator transformer is peaked for max negative voltage and the burst transformer is peaked for maximum voltage. The Magnavox Tabs Manual explains the theory of the phase detection circuit too, that is, the burst is supposed to be 180 degrees out of phase with the oscillator, and they are supposed to be approximately matched in amplitude. So peaking the oscillator transformer should make the test point voltage as low as possible, and peaking the burst transformer should the raise the test point voltage as high as possible.

The naive reading of the Sams instructions results in the opposite, which minimizes the ability of a phase shift in burst to phase shift the oscillator. This would also explain why a weak burst amplifier tube would cause limited range for the tint control. It also suggests that peaking both transformers isn’t ideal. It is just a starting point, and it is best to touch up both transformers so that oscillator wave and burst wave are the maximum amplitude for which they can be adjusted to also be equal amplitude.

http://videokarma.org/attachment.php?attachmentid=206174&stc=1&d=1698818419

I believe any tuning to make the oscillator and burst inputs to the phase detector *equal* amplitude is irrelevant. The important thing is that both the oscillator output coil and the burst input coil are tuned for maximum response at 3.58 MHz. Unlike the oscillator zero beat tuning, these two are relatively broad peaks of the resonances whose purpose is to get maximum input to the phase detector from both the burst and the oscillator.

I believe any tuning to make the oscillator and burst inputs to the phase detector *equal* amplitude is irrelevant. The important thing is that both the oscillator output coil and the burst input coil are tuned for maximum response at 3.58 MHz. Unlike the oscillator zero beat tuning, these two are relatively broad peaks of the resonances whose purpose is to get maximum input to the phase detector from both the burst and the oscillator.

I guess I just didn’t realize that burst and oscillator waves were entering the phase detector 180 degrees out of phase with each other. So I thought peaking the transformers would pull the test point voltage in the same direction for both, and therefore I am pretty sure I tuned the burst transformer for the minimum resonance.

Aligning the burst amp in the chroma AFC didn’t fix the hue, but the tint control seems to have the ability to shift the phase by about plus or minus 20 degrees now. The problem is that the hue is 90 degrees out of phase, so the range of the tint control is not sufficient to correct a 90 degree phase shift.

Correct Test Pattern Hues:
http://videokarma.org/attachment.php?attachmentid=206180&stc=1&d=1698975771

Notice the 90 degree phase shift:
http://videokarma.org/attachment.php?attachmentid=206181&stc=1&d=1698975771

http://videokarma.org/attachment.php?attachmentid=206182&stc=1&d=1698975771

Here is my current theory. The burst and chroma signal travel through separate paths. The red path is the burst. The blue path is the chroma signal. It passes through A14, A13, and A12. These 3 coils are what are tuned during chroma bandpass alignment. They are causing the 90 degree phase shift. So I will first try to adjust A14 to get the correct hue, and then redo alignment but only changing A13 and A12.
http://videokarma.org/attachment.php?attachmentid=206183&stc=1&d=1698975771

Don't go ruining your bandpass to center the tint.

Is it correct that L31 connects to the bottom of A17 (doesn't jump over it?).
If so, this is the suspect area. I would first try setting the tint control to center range and tuning A17 to see if that will center the tint.
Also check if C138 (? can't read it clearly) on bottom of A17 is good.

Of course, ther is always the possibilty that the aligment procedure for A15 and A17 isn't right.

There are some inductors in this circuit that look like ad hoc fixes because the phase didn't come out right in the first design: L35, in series with the color control output; L32, in series with the CW feed to the phase detector.

Is the 4700 ohm from L32 to ground good? Are the supposedly equal-value components in the phase detector all good?

Don't go ruining your bandpass to center the tint.

Is it correct that L31 connects to the bottom of A17 (doesn't jump over it?).
If so, this is the suspect area. I would first try setting the tint control to center range and tuning A17 to see if that will center the tint.
Also check if C138 (? can't read it clearly) on bottom of A17 is good.

Of course, ther is always the possibilty that the aligment procedure for A15 and A17 isn't right.

I tried the entire range of A17 and it doesn’t get the hue to the right phase. The reason I am going back to adjusting the chroma bandpass is that there are many variations of the bandpass slugs that create the right bandpass shape. Before I did the alignment, the bandpass slugs were set to peak max gain at 3.58mhz. The bode plot looked like a very tall and skinny triangle. That had the right hue, but caused the horribly bad smearing. The Pb and Pr bandwidth was far less than the 0.5mhz that it should have been.

If I could observe the phase angle and the bode plot of gain at the same time, I’d be able to align the band pass and land on a phase angle close to that of the color burst amplifier output.

L32 and the 4700 ohm to ground look suspiciously like a phase shifter. What happens if you short L32?

http://videokarma.org/attachment.php?attachmentid=206193&stc=1&d=1699227531

http://videokarma.org/attachment.php?attachmentid=206194&stc=1&d=1699227531

Top is before alignment. Bottom is after. The hue was 90 degrees out of phase when the chroma bandpass is optimally aligned. De-tuning it slightly allows for the tint control to get the correct hue when the tint capacitor is turned to minimum capacitance.

A better design would be for Magnavox to have chroma tint control after the chroma bandpass, so that alignment of the bandpass doesn’t impact the hue.

...A better design would be for Magnavox to have chroma tint control after the chroma bandpass, so that alignment of the bandpass doesn’t impact the hue.

Phase of the burst signal is what the tint control has to regulate, thereby the phase of the 3.58 oscillator. Its output is then referenced to the wideband chroma bandpass signal, to feed the demods.

If it doesn't throw the set off too badly in response you want to have the control be correct at least a few degrees away from it's stop. Not all sources have correct or matching tint settings and if one of your sources needs a few degrees past the stop it will drive you nuts in the future.

Phase of the burst signal is what the tint control has to regulate, thereby the phase of the 3.58 oscillator. Its output is then referenced to the wideband chroma bandpass signal, to feed the demods.

I got the oscillator to the point where it had its full range of plus or minus 30 degrees. The problem is that the difference between the color burst phase and the chroma signal after bandpass ends up being 90 degrees different. So the 30 degrees is not enough.

The chroma demod acts as a differential amplifier and so it is the difference in phases between the oscillator and the amplified chroma that matters. That difference cannot be more than plus or minud 30 degrees. As the chroma bandpass is adjusted, the chroma signal’s phase is shifted. So having tint follow the bandpass would have made alignment not impact tint since both the burst and the chroma signal’s phases would be shifted together.

...having tint follow the bandpass would have made alignment not impact tint since both the burst and the chroma signal’s phases would be shifted together.

Only thing that approach is gonna do is change the shape of the bandpass curve, ie, its swept amplitude curve - not its phases. The chroma carrier is not a single, fixed-amplitude sine wave like the reference/burst signal is.

EDIT. Just for the heck of it and an experiment, can you reverse the connections to the primary of A17/L30? That would reverse the phase of the burst signal.

Only thing that approach is gonna do is change the shape of the bandpass curve, ie, its swept amplitude curve - not its phases. The chroma carrier is not a single, fixed-amplitude sine wave like the reference/burst signal is.



Incorrect. The chroma bandpass is a tuned circuit whose phase shift is affected by the amplitude response tuning and not necessarily zero at the subcarrier frequency, and thus can affect the tint if the burst takeoff for the oscillator control is before the chroma bandpass. However, if the chroma bandpass response is nominal, the phase as well as amplitude should be close to as-designed. A 90 degree shift is much more than woud be expected from a slight bandpass misalignment. That's why the search should be for something that shifts the relative phase of the signal and oscillator paths, and it is more likely to be in the oscillator path. This could occur in the feedback of CW from the oscillator to the chroma phase detector as well as in the burst takeoff path. That's why I suggested investigating the L-R circuit in the path from oscillator output to phase detector. The problem could also be caused by the oscillator output transformer being mistuned.

Thank you for the correction, OTN. So noted. :)

Sounds so complicated and scary! IF /chroma bandpass alignments, where all NOOBs fear to tread!:tears: :nono:

Sounds so complicated and scary! IF /chroma bandpass alignments, where all NOOBs fear to tread!:tears: :nono:
Well, is a great work! Difficult for sure; patience is a virtue.
And, for some people like me, that worked only with TV's from ceramic filter era ahead (probably some members here are from more recent era also), never touched alignment when at work so don't have the field expertise, will be scary, but a interesting thing to learn. IF the core from these delicate coils not end stuck or destroyed...
So, watching this with curiosity.

Well, is a great work! Difficult for sure; patience is a virtue.
And, for some people like me, that worked only with TV's from ceramic filter era ahead (probably some members here are from more recent era also), never touched alignment when at work so don't have the field expertise, will be scary, but a interesting thing to learn. IF the core from these delicate coils not end stuck or destroyed...
So, watching this with curiosity.

Yes, the scariest thing to me is breaking a core!
I'd also note that the higher frequencies in the tuner and IF amplifier make lead lengths and physical probe design more critical, but chroma should be relatively insensitive to the connections of the alignment equipment.

The only experience I have had so far with IF alignment is on my Arvin TV, and that took 4 passes of adjust for Max/Min levels with an RF generator and volt meter to get NEAR OK! :D:yes:

I keep forgetting to add that all this analysis of what might be wrong has been based on all the bypass caps in the chroma and oscillator circuits being good. If one is open, that might mess up things.

I have now reached the stage of learning where sweeping RF, IF, and chroma is fun. Possibly because I now know what I am doing and I am getting good results.

After digging through 1960s and 1970s issues of “Electronic Technician” that are available as free scanned PDFs on worldradiohistory.com, I learned that the typical artist renditions of the alignment bode plots are not accurate in their shape. There are actually two sphapes that an alignment bode plot should have. It depends on the TV model: either a flat top bode plot or a “haystack”. The authors in Electronic Technician say that you will know which shape your TV uses because one will fall into place more naturally whereas the other shape will fight you as you try to align it. For the T933, the bode plot is the “haystack” type.

http://videokarma.org/attachment.php?attachmentid=206211&stc=1&d=1699715559

Here is a before vs after alignment. Note the improved sharpness and decreased color smearing.
http://videokarma.org/attachment.php?attachmentid=206212&stc=1&d=1699715559
http://videokarma.org/attachment.php?attachmentid=206213&stc=1&d=1699715559

Colored text in the upper left corner is a real stress test due to dynamic convergence, focus, degaussing, and RF/IF/chroma alignment. I wish I had a before picture of “red” at the very start of this project before I fixed the focus and dynamic convergence problems. I am very happy with this restoration progress.

I will be watching old Christmas movies this year, on this Magnavox.

http://videokarma.org/attachment.php?attachmentid=206214&stc=1&d=1699716204

http://videokarma.org/attachment.php?attachmentid=206215&stc=1&d=1699716152

Great so far, but the off-center tint control is still a problem.

Great so far, but the off-center tint control is still a problem.

I haven’t fixed the root cause, and I am sure that when I do fix it, I will be able to get an even better picture due to being able to use a better chroma bandpass bode curve than I am using now. However, these amazing analog TVs are so malleable and organic that they can handle significant component drift and failure.