VHF Tuner Alignment: where is the overall oscillator adjustment?

[LukeSimon]

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?

[ARC Tech-109]

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.

[Yamamaya42]

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. :/

[zeno]

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
LFOD !

[Yamamaya42]

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.

[LukeSimon]

Originally Posted by Yamamaya42

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?

[zeno]

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

[LukeSimon]

Originally Posted by zeno

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

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?

[Yamamaya42]

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.

[LukeSimon]

Originally Posted by Yamamaya42

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.

[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

[LukeSimon]

Originally Posted by 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.

[zeno]

Sight uneen go to color demod area first.

Zeno

[LukeSimon]

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.

[zeno]

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
LFOD !