VHF Tuner Alignment: where is the overall oscillator adjustment?
 

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/attachmen...1&d=1693027343

https://www.videokarma.org/attachmen...1&d=1693027343

https://www.videokarma.org/attachmen...1&d=1693027343

https://www.videokarma.org/attachmen...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.

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:

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.

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

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...1&d=1693860185

http://www.videokarma.org/attachment...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...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 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.

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...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...1&d=1693943703

T933’s color smear goes away if picture is tuned to be wormy.
http://www.videokarma.org/attachment...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...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.

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...3&d=1693860184

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

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...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)?

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.

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/wikiped...t_plot.svg.png

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 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.