Sylvania 336mu

[sylvania]

Thanks for help, I have to make adjustments to video and its pretty messed up great sound. Can not get picture to focus enough to see it clearly

[sylvania]

I have got the picture on a test CRT, seems the picture not very bright.Have I got used to new LCD tv's and forgot how much dimmer pictures were on old B&W tv?

[Electronic M]

I don't have a feel for the relative brightness, but the max available brightness depends on the amount of HV you have and the CRT emission capabilities (ie how good the CRT tests), and how well the circuits driving the neck are working (and position of the ion trap if present). On most sets made after 1952 you should be able to watch the set in a sun lit room (pix may not be optimal in this case) as long as the sun is not directly shining or reflecting onto the screen....Less brightness than that indicates significant miss-adjustment, wear, or correctable defects.

It looks like you have very weak contrast. Is the contrast control maxed and have you adjusted the AGC control properly?

[sylvania]

I have the contrast turned way down, it washes out the pic if I go higher. The AGC pot does not seem to be working because when I adjust it I see no difference,

[Electronic M]

If you crank the AGC from min to max you should go from blank raster to weak contrast to strong contrast to wildly excessive contrast/loss of synch to negative video and or useless chaos on screen. If you don't have that then the AGC circuit is not properly functioning.

More contrast should not wash out the picture, less should. More should make the darks darker relative to the brights areas. The brightness control should normally be able to washout the image when maxed out.

Have you ever put that test CRT you're using through a proper CRT test using a CRT tester? If it is in bad shape it could be contributing to things.

[Username1]

Originally Posted by sylvania

I have the contrast turned way down, it washes out the pic if I go higher. The AGC pot does not seem to be working because when I adjust it I see no difference,

Hi !

Before going into any further guessing about a possible cause of your problem.....
I think we should clarify what you are seeing..... You say you have the
contrast turned way down, (to the left) but to turn it up, washes out the
picture.... To me washing out the picture means almost no contrast, as if
the picture were almost all gray, no sharp black and white....

This action is almost the exact opposite of what we expect when turning
either the contrast knob to the right (up) or turning the AGC knob up.....

Turning either of those knobs should produce an almost "overloaded"
picture, too much light for the bright areas, and too much dark for the
dark areas.....

When you turn the contrast "up" would you still say that the picture gets
even less contrast...?

.

[sylvania]

What I am seeing is if I turn contrast up (making it brighter) it washes out picture.If I darken it (less bright) I can see it clearer. The AGC does not seem to effect picture adjusting to right or left. The test CRT was supposed to be new old stock and I tested it and it showed good emission. I do think the AGC circuit my not be functioning properly very little response from adjustment on back of chassis.

[Username1]

Ok, well the AGC is a kind of Automatic signal amplifier adjustment, so with
no response from that control, you most likely have - or possibly have some
signal loss along the IF section..... But before we go there, what is the quality
of the sound....? Good full tone, loud, or is it also kind of washed out, and
have some noise in it.....?

First, you should leave the set on, and locate the tubes in the tuner, and IF section
and first, second Video IF and wiggle these tubes in their sockets, tap with a wood
stick the circuit board, or chassis near these tubes, and look for any noise, or
jumping in the picture..... Possibly a momentary better picture....

.

[Electronic M]

You sure you don't have the brightness and contrast controls mixed up?

Contrast should not change the average brightness.

[sylvania]

I could of done that, I am using sound control pot to make it brighter which washes out most of pic i can still see it but very dim so I turn brightness all the way down. Contrast adjustment helps a little.Definitely AGC not responding I can turn it all the way right or left just get a little flicker at end of travel of pot. I have very good sound quality. Possible AGC
pot is bad it feels like it has no resistance when turned.

[Electronic M]

Originally Posted by sylvania

I could of done that, I am using sound control pot to make it brighter which washes out most of pic i can still see it but very dim so I turn brightness all the way down. Contrast adjustment helps a little.Definitely AGC not responding I can turn it all the way right or left just get a little flicker at end of travel of pot. I have very good sound quality. Possible AGC
pot is bad it feels like it has no resistance when turned.

Is the 'sound control' still wired as a sound control or has it been substituted in another circuit?

If it is still wired in to only the sound system, then that behavior is both a significant problem and a clue......A large number of sets used what IIRC is referred to as stacked B+. Basically they connect the audio output plate to one of the highest B+ rails and use the tube as both a power amplifier and a power supply voltage divider resistor. The tube draws a fixed range of current (and variations within that get filtered out) and has a fixed plate and transformer impedance so it acts like a resistor. A lower B+ rail likely is supplied by the cathode of the audio output tube, and that rail likely feeds the video output tube, brightness control, and possibly the AGC circuit. If the audio output and driver circuits are not working just right then it can upset the cathode output voltage of the stacked B+ and disturb the circuits driven by the lower voltage rail it supplies....Troubles in the circuits driven may also cause problems. IMHO before you go after the AGC and brightness/contrast issues it would be wise to fix the 'sound control'/picture interaction (assuming the assumption made in the beginning of this paragraph are correct).

[Username1]

What he said....... Yah!

.

[sylvania]

Thanks

[sylvania]

I guess it was cheaper to wire the sets that way out the door more production at lower cost.So it would be best to supply a different path to supply voltage to that section.I could understand what a load is being put on that tube.

[Electronic M]

Stacked B+ ain't a bad thing and worked well (otherwise it would have been a short lived design), but it is a bit more involved to trouble shoot than a traditional resistor based B+ divider.