1973 Zenith B&W Set - Good Audio, Plenty of Snow, No video through VHF

[Electronic M]

Originally Posted by LowHeadRoom

First off,

Thank you for the great advice so far!

The screen is supposed to display video from a vintage game console. I was using NES Tetris and can get full audio from the console.

I have switched between channels 2, 3, and 4 and can get audio through those channels but nothing that even slightly resembles the Tetris game.

I have swept through the fine tuning, turning it left and right for a full sweep. No combination of channel selection and using the fine tuning brings up the Tetris game display.

I have also swept through the AGC control back and forth but do not get anything resembling the Tetris game.

Here are some videos to show how the TV is functioning. I am using the RF modulator this time instead of the NES. It has the same symptoms:

Contrast, fine tuning, vert hold etc:

https://photos.app.goo.gl/Sk8SwCkAQ4RTVSng7

AGC:

https://photos.app.goo.gl/vz4ZQT6KNu5iHCBH8

Snow:

https://photos.app.goo.gl/wtbxbau9DM8WFckZ7

THANKS AGAIN FOR ALL OF YOUR HELP!!

I am thinking it is a bad video detector diode as well.


Thank you,

In your first video the TV had good video on screen (those nearly horizontal bars), but the horizontal hold adjustment was WAY off. You need to duplicate how things were in that video and turn the horizontal hold coil 3 turns in either direction. Whichever direction reduces the number of horizontal bars is the correct direction, you'll need to keep turning it in the correct direction until you get a picture could easily be 5-10 turns. Some Zenith hold knobs have stops that limit rotation to 1 turn...To bypass the stops pull knob ~1/4" out so it's pointer clears the stops...Push it back in once it's adjusted correctly.

I'm guessing you've never experienced a tube TV with the Horizontal hold way off before.

[old_tv_nut]

I think the snow indicates the video detector is OK.

I agree with Electronic M that the horizontal frequency is off. If this is due to a bad component or bad solder connection, you may not be able to get it correct with the adjustment. Once you do get the horizontal frequency correct, you will be able to see if the video is actually OK.

[LowHeadRoom]

Originally Posted by Electronic M

In your first video the TV had good video on screen (those nearly horizontal bars), but the horizontal hold adjustment was WAY off. You need to duplicate how things were in that video and turn the horizontal hold coil 3 turns in either direction. Whichever direction reduces the number of horizontal bars is the correct direction, you'll need to keep turning it in the correct direction until you get a picture could easily be 5-10 turns. Some Zenith hold knobs have stops that limit rotation to 1 turn...To bypass the stops pull knob ~1/4" out so it's pointer clears the stops...Push it back in once it's adjusted correctly.

I'm guessing you've never experienced a tube TV with the Horizontal hold way off before.

Thanks so much for this !

You guessed correctly. This is the first tube set I've ever owned.

I tried pulling the nob out 1/4 inches but it's locked at one turn. I guess this means that the cover needs to come off for the adjustment to occur.

Is the stop easily bypassed with the back cover off of the television?

Thanks in advance.

[LowHeadRoom]

THANK YOU so much for helping me with this simple fix.

I carefully took the cover off, removed the knob, put the cover back on, and adjusted it successfully (almost).

Here's a video:

https://photos.app.goo.gl/J2RWjAgTwUAXYFRb9

I cannot get the vertical to lock completely.

Horizontal seems to behave ok.

Any advice on the vertical?

Edit: After tweaking the AGC, the vertical locked!

Thanks again!

[zeno]

Good example of a COMMON problem in the day. Kids would force the
H hold past the stop. Then they couldnt get it back.
The vert jitters were from too much gain, thats why AGC.

BTW you did a great job describing the problem. Wish they were all as well done ! Now go find a few more to work on.

73 Zeno
LFOD !

[LowHeadRoom]

Thank you! I will definitely be looking on FB market for more of these incredible machines.

Old tech troubleshooting kind of represents the ultimate mental challenge for me.

I look forward to working on more.

This community, and the community on the Antique Radio Forums have been extremely helpful.

I really appreciate it!

[Notimetolooz]

Having a good understanding of radio repair is a very good start to understanding TV.
It be a good idea to download a book or two about TV repair.
It seems that many people are not aware of the huge amount of knowledge available online. Ask and download and you can get a library that would have cost thousands of dollars.
Look for "How to Troubleshoot a TV Receiver" by J. Richard Johnson, about half way down the page, blue cover. This edition was published in 1953 so it will not contain info about transistor circuits but the basic function of a B/W TV is the same. Some test equipment is different also.
https://worldradiohistory.com/BOOKSH...helf_RIder.htm

Finding info about solid state radio and TV circuits is a bit harder to find. In later years basic practical solid state troubleshooting was published in magazines. Magazines are available also but the info is spread out among them.