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

[LowHeadRoom]

Hello all,

I am brand new to this but have been researching and getting interested in Vintage TV's. I have been working with old Tube equipment for the past year or so, so I guess I saw Vintage TV's as the next logical step.

I bought a 1973 Zenith B&W set. I really like the aesthetic.

https://photos.app.goo.gl/HaZFe1prSptk9WZy5



It will not, however, display any expected output from either my RF modulator OR my vintage game systems.

I have tried various adapters to rule out the possibility that the RF modulator or adapters or consoles are bad.

I am able to get audio from both the RF modulator AND any vintage game console I attach to the VHF input. I can hear the vintage game music and the audio from my RF modulator!

The only thing that seems to appear is a greyish/white screen when I turn either the RF modulator on OR attach a gaming console. The snow goes away and is replaced by the grey/white screen:

https://photos.app.goo.gl/WH7BQZ8ULTh5VkGw6


Other details:

1. It comes on and has a very bright snowy image
2. The vert hold works, the horiz hold works, the agc works, etc
3. The audio works
4. The channel selector works and changes between channels
5. The fine tuning seems to work
6. Brightness and contrast seem to work fine


I realize that TV sets are all unique and incredibly complex.

I went and bought the Sams' Photofact on this particular set:

https://drive.google.com/file/d/1QDH...ew?usp=sharing

I was wondering if any of you with experience have run into this type of problem before. I was thinking I would start testing capacitors and transistors near the video amp and video ouptut, but would like any advice you can offer.

Thanks for taking the time to read this!

[nasadowsk]

Start off safely - you MUST use an isolation transformer with this set.

Once you're isolated - check the power supply voltages BE CAREFUL! there's some pretty high ones near the horizontal section. If you have a scope, check the ripple. It should be pretty low. I've seen sets with marginal power supplies act 'dead', and still show a picture. If you get a hum and no 'static' noise, it's probably in the power supply.

Your set has a good number of transistors in it - so most capacitors or resistors that need replacement will be easier to source.

Also, once you've discharged the CRT, pull ll the tubes out carefully, and clean them. The high voltage rectifier and horizonal output in particular can get sensitive to junk in there.

[old_tv_nut]

What is the picture supposed to be when you get that gray screen with spots? Is there any hint of the picture?

Wondering if the fine tuning is way off and/or the tuner is on the wrong channel.

Is there an external AGC control? If there is, try turning it both ways.

Ditto on safety - before you open it up, get an isolation transformer.

[Electronic M]

I think the video detector CR101 may be bad. If it shorted you would still get audio, possibly slightly weak, and a blank raster with a signal, and snow with no signal. I've seen this problem on older sets before.

If the diode opened and there was some capacitive coupling for the signal the symptoms would be the same, but audio would be MUCH weaker.

[zeno]

Easy first.
The fine tune can do it. Keep in mind if the TV is on ch3 the range on
most sets will be ch2- ch4. The FT is multi turn.
If the little PCB has plug in transistors pull & clean the sockets.
The little electrolytics are known to go & cause AGC problems.
IIRC this set has 2 AGC adj. AGC & AGC delay.
BUT symptoms say its after the 1st video amp.

73 Zeno
LFOD !

[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,

[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.

[AlanInSitges]

Congrats on your first fix, easy though it was!

You'll probably get a much better deal on old sets like this if you look at garage and estate sales, or CL or OfferUp rather than FB.

[Tube TV]

Glad you got it sorted out. That's a nice little Zenith.
Welcome to Videokarma.