I have been trying to help an elderly friend of the family with her Zenith System 3 table top 13" set from the early 80s (1982?). It is a Space Command set with on screen display.
She would like to be able to have this set working because she worked for Zenith in Chicago for over 30 years, and therefore the stuff she has from Zenith has sentimental value to her.

The problem with this set is that sometimes it will not turn on. It you hit the top of it (think Onslow on Keeping Up Appearances...maybe I'm the only one who watches that show...), it will usually turn on. Other times, when you try to turn it off, the screen will go blank and the sound will cut off, but the electronics will stay on - the CRT filament is lit and you can hear the flyback running.
Twice I have tried to take the set apart and clean as many as the connections as possible, thinking this problem was due to a bad connection somewhere. This set is full of wires connecting the different boards.
This seems to make the set work well for about a month, then it goes back to the same problem again.

I know that problems like this are probably very difficult to solve, but I thought the fact that it does not always turn off completely might narrow it down and somebody could offer something specific to look at.
I'm sorry I don't have more pictures of the set or a model number, it is not here right now, but I will probably be visiting the owner over the weekend, and can bring it home to try to get it going again if anybody has any suggestions.

http://homepage.mac.com/brameika/ZenithSystem3V/chassis1.jpg

http://homepage.mac.com/brameika/ZenithSystem3V/chassis2.jpg

Thanks,
-- Max

I changed out a lot of those motherboards back when these were new!

I would suggest pulling the board out completely and going over it carefully looking for bad Solder joints.

Resolder the Flyback and any high watt resistors you can find, also the grounds.
Your problem is almost certain to be bad solder somewhere.

Two things:

1. A sticking on/off relay. I've seen the contacts stick and keep power on the main chassis. The tuner and LED readout would power on and off normally because it uses transistors for switching on and off. I've seen the problem many times on many different brands of TVs. The best solution is to replace the relay. If that's not possible, pop the cover off and clean the contacts.

2. Bad connections on the 9-160 power supply/horizontal sweep/HV module. As you can tell, this board has many heavy parts mounted on it. You need to pull the module and resolder everything on it.

Without a doubt, reflow/add solder to basically the power board(s) whichever it uses, can't tell. Most of the 9-181 and 9-160s have erratic issues due to the loose connections. Clean all the snap-in made in Mexico's too. That can always help.

However, on my 1979 25" system 3, i can't recall what board # it was, but it had the standby ckt transformer and zoom all in the same board under a metal cover. Well the power relay was screwed up so I cleaned the contacts with sandpaper and bent them ever so slightly and it brought er back.

I was thinking it may be a relay, since when it doesn't turn off all the way, I don't hear any relay click, but you do when it does turn off right. Would the relay be that white box in the second picture? As I recall, that was the only relay I saw in the set, but I don't have it now, so I can't be sure. The last time I had it I tried to clean as many of those connections as possible. I think I looked over the solder joints, but will more carefully the next time I see it.
I actually believe the flyback was replaced in this set at one time, there is a tag on the back from a repair shop and I noticed the soldering looked a little sloppier under there than the rest of the board, I will have to check that out.

Thanks for the help.

Yep, that's the relay. I'm not surprised that the flyback was replaced. Those were bad about arcing. They sounded like fireworks going off when it did, too. I've replaced a few of those flybacks; but, I always got one from a junk board.