Why is the CRT taking longer to light up?
 

I have been enjoying my 1960 Magnavox set for a while now,
but recently, when I turn it on, the CRT takes a few minutes, sometimes longer to light up...the sound is there right away though.
Any simple fix?

Is the CRT taking longer, or do you mean the display takes a long time? If the heater isn't drawing enough current to get its temperature up, that's one thing. If you are referring to the delay before a picture appears, that's entirely different.

Could be a number of things without getting some test readings, but first thing that comes to mind would be a weakening CRT.

When it lights up, is it sudden, or does it slowly come up over the now increasingly gradual period of time?

EDIT: Basically echoing Bob's input

What I mean is the display is taking a longer time than normal.
No light at all on the screen... then it pops on.
It is sudden, not gradual.

There are two things that can cause that, either the CRT is reaching normal emission level slower(which is either the CRT going bad or the electron gun drive circuits), or the HV supply is taking longer to come up.

If you have a hv probe, you could easily test the hv as the set comes on....

We had a similar looking set when I was a kid, The picture got really light in contrast, like the one you posted.... Dad changed the picture tube, no fix, changed tubes, nothing, then after a while we took it apart, made a storage cabinet out of it, I was not old enough to know what to do to fix it at that time. Today of course I would go to the video amplifier area and get cracking...

That set has a vertical chassis with a hole around the picture tube, little black box lower right I think for the flyback.... With a overhead pull light chain attached to the cover....

Sounds like a bad connection of some sort; dirty tube socket, or even a dirty brightness control. I'd try tapping around on the chassis, and see if it makes the picture pop on & off. IIRC, that year of Magnavox (higher end models) also has an automatic room light sensor, which can cause issues. I don't remember what Magnavox called it, but it was some catchy name indicating auto brightness. Try shutting that off (switch on the back, usually near the top with the vertical hold, etc).

Charles

Look at the chassis with the back removed, in a darkened area. Take notice if the horizontal output and the damper tube is lighting properly, as well as the CRT. Also, make sure the horizontal output isn't red-plating, indicating the horizontal oscillator, isn't starting up, as fast as it should.

I believe that room light sensor was called a Lumisponder.

Make sure that the C.R.T. plug has not loosened and causing the tube's heater not to come on perhaps until the warmth of the other tubes or movement, shaking or something else completes the circuit. This is very common and normally easy to fix.

The "plug" is the plastic end of the C.R.T. with the pins sticking out. The socket is the hard wired piece that connects to it. Just see if it (the plug) feels loose, but be gentle. If it is loose and the set is powered on, then you can move it slightly and see the darkness turn to light when you hit the right spot.

If this is your problem then we can go into detail on possible solutions. Good luck!

Find out if its the HV or video first.
When on but "acting up" put the back of your hand near
the face of the CRT. If you have HV you will feel the static pull
on your hairs. Try it when working also to get the feel of it.
Its Gods built in free HV probe !!
If its a video or CRT problem you will always feel the pull
after the normal warmup time.
Try it & get back to us.
Remember TV repair is "divide & conquer".

73 Zeno:smoke:

Thanks for all the replies,
after tapping on a few things in the back,
the screen now comes up right away...
Not sure what it was, maybe the CRT plug as Tubejunke thought,
though it was on tight.
Well, glad it's fixed, Thanks for all the help!

Magnavox300

This may have been one of the most valuable lessons that you will learn in this hobby, and frankly I think you learned it the easiest way; with a c.r.t. I say this because with a c.r.t. you get to see the fix on the screen. Tubes have a pretty big design flaw being pins and sockets made of varying qualities of metal, most of which oxidizes over time.

Everything can appear to be "tight" and is in many cases like this where the plastic locating plug is the primary anchoring point. Meanwhile you have electronic connections often marginal at best or even open as would be the case here with the final possibility of voltage dropped to a point so low as to not be able to rated potential.

With any tube equipment it is essential to get a cleaner for electronic parts and go through all tubes cleaning the tubes and the sockets. They used to make cleaners especially for this along with tuner contact cleaner which worked just as well like DeOxit and such. The trick is to take an old pocket knife or what have you and scrape the tube pins until they shine. It's not hard; just a few swipes and you can often cover more than one pin at a time. Spray the socket and insert, remove and reinsert the tube several times to help clean the socket. You will be much less haunted with misguided troubleshooting this way. Hope this helps! :thmbsp:

Thanks, good advise!
I think I'll take an hour and clean the tube pins, and sockets with Deoxit...
I remember once reading you shouldn't scratch the tube pins, since they are coated? I would rather go with your suggestion, and get them bright again and not worry about it, I'd rather have a good connection...
You said use a pocket knife or something and scrape them till they shine,
are you sure that's okay to do?

I had trouble with my Admiral bakelite console, in that the CRT would go dark from time to time. I cleaned the pins of the CRT some years ago, and this problem hasn't reappeared. :banana:

I have learned to clean the tube pins (and test tubes) first, before attempting any service. As Tubejunke points out, that can save embarrassing head-scratching later on.

http://antiqueradio.org/FirstStepsInRestoration.htm

Tube pins aren't so delicate that you need to fear cleaning them. I have brightened up many, many of them using fine sandpaper or a scraper, with no problem.

Phil Nelson
Phil's Old Radios
htt[://antiqueradio.org/index.html

Thanks, I will clean all the pins thoroughly, I am sure they need it!

I think my reply is a little late in the game and you have probably already done the right thing. Actually, I found it interesting that you stated that you read that the tube pins were somehow coated. I'm not saying that there is no truth to that; maybe some special applications or something, but I have never heard of any coating. If they did coat them with say some conductive anti-oxidant or something, then it didn't work. Gold would be the way to go, but counter cost effective. Silver is the best conductor, but it tarnishes which would give us what we have. The pocket knife (cheap) or whatever implement you use to me is one of your most valuable service tools; right up there with your V,O.M.

Also, I think the topic of pins being delicate came up and someone correctly put any fears there at ease. In fact, they are very rugged in my experience. Even the small pins on say a seven pin miniature can be bent and straightened a number of times without issue. Actually, I have never broken one. The most likely to break would be something like an octal, and they would break from the plastic base. C.R.T. pins have a bad habit of needing to be re-soldering once the epoxy (or whatever glue they used) gives out and the plug is left supported by the wires protruding from the glass.

Don't know if this relates to your set, however, sometimes crts develop an intermittent open cathode, where the picture blacks out, and if you tap the neck of the tube lines of the raster will flash intermittently with the tapping and sometime it cures the problem for a while. Only real cure is to replace the crt. Just don't hit the next of the tube too hard. In color crts the same thing can happen, only usually just one color is involved, R. B or G

I never saw a case of intermittent cathode in a color CRT (not saying it hasn't happened, though). But saw oodles of heater-cathode shorts that show up intermittently when tapping the neck. (H-K shorts kill the luma while leaving the chroma intact, since chroma goes in on the grids.)