Advent TV

[Carmine]

The TV works great for the first hour or so and then the picture disappears but the sound stays on. I then could turn it off and then back on and the picture usually comes back on for a while. I don't have a service manual so I guess what I am asking for is help with troubleshooting, Service Manual and where can I find the part(s) to fix it.
Thanks!

 

[The_Penguin]

What kind of TV? LCD, Plasma?

If it's LCD it could just be the backlight circuit shutting down, either due to a bad lamp, or bad components in the inverter circuit.

 

[Carmine]

Unfortunately it has a CRT.

 

[The_Penguin]

Unfortunately it has a CRT.

OK that changes things a bit.
Before we go much further, what is your level of knowledge/experience with electronics in general, and CRT TVs in particular.

It may be a relatively easy fix, but quite likely in a very dangerous area of the TV.

 

[Carmine]

I played around working on Televisions years ago. I am an electronic technician now making Thermal Imaging Cameras. Worked on computers for years. I am a bit familiar with High voltage and most of the precautions that I should take to be safe.

 

[The_Penguin]

Ah good to know. Didn't want you poking around in areas that could radically alter your hairstyle
In my experience with CRT-based TVs (it's been a while now)
Loss of picture is often due to the high voltage disappearing.
An easy way to verify, I run the back of my hand across the front of the screen, and if I don't feel the static field, I assume the HV is off.

Because it's intermittent, it's probably not a bad horizontal output transistor, so I'd suspect a bad solder joint in the high voltage section. Have a close look at the flyback and horizontal sections for cracked joints.

Also, since it's intermittent, when the picture disappears, try tapping/flexing the PCB with a long non-conductive tool or wooden stick, to see if you can determine if it is a bad joint.

 

[(*steve*)]

Since the fault is initially caused by the set operating for some time, and considering that turning it off and back on allows it to recover for a short time, I would be looking for a fault with a thermal nature.

Is something getting a lot hotter than it should be.

You know, if you had a thermal imaging camera that would be a great diagnostic tool. But they're expensive and almost nobody has one just hanging around for them to use...

 

[The_Penguin]

Since the fault is initially caused by the set operating for some time, and considering that turning it off and back on allows it to recover for a short time, I would be looking for a fault with a thermal nature.
.

A good point, though mechanical (solder joint) issues can have thermal behaviour.

I had an Alinco mobile ham transceiver that had a very strange squelch threshold issue that behaved like a thermal issue. After much beating my head against the wall, I found a SMD resistor that wasn't properly soldered on one end, As it warmed up it made proper contact, but when cold it got flaky.

 

[(*steve*)]

A good point, though mechanical (solder joint) issues can have thermal behaviour.

True.

I'd be seeing if the fault had any mechanical sensitivity first, because that *may* be easier to track down. If it doesn't, it may be purely thermal.

A thermally sensitive solder joint is likely to be very mechanically sensitive as well.

 

[KrisBlueNZ]

Yes, and it could be a lot of things, not necessarily the line output stage. It could even be an IC in the signal processing chain.

To the OP, there are a few things you can check for easily.

Can you hear the squeal from the line stage when the set is running? If not, ask a child; they have better high-frequency hearing. When the picture disappears, is that squeal still present?

When the picture disappears, does it just suddenly go black? Or does it fade out? If so, does the image get larger or smaller as it fades out?

If you turn off the lights in the room and adjust the brightness, can you see any light at all from the screen?

Oh, and what's the model number?

 

[jcurrie]

you might try freeze mist if it is indeed a heat related problem.

jc