TV set won't stay on one channel

[Matt]

Hi Folks,

My TV works fine except that it won't stay on the channel it's set to.
It keeps cycling through the channels as if the channel-up button is
being pressed. The problem is not in the remote, since it happens when
the batteries are removed from the remote and the remote is put in a
different room.

The model is Panasonic CT-20G21, a 21" box from 1996.

I have little test equipment beyond a multimeter. I am able to solder,
and I have some understanding of electronics, but no experience working
on TVs.

Please advise.

 

[Meat Plow]

Hi Folks,

My TV works fine except that it won't stay on the channel it's set to.
It keeps cycling through the channels as if the channel-up button is
being pressed. The problem is not in the remote, since it happens when
the batteries are removed from the remote and the remote is put in a
different room.

The model is Panasonic CT-20G21, a 21" box from 1996.

I have little test equipment beyond a multimeter. I am able to solder,
and I have some understanding of electronics, but no experience working
on TVs.

Please advise.

Cover the IR window on the tv with black tape and advise.

 

[none]

Hi Folks,

My TV works fine except that it won't stay on the channel it's set to.
It keeps cycling through the channels as if the channel-up button is
being pressed. The problem is not in the remote, since it happens when
the batteries are removed from the remote and the remote is put in a
different room.

The model is Panasonic CT-20G21, a 21" box from 1996.

I have little test equipment beyond a multimeter. I am able to solder,
and I have some understanding of electronics, but no experience working
on TVs.

Please advise.

Just a wild guess: Check the channel-up and channel-down buttons on your
TV. They should "click" when pressed. If the button is stuck down, or
the switch below the button is damaged, you probably won't feel that
"click", or it might have a "mushy" feel.

 

[Matt]

Cover the IR window on the tv with black tape and advise.

Okay, tried that. No difference.

 

[Matt]

Just a wild guess: Check the channel-up and channel-down buttons on your
TV. They should "click" when pressed. If the button is stuck down, or
the switch below the button is damaged, you probably won't feel that
"click", or it might have a "mushy" feel.

The buttons are okay. I suspect an electronic defect in the tuner.

 

[Meat Plow]

Okay, tried that. No difference.

Does the set have an OSD and do the channel numbers change when it changes
channels on its own?

If they do not change it /could/ be a problem with the tuner but no
guaranty.

I'd look more into the device(s) that decode the IR
and or those that interface the buttons to the sets logic control if the
OSD changes channels. Without a schematic/service literature this may not
be something that you can perform due to your limited knowledge.

 

[clifto]

Just a wild guess: Check the channel-up and channel-down buttons on your
TV. They should "click" when pressed. If the button is stuck down, or
the switch below the button is damaged, you probably won't feel that
"click", or it might have a "mushy" feel.

As I recall, years back we had a KV25XBR with a shorted channel switch
that felt just fine mechanically.

 

[William Sommerwerck]

This could be caused by what I call "MOSFET lockup". (I've seen things like
this in other products.)

With the set turned on, pull out the power plug and let the set sit for
several minutes. Then see what happens.

 

[JANA]

Check the channel up and down switches on the front panel in case one of
them is making contact. It can also be a fault in the uPC.

--

JANA
_____

Cover the IR window on the tv with black tape and advise.

Okay, tried that. No difference.

 

[none]

As I recall, years back we had a KV25XBR with a shorted channel switch
that felt just fine mechanically.

Such things do occasionally happen. And that is why I used the term:
"Probably"...

 

[b]

Hi Folks,

My TV works fine except that it won't stay on the channel it's set to.
It keeps cycling through the channels as if the channel-up button is
being pressed. The problem is not in the remote, since it happens when
the batteries are removed from the remote and the remote is put in a
different room.

The model is Panasonic CT-20G21, a 21" box from 1996.

I have little test equipment beyond a multimeter. I am able to solder,
and I have some understanding of electronics, but no experience working
on TVs.

Please advise.

let's start eliminating the easy stuff. if the IR sensor is covered
and the set keeps changing, remove the back, slide out the pcb a few
inches, stay away and power up.Observe. sometimes the plastic keys on
the set deform and press the microswitches. if this does not cure it,
desolder the microswitch for ch+ and see what happens.

-B

 

[Ken G.]

If someone sprayed something on the screen to clean it , or kids with
something .. it could run down inside and drip on those buttons . Take
the board out and inspect .
Take care not to get shocked .