random color loss

Hello. I have a Toshiba model CX2667 made oct 1987. The picture randomly loses all blue color. I tried opening the case and cleaning out any dust and it seemed to work for a while but then started to lose color again. Then I tried un-plugging and cleaning all the wire connections. Again it worked for a few days, then started to lose color again. I understand that the cause may be a loose connection that looses contact with temperature change, but I do not know where, or how to fix it. Can anyone help me find a permanent solution to this color loss?

 

Good Morning, Marauder Moe:

You've done well to get 12 years of service from the unit. From the indications you described, it is very possible that the set is suffering from a bad solder connection on the circuit board. Also, it would not be out of the question to find that the blue gun in the picture tube is bad.

Let's look for the loose solder connection first. Since you have had the back off the set before, this is nothing new for you. So, do this:

1) Remove the back of the set again. Plug it in, and turn it on with the back removed. Go have a cup of coffee for about 30 minutes while things warm up.

2) If you have a large mirror available (or even a smaller one), prop it up in front of the set on a chair so you can see the screen from the back of the set.

3) Use a non-conductive wand of plastic or wood and tap around the circuit board looking for a sensitive spot. This is the spot that will cause the color to appear and disappear as you tap on it. Mark this location physically.
NOTE: A WORD OF CAUTION HERE. SOME FOLKS TRY THIS WITH A PENCIL. PENCIL LEAD IS CONDUCTIVE. NOT A GOOD CHOICE!
NOTE: STAY AWAY FROM THE NECK OF THE PICTURE TUBE. IT IS VERY FRAGILE.

The area of concern is usually located directly under the picture tube if the circuit board is parallel to the floor.
This is normally where your color amplifiers are located.

4)Unplug the set and let it sit for an hour to let voltages
drain off from the power supplies.

5) Access the solder side of the circuit board. Use a low heat soldering pencil (not a soldering gun) and a little 60-40 electronic solder to just touch and remelt the solder connections. Solder in and around the physical area you located.

You may not actually see the loose connection. But heating and remelting the solder connections is usually sufficient to restore the bad joint.

Use solder sparingly to avoid blobs and creating short circuits from one solder trace to another.

Repeat this process until you locate the bad joint. Once it is corrected, tapping with the wand will produce no changes in the picture.

Put the back on the set and return it to normal operation.

Come back and let me know if this worked out for you.

Smokey

 

Actually, I bought it at a yard sale for $20 a bit over a year ago, but only just now decided to make a serious effort at repairing it.

Anyway, I took off the back and let it warm up. But after 3 hours there was still no color loss. Then, I put the back back on, but left the screws out. After about 20 minutes it got nice ant hot inside the case and the picture lost blue color. However, as soon as I opened the back to probe the circuit board the color returned. I tried letting it warm up with the case on a few more times, but each time I take the case off it starts working again. So it seems I cant test for any bad solder joints because at room temperature whatever connection that is at fault conducts.

Perhaps what I should do is saw a hole in the case and install a computer fan or something. Anymore suggestions?

 

Touch up all the solder joints in the CRT driver/color decoder. Loss of blue does kinda look like lost color
(I know, 90% of the time, our TV has no blue, we just tolerate it.)

 

Gary isn't too far off in his answer.

Obviously, the problem is heat related. When the set heats up, the circuit boards flex a bit and cause the poor connection to occur. Conversely, the poor connection will remain in any condition, either cool or warm.

Change your tactic to probe the board to make the color disappear when you have the back off. If you probe the right component or the right spot, you will simulate the heated condition and the color will disappear.

Same operation only in reverse.

Smokey

 

Ok, I've been poking for about an hour, tried just about every component in the whole set. I can't seem to get it to lose blue color. Should I just go ahead and retouch certain solder connections? Are the color drivers on the main board, or that smaller board attached to the tube?

 

Good Afternoon, M+ Joe:

Yes, go ahead and resolder the areas of electronics directly below the neck of the picture tube. Reason? Sometimes we run into "layered" electronics where two circuit boards are married together. A cold solder joint is hard to find in this situation.

Question: Have you tapped the neck of the picture tube with the back off? There is a possibility that the picture tube may be at fault. If tapping the neck of the picture tube restores or removes the color, there is a problem we haven't discussed. Remember, the neck of the picture tube is fragile. But taps on the side of the tube and at the end of the tube will show results without tube implosion.

Be careful.

Smokey

 

I'm a bit confused about what you mean by "directly below the neck of the picture tube."

This TV is a 25" set. It has two circuit boards, one on the bottom of the set, paralell to the floor, and another smaller one stuck to the back of the picture tube, paralell to the screen. Neither of these circuit boards touch, they are connected by a bunch of wires. The smaller board has the knobs to adjust the RGB drive/bias as well. Should I try fix solder joints on the small board attached to the tube? Or parts of the larger one on the bottom?

Also I tried tapping the tube itself, but that also failed to produce a color loss. Then I tried looking at the tube right after opening the case slightly, even though the picture had no blue color, I could still see all 3 filaments glowing. I dont think that its a problem with the blue gun fortunatly.

 

Good Evening, Marauder Moe:

I would advise that you concentrate on the board that is parallel to the floor. Usually the stuff on the picture tube deals with convergence and such. Color drivers and bad solder connections are associated with the bigger board.

Smokey

 

Most recent TVs I've seen have the driver transistors
on the board attached to the CRT neck.

 

Sorry I haven't posted any updates but I've been busy and haven't been able to work at the TV much until recently.

Anyway, I have tracked the problem to something on the larger main-board, and apparantly heat isnt as important as I thought as to creating the color loss. With the set open and with the back off the color returns since the main board bends downward without the case to support it, but if left running for a while with the back off the color fails again. I tried again probing with a plastic rod, but so far I havent had any success with that method at all. I could go on re-soldering a bunch of connections on the board, but half of it is covered by the front casing of the set and I would have to remove the whole circuit-board to get to it. Any suggestions on what to try?

 

No, we've pretty much covered the topic and I am still of the mind-set that there is a bad solder connection or ground on the affected board.

If you feel uncomfortable removing the board to access the bottom solder connections, then by all means leave it to a professional.

Smokey