Discharging a television tube (and other parts) prior to repairing the set

Correction to (1) above: I meant to say, "... shorter duration on warmer days, longer duration on cooler days..."

The vertical amp has nothing to do with the CRT. Why would you want to discharge the CRT to work in the vertical amp section?

Your problem is most likely one or more capacitors going bad in the vertical deflection amp section.

From the sound of your question, you should be giving the set out for service. There are also serious safety issues when working on a TV set.

--

JANA _____

Here are the details of my situation (long!):

I have a Zenith 27" television set circa 1995 that works perfectly, for the most part. The only problem is that, as it's warming up following power-on, it goes through three distinct phases:

1) A normal picture, usually for 2 or 3 minutes (shorter duration on warmer days, longer duration on warmer days)

2) The lower half of the picture collapses to a center horizontal line, and the top half remains mostly intact but is pulled downward a few inches toward that center horizontal line (this usually lasts 5 to

10 minutes, depending on room temperature)

3) The picture twitches a few times from normal to collapsed, and finally settles in at a perfect picture, remaining there until I turn off the set.

I removed the back cover from the set to see what's going wrong. I'm guessing there's a cold solder joint or other 'mechanical' problem with the Vertical amp section on the main circuit board. Sure enough, when I very gently touch (using a long wooden chopstick!) the S-shaped heatsink that holds the Vertical amp IC, I can make the problem come and go at will.

So now I'd like to resolder all the pins on the Vertical IC, and I'd like to do it safely.

I'm good with digital electronics, but a total newbie at television repair. I understand that high-voltage safety precautions are needed when repairing a TV, so I'm hoping to get some help in this area.

Can anyone lend me some tips and advice here? Any help is greatly appreciated!

Dave

Gotta disagree with you on that one. What part of making the problem come and go at will, by poking the V. output chip's heatsink with a chopstick, indicates a cap going bad ? Seems to perfectly back up the OP's diagnosis of a bad joint at the chip's pins, which is a common enough failure on every CRT TV and monitor made in the last 20 years.

As to the original question, nothing really should need 'discharging' prior to working on the set. There are no issues with the switch mode power supply's main filter cap staying charged in this case, as the power supply is working ok. Whether or not the tube stays charged, should not be an issue, unless either the high voltage poppy connector, or any ground wires to the tube's 'dag or rimband have to be disconnected to remove the chassis to get at the underside, in which case discharging the tube would not be a bad idea. In theory, this should be done with a string of several 1 meg resistors, but most engineers will tell you if pressed, that they do it with a couple of screwdrivers between the cavity connector and the 'dag ...

As to remaking the joints on the chip, if you have desoldering equipment available ( solder wick braid or a de-soldering pump ) you might want to take the solder off of all the joints, and remove the IC completely, so that you can clean off the old heatsink paste, which has probably gone chalky and lost some of its effectiveness, and put new on it, before refitting and re-soldering.

Before re-starting the set, make absolutely sure that you give the chassis a thorough inspection to make sure that you have not left anything disconnected, or that nothing else has become disconnected as you moved the chassis around to work on it.

Arfa

John :-#)#

--
  (Please post followups or tech enquires to the newsgroup)  John's 
Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9      Call 
(604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)            
        www.flippers.com              "Old pinballers never die, they 
just flip out."

I doubt you'd really need to discharge the picture tube just to re-solder the pins on the vertical deflection I.C. But if you have to remove the anode cap at the end of the flyback's red wire in order to get enough length to get the chassis out so you can work on it, then read the procedures others have posted. If you can get to what you need without having to remove that big red lead, then leave it alone.

Ideally, when power is removed from a TV set, everything should bleed down to a safe level pretty quickly. Then again, there's no guarantee, so I'd be suspicious of all of the big electrolytic capacitors until I've satisfied myself with a voltmeter that they're discharged. That's easy to do. If, however, you find a big cap that still has a healthy charge on it even with the power cord removed, then discharge it safely by using a power resistor of about 470 ohms, at least 1 watt, to bleed it dry. Just temporarily short the resistor across the cap leads for a few seconds. Then remove the resistor and check what voltage is left on the cap with the voltmeter. Do this for all of the physically large electrolytic capacitors, as they are in the main power regulation circuitry. The vast majority of all the other electrolytics are physically small and don't have nearly as high voltages appearing across them.

If the solder joints on the vertical deflection chip have become intermittent over time, then I suggest replacing the electrolytic capacitors close to that chip while you're at it. Those caps fail somewhat faster than other caps in a TV set, and they cause weird things such as picture shrinking, "foldover," and other things related to vertical deflection. They may have flaky solder joints on them, too. They're cheap.

Also take a bright light and inspect the solder joints in the immediate area of the vert IC.