Sweet little old ladies and TVs from attic

(Original request from rogerj@apex.com):

"A sweet little old lady has duped me into repairing her old GE 13" color TV. She wanted me fix a bad volume pot. "Oh it has such a good picture", she says.

Stupidly without even turning it on, (big mistake) I begin to open the set. After 15 to 20 min. of travail, I discover that a previous "repairman" has glued the case shut! (I wonder the sweet little old lady was in the habit of tweaking everything inside!:) --- Sam.)

Now with the set open, I turn it on and this picture is LOUSY. Bad color, and very poor convergence. But I don't know if I'm to blame for banging it around trying to open it up. Also, no horizontal or vertical hold. (fixed that with a few caps). This thing has probably been sitting around for a few years."

Well, you certainly did not kill the caps. Anything that sits for a few years - probably in a damp unheated attic - is suspect.

Did you find the adjustments on the yoke assembly tight? If so, you probably did not move anything very much either. She may remember the good picture it produced before being stuffed away in the attic.

(From: Roger.)

"Anyway after going through all the adjustments, the convergence at the sides is still bad and the horizontal size is a tad insufficient (and no adjustment available)."

It could be that the convergence (including pincushion) circuits are still faulty - not just misadjusted.

Other things that can effect horizontal size while still giving you a complete picture:

1. Voltage to horizontal output transistor low. Is there a voltage regulator in your set? The one I have has none. I assume your line voltage is ok.

2. Increased resistance or inductance of the yoke windings. For all you know, the yoke may have been replaced with the wrong part.

3. Yoke improperly positioned on tube neck.

4. Excessive high Voltage. This is usually not adjustable.

I bet the thing hasn't worked properly in 10 years! :)

Phantom spot or blob on CRT after set is shut off

Why is there a splotch of colored light at the center of the CRT after I kill power to my TV? Why does this not happen if the plug is pulled instead? It seems to last for hours (well maybe minutes at least).

A broad diffused glow (not a distinct spot in the middle of the screen) that lasts for a few seconds to minutes is called 'afterglow' and may be considered 'normal' for your model. The warm CRT cathodes continue to emit electrons due to the high voltage that is still present even though the signal circuits may have ceased to operate.

For more sharply defined spots there are two phenomena:

1. Thermal emission from a cathode that has not yet cooled off (and this could take several minutes) gives a more or less circular spot near the centre. It is actually 3 spots from the 3 cathodes, we at Philips call them 'Christmas balls'.

2. Field emission from sharp whiskers on any electron gun part gives a much sharper spot, sometimes with a moon-shaped halo around it. Even with the filament off, there may be some electron emission from these sharp points on the cold cathode(s) due to the strong high voltage (HV) electric fields in the electron gun. I do not know how likely this is or why this is so.

The shape of the spot is an inverted image of the shape of the emitting area(s) on the electron guns cathodes.

The visibility of both effects depends in the same way on the decay time of the high voltage (HV/EHT) on the anode.

When turned off with the remote or front panel button, you are not actually killing AC power but are probably switching off the deflection and signal circuits. This leaves the HV to decay over a few minutes or longer as it is drained by the current needed to feed the phantom spot or blob.

When you pull the plug, however, you are killing AC input and all the voltages decay together and in particular, the video signal may be present for long enough to keep the brightness (and beam current) up and drain the HV quickly. Whether this actually happens depends on many factors - often not dealt with by the designers of the set.

A proper design (who knows, yours may simply have been broken from day 1 or simply be typical of your model) would ensure that the HV is drained quickly or that the other bias voltages on the CRT are clamped to values that would blank the CRT once the set is off. If the problem developed suddenly, then this circuitry may have failed. On the other hand, if it has been gradually getting more pronounced, then the characteristics of the CRT or other circuitry may have changed with age.

In most sets it is left to chance whether the picture tube capacitance will be discharged by beam current at switch-off. It may simply be due to the behaviour of the video control IC when its supply voltage drops that causes the cathodes to be driven to white and this may not be formally specified by the manufacturer of the IC. Some of of the latest sets have an explicit circuit to discharge the EHT at shutdown.

As noted in the section: Safety guidelines, the HV charge on the CRT capacitance can be present for a long time. A service technician should be very aware of that before touching HV parts!

Interestingly, most sets for the Asian Pacific market have a bleeder resistor built in that will discharge the EHT without the need for a white flash at switch-off. These will in fact drive the beam to black at switch-off via a negative voltage to the CRT G1 electrode. The AP market is very sensitive to proper set behaviour, they don't like a white flash.

In short, it all depends on the demands of the particular market, the chance of the picture tube producing a spot/blob, and the mood of the designer.

So, it may not be worth doing anything to 'fix' this unless the splotch is so bright (more so than normal video and for an extended time) that CRT phosphor damage could result. This is usually not a problem with direct view TVs but would definitely be a concern with high intensity projection tubes.

On the other hand, your phantom blob may provide for some interesting conversation at your next party!