Question about Likom ViewMate LP995 (White) 19 in.CRT Conventional Monitor

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Arcing around neck board

Likom ViewMate LP995 19 in. CRT several years old. Powers up OK, but hear "snap" and video drops until recharged, repeats. Took off cover, flash of arcing around neck board, not sure, but looks like might be arcing inside CRT tube's clear neck. Is it an over-voltage problem, or is the CRT tube bad, or...?? I have a HV probe good to 40kV, would it help to measure a voltage somewhere?

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Re: Arcing around neck board

There is a crack in the neck of the tube and the vacuum inside is lost. There is already air inside the tube that is why the sparks are going to the pins. The TV must have been dropped or someone accidentally hit the neck of the tube

Posted on May 31, 2007

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Re: Arcing around neck board

24 seven is probably dead on. CRT is likely gassy. While an overvoltage condition is possible, the overvoltage sense circuit should shut the Hi-volt circuit down, and you'd have no Hi-v or display at all. With the availability of good used monitors & flat screens, repair is not economical at all.

Posted on May 30, 2007

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Re: Arcing around neck board

Sounds like bad crt if you saw arc inside economical fix-replace the monitor

Posted on May 30, 2007

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Monitor display is fuzzy and pixelated


Focus voltage on the CRT is usually in the range of 2-8 kV DC and should be controllable over a fairly wide range by the focus pot - usually located on the flyback or a little panel in its vicinity:

If adjusting the pot results in a position of acceptable focus, you may be done. It is not unusual for the focus setting to drift a over time.

If the setting is already as good as possible but not really good enough, the CRT may be tired. Alternatively, the filament voltage may be too low. Check for bad connections in the filament circuit.

If the optimal setting is out of range of the focus pot, the problem is likely leakage in the focus divider in the flyback or one of the components on the CRT neck board.

Also see the sections: "Focus adjustment" and "Focus drifts with warmup".

The focus wire usually comes from the flyback or if the general area or from a terminal on a voltage multiplier module in some cases. It is usually a wire by itself going to the little board on the neck of the CRT.

If a sparkgap (a little 2 terminal device with a 1/8" gap in the middle) is arcing with power on, then the resistive divider has shorted inside the flyback, focus board, or HV multiplier - whatever you TV has - and the this unit will need to be replaced. Ditto if the SCREEN control affects focus and/or vice-versa.

Using a suitable high voltage meter (range at least 10 kVDC, 1000 M ohm or greater input impedance), you should be able to measure it connected and disconnected. The ground return will be the outside coating of the CRT which may or may not be the same as the metal chassis parts. If the voltage is very low (less than 2 kV) or too high and the pot has little effect:

When measured right off of the source disconnected from the CRT neck board, then the problem is probably in the focus network in the flyback (or wherever it originates). Sometimes these can be disassembled and cleaned or repaired but usually requires replacement of the entire flyback or voltage multiplier. Note: you may need to add a HV (10 kV) capacitor between the focus wire and DAG ground to provide filtering so you get a DC level for your meter.

When measured with the focus wire attached to the CRT neck board with the CRT connected but reasonable with the CRT unplugged, there is probably a short between the focus and another electrode inside the CRT. See the section: Rescuing a shorted CRT.

When measured with the focus wire attached to the CRT neck board with the CRT unplugged, there is likely a component on the CRT neck board that is leaky or breaking down. Also, check for decayed (tan or brown) glue which may turn leaky with age.
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There are a couple of possibilities and none of them lend themselves to DIY repair. It can come from arcing in a component trying to fail; you may be able to hear this (if present) by listening closely near the rear of the housing, preferably in a quiet room and near the cooling slits.
Another potential source is what is called a 'cold solder joint' that may look and even measure OK until a little more current is flowing through it.
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You will see a fairly fat cable leading from a black lump on the circuit board; this is a transformer responsible for producing the 20-25,000 volts. If the unit has been off overnight, you can clean enthusiastically around the rubber cap you see on the side or top of the picture tube; this is a good leakage point for high voltage.   
If you do this and it didn't help, you will have to find a surviving CRT monitor tech to take it from there.   

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