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Old 12th Feb 2019, 7:07 pm   #1
AngelBryan
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Question Help fixing black & white mini CRT TV.

Hi. I got a mini black & white CRT TV but the display is broken and i don't know what could be the reason of it.

The issue seems to be on the TV itself since I already set an analog signal to the TV and still looks bad.

What could I do?

I also already used the V.hold controller on the back, without succes. Any help will be appreciated.

Here are some pictures detailing the problem:-

********************gallery/JfC0Nvt

And here is a video:
https://youtu.be/X-j_Mvut7zk
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Old 12th Feb 2019, 7:24 pm   #2
19Seventy7
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Default Re: Help fixing black & white mini CRT TV.

Hi angel, good to see that you took my advice (from YouTube)- being on here will give you a MUCH better chance of fixing it!

Goodluck!
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Old 12th Feb 2019, 9:53 pm   #3
Jac
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Default Re: Help fixing black & white mini CRT TV.

Could be some bad capacitors in the vertical deflection circuit.

Jac
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Old 12th Feb 2019, 10:19 pm   #4
Argus25
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Default Re: Help fixing black & white mini CRT TV.

There is a lot of information in the raster scan. Firstly the width looks normal, probably the power supply and its capacitors are fine.

Looking at the vertical scan the frequency is right or close, so likely the vertical oscillator is ok. The vertical oscillator produces a saw-tooth like voltage, that is amplified up by an amplifier circuit (not dissimilar to a small power audio amplifier) and that applies the voltage to the vertical deflection yoke coils. Looking at that vertical scan you can see how the lines are stretched apart at the top, and compressed together at the bottom and in fact at the bottom have a folded over look. Near the bottom is the part of the scan with the highest peak current.

So this means that the scan voltage & current applied to the vertical yoke coils is distorted. Most likely, this problem will either be caused, as noted already, by faulty or dried out electrolytic capacitors. There is often one directly in series with the vertical coils to block the amplifier applying a DC voltage to them. Or another dried out capacitor (with high ESR) in the vertical amplifier circuit itself. Also though, the vertical output stage transistor could be failing, but this TV might use a vertical output IC where the oscillator and IC are in one package.

In any case, to fix this you need at a minimum two things, the circuit diagram and an ESR tester to test the capacitors in the vertical scan stages. Also, sometimes a scope is needed if the problem is more elusive.
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Old 12th Feb 2019, 10:51 pm   #5
Argus25
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Default Re: Help fixing black & white mini CRT TV.

........also I could have mentioned, one way that sometimes works (unless the faulty capacitors are particularly leaky) is to get some electrolytic capacitors of about the same uF value and voltage rating and connect these in parallel with the existing capacitors in the vertical circuit, one by one (observing the correct polarity) to see if the fault clears. That is the kind of "trick" that can sometimes work, in the absence of proper test equipment.

The proper fault finding approach though, would be to go over the working vertical circuit with a scope and from the waveforms obtained, deduce the faulty component.That of course requires familiarity with oscilloscopes and knowledge of circuit theory.
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Old 13th Feb 2019, 12:30 am   #6
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Default Re: Help fixing black & white mini CRT TV.

A quick fix can be done by first looking up the data sheets for the suspect vertical amplifier chips until you find it.
They normally have a cooling fin or two and are not very close to the place where the high voltage cable comes from that supplies the picture tube.
Working from the scan coil cable google the chip numbers that have cooling fins until you find the data sheet with frame or vertical scan in the title.
Buy two or three capacitors of the values given in the example circuit shown in the data sheet.
Then carefully gain access to the track side of the board and change them identifying each one by following the tracks on the board.
If you do them one by one start with the one closest to the cooling fin because they often dry out due to the heat.
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