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Old 13th May 2017, 11:15 pm   #1
Retroman
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Default Philips G8 Blank Raster displayed

Hi Everyone.
I hope someone might be able to help me. I have basic knowledge of tvs having repaired a fair few in the 1970s.
This 26" G8 (527) I have had for a few weeks, bought it locally. When I got it there was no colour. I replaced the IC TBA540 found on a website of G8 stock faults which lucky cured my problem. The CRT is very good so gives a great picture. However switching it on yesterday the sound is OK but I was greeted with a blank raster, also the Brightness control has no effect. I have a spare working line panel and HT panel so tried both of these which made no difference, so the fault must be on the combined IF decoder panel. Any thoughts on where I could start fault finding? I had an idea if I remember I needed to check for video voltages on the CRT and work back. Is this correct ? Has anyone got a circuit for the combined panel? On the web pages I can only find a circuit for the older G8 tvs with the two single panels.
Thanks
John
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Old 14th May 2017, 12:16 pm   #2
TV_Madness
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Default Re: Philips G8 Blank Raster displayed

Hi John

The G8 really is a superb set when working properly.

Although I own a later G8, I'm still not 100% familiar with it. The combined signals panel has a regulated 12V supply for all of the ICs on this board. Maybe worth checking that? It is located at the front of the board on the far right hand side (looking from the back). What do you see at the cathodes on the CRT? It is worth starting there and working back to the TBA530 RGBY Matrix IC.

Is there any EHT also? It may be possible that the tripler has failed in action.

Kind Regards

Matt
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Old 14th May 2017, 12:31 pm   #3
Retroman
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Default Re: Philips G8 Blank Raster displayed

Thanks for the reply Matt
Yes EHT is fine I have a nice bright raster which fills the screen Will check the 12V supply to start with. Which is the video chip on the board as a matter of interest
John
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Old 15th May 2017, 8:34 pm   #4
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Default Re: Philips G8 Blank Raster displayed

Hi John

My apologies. I jumped to the conclusion that you were looking at a black screen!

A bright white raster is the exact same fault that I have on my G22K550 G8. The TBA560 contains the luminance output and black level clamps (as well as the chrominance amps / burst gate). The output from this is then fed to the TBA530 RGBY matrix.

On mine, I got as far as replacing the TBA530 which made no odds at all, so I shelved it pending further investigation.

Failing the TBA530 being the culprit, my money would probably be on the 560 earlier in the circuit.

Thanks

Matt
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Old 15th May 2017, 10:17 pm   #5
Retroman
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Default Re: Philips G8 Blank Raster displayed

Thanks Matt
Think it will probably end up me changing these two chips see what happens
John
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Old 18th May 2017, 12:02 am   #6
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Default Re: Philips G8 Blank Raster displayed

Hi

It might be worth checking the voltage conditions around the blanking circuit on the timebase board. T4488 (BC148) is the field blanking transistor and T4524 (BC148) is the line blanking transistor.

Regards
Symon.
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Old 19th May 2017, 4:38 pm   #7
Retroman
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Default Re: Philips G8 Blank Raster displayed

Interesting thanks Symon. I also have a spare time base board I could try.
Thanks for that having another look this weekend.
John
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