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Tech Chat G11 at switch on bright picture

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Stevesje71
(@stevesje71)
Posts: 9
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Topic starter
 

Hi all

my Philips G11 has a fault in that the picture brightness is varying. At initial switch on the picture is bright with black areas no longer dark. The picture is viewable and only noticeable on dark scenes. After 5 minutes or so the brightness returns to the setting it was at the last time it’s was switched off. I’ve tried everything, measured test points voltages from the brightness control on the decoder panel, HT check, with no fault found. The CRT is 100%. I’m guessing my next move should be to look at the CRT base panel components? 

 
Posted : 03/04/2022 10:58 am
Nuvistor
(@nuvistor)
Posts: 4594
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Check the voltages on the CRT, turn the colour off so all guns are using the same signal and check which voltages change from fault to normal condition. You may lucky and find the voltage rail feeding the A1’s in the CRT is varying and causing the problem. If it’s the  cathode/grid voltages then work back from there.

I only ever saw one G11 so I’ve no experience with them but the reservoir/smoothing caps are notoriously poor and were upgraded, confirm you have the correct type or faulty ones can cause lots of damage, though probably nothing to do with this fault.

Frank

 
Posted : 03/04/2022 11:34 am
slidertogrid
(@slidertogrid)
Posts: 990
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@stevesje71 There was a modification to the beam limiter circuit. It is decades since I did one but I remember removing the can of the module on the RGB panel and lifting one end of a diode from the print and soldering it onto the top of one of the resistors mounted on end. This was to cure random brightness 'jumping' on scene changes. It might be worth seeing if you can find details of the mod and seeing if it makes any difference. 

Rich

 
Posted : 03/04/2022 1:19 pm
Mikey66
(@mikey66)
Posts: 201
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Here's a link to the G11 beam limiter mod I posted on UKVRR a long time ago....

https://www.vintage-radio.net/forum/showthread.php?t=61374

 

Regards

 

 

Mike

Best regards

Mike

 
Posted : 05/04/2022 5:11 pm
Stevesje71
(@stevesje71)
Posts: 9
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Topic starter
 

@nuvistor many thanks for this. I’ve already tried this, all voltages are stable. There’s a 12 reg even that which is prone to failure seems fine. 
I’ve found the cause, the TBA560C luminance/chroma IC on the decoder panel. 
thanks for your post 

 
Posted : 11/04/2022 5:11 pm
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