Ceronix 2793 fades out.

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Started by LYNKKK -


I have a few 2793 Ceronix chassis that will fade out to total darkness after 20 to 30 minutes, I think the vertical output IC377(LA7838) is getting hot which is affecting the CRT Auto BIAS/Brightness circuit. After the screen fades out I can aim a fan at the LA7838 heatsink and the screen will come back to full brightness within seconds. I've changed the LA7838, FBT, neck board, all the other ICs, all electrolytic capacitors, and checked and tested many other components in the vertical circuit.

Has anyone had the same problem?

Thank You.

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By Cactusjack -


If you think its heat related (which it sounds like you have already proved that), you might want to try freeze spray.  The Electronic stores use to sell it (might not be planet friendly any longer) but I think the "Air in a can" for dusting your keyboard is the same thing but used upside down so it spits liquid.  Hit a couple of suspect comonents and let the "freeze" soak in for a short while - it doesn't always act instantly.  You can re-heat the area with a blow dryer to keep repeating the fail-fix process.

Usually, I will cool down a large section and see if it fixes things.  If so, divide the area in half and try again.  Repeat until you have narrowed it down to one component.  You might want to re-flow the solder before changing the part first.  Could be a cold solder that is not obvious to the eye.  Sometimes, the solder doesn't bond to the lead even thought it looks good to the eye.

Although, fading in and out slowly often indicates loss of Filament voltage.  Does the neck stay glowing even when the screen fades away?

CJ

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By LYNKKK -


Thanks for the freeze tip, I'll have to try that but I think that there is a component with the wrong value and it's not getting hot. I've measured the temperature of a lot of other components and have found no other rise in temperature's, but I will try to freeze some individual parts.

The neck still glows during the fade out.

Thanks.

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By Habu -


I suppose this problem is ancient history by now but if it isn't check component 945 on the neck board (freeze spray). This caused the same symptoms on a 2093, don't know how similar a 2793 is.

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By LYNKKK -


Sorry I haven't replied back earlier. I did figure out my problem. The fix was easy  , I just needed to set the G2 voltage to 4.6 volts measured from IC920 pin 8 on the neck board.

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By Habu -


DOH!...

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By ESDAVE -


hi check your tube heaters for fading out if you put a meter across the htrs pins of the tube approx 6v3 could be ac or dc . if htrs are ok you could have a beam limiter fault , if it was your frame fault shutting tube down via tube frame guard turn screen on line transformer control clockwise/right and see if you have line across if so frame fault. but sounds more like htrs , check in a darkend room if so trace it back to line transformer, if you need more info let me know. esdave uk engineer.