25" K7000 Problem with Red

tec9

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Hey all-
I'm bored at work here, and just wanted get some opinions advice on the problem I am having. I have a 25" K7000 that I am rebuilding for a cousin. I've got it to power up, recapped it, and it has a clear stable picture, but it appears to have a issue with the red. The red is missing I beleive. If I play around with ALL the color drive/ cutoff pots I can sort of get a maroon color. I checked the 3 transistors on the neck board and the 3 on the chassis, and they all read the same. I checked the pots, and they all seem ok. I have some parts chassis, and another 25" K7000 in another machine. I was thinking of swapping this problem chassis with the known working tube, and see if the problem follows the chassis. (are all 25" K7000 tubes compatiable?) Any other suggestions??
Thanks-
Mark
 
It sounds like you need to test the tube with a rejuvenator. The 7191 and 7193 (these are what are typically referred to as 25" K7000) are compatable, do not swap it with a 7400 or 7500.
 
It could very well be the tube... but something else to check - capacitor C4 on the board. It's one of those little 10uf nonpolar ones. Swap that cap with C5. Those little caps are in the signal path, and I've seen them fail before. If swapping the part moves the problem over to green, then you've found your culprit.

-Ian
 
Another quick test to help rule out the picture tube, you can use a piece of wire to ground the metal tabs on the video transistors on the neckboard - do one at a time. Each one should produce a bright screen of it's associated color. If you ground the red transistor and get only a dull screen, then it's the tube.

-Ian
 
I also forgot to add that I did reflow the header pins on the chassis, and I also reflowed the pins that connect the neckboard to the chassis. Thanks for all the suggestions!! Keep em coming!
-Mark
 
PROBLEM SOLVED!!!

I first grounded the tab of the "red" transistor on the neckboard (Q201)
The screen gave me a flash of red! Good...the tube isn't shot!
I double checked both the transistors for the red (Q201 + Q1) and they checked good. I decided to replace them anyway, and as Retrohacker suggested I replaced C4 (non-polar cap). So I sorta shotgunned it with three parts, and when I fired it up- it was fixed. Not sure what component fixed it, but I would guess C4.

Thanks!!
Mark
 
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