K7400 wigs out after a few seconds

I have one that did something similar that seems to go away for a while when I play with the pots on the flyback...

I'd try a new flyback.
 
If you have another K7400/K7500/U2000/U5000 lying around, try swapping the neckboard (if it has the same number on it). There is a chip on the neckboard that can cause some strange stuff like this if it goes bad...
 
OK, new flyback and it does the exact same thing. I don't have the same neckboard to test out with. Any ideas where to go next? modessitt, you mentioned the chip on the neckboard? Is there a place that stocks those?

Ed
 
i dunno but heres the info from wg on the neckboard. Used to be you would buy these new, now it just says call:

054A7401-020
Neck Board for Model K7400

img_314200631006pm.jpg


Enlarge Image
Call 1 (800) 336-6630 for pricing
Used on: WGM2774-U0TS01K
 
looks like the verticle is pulling down when it starts in the video.....but my first question is what voltage do you have on the B+ line?
 
This seems off to me, but I don't know.

One leg is showing 5.6 VDC and the other is showing 19.8 VDC.

One leg of what?

Test the B+ at TP202 - red on the test point and black on the monitor frame...
 
Oh dang, my bad. I was thinking it was the big white resistor.

TP 202 starts at 110vdc and drops to about 99 when the screen starts freaking.
TP 203 (stated as video b+) stays around 172 vdc

Thanks so much.

Ed
 
TP 202 should be 117VDC and stay steady.....since you did a cap kit, check r120. It is a 68k reisitor and is known to go bad. Also, check 1.2 ohm resistors by the flyback as they go bad as well.
 
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TP 202 should be 117VDC and stay steady.....since you did a cap kit, check r120. It is a 68k reisitor and is known to go bad. Also, check 1.2 ohm resistors by the flyback as they go bad as well.

823 is at 1.6 (Schematic shows 1.8 I think, scan is pretty rough).
303 also reads 1.6 (Schematic shows 1.2)
The resistors are the same based on their banding.

I tested them still on the board though, so I am going to remove them and test again.

Also, the cap that was not replaced via the kit is C729, a 10uf 50v

Ed
 
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Should I replace C729? It's a big boy, 10uf 50v.

Could it even cause this issue? Smalltownguy, what have you done to yours that is doing the same thing?

Ed
 
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