k4900 Fireworks!

Arcade22

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Donor 3 years: 2012-2014
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So, recently I capped out a poor loking Wells Gardner k4900 19in. monitor and got it looking really good. It was Mfg. in 1981 and had never been worked on before. The monitor worked great for a little over a week and then suffered vertical collapse. I got around to going through and inspecting the caps, work and checking all joints for cold solder, etc. Popped the chasis back in for a re-check and still no change. Still vertical collapse. Poking around, I tapped the tip of a screwdriver on the outer casing of the STR381 voltage regulator on the outer left side of teh chasis (looking at from the back)... and POW!! Sparks and Fireworks everywhere! Good thing I didn't touch it with my hand! Fortunately, nothing else was damaged and nobody was hurt. Still same vertical collapse on the screen afterward.

Guess that means the STR381 voltage regulator is shorted/bad? Def. gotta replace it, but what about any other components that would be good when placing my parts order? There is a 2SD869 HOT on the right side (looking from the back). Should I replace that as well?

As I said, it's got all new caps and was working fine for over a week until the vertical collapse. No caps appear to be damaged, but I'm sure at least the STR381 voltage regulator is toast.

Any help would be greatly appreciated.

Thanks!
 
test the vertical transistors there are two of them by the rear of the chassis mounted to heat sinks, test the vertical pots they can also be a problem.
 
I assume you're talking about the ones located at Q302 & Q303? Listed as replacement transistors NPN 2sc2073 in the manual?

(or sub in NTE398 or sk9363)
 
Yes you are correct. Also the voltage reg and HOT are fine since the monitor is working. A word of advice, use plastic tools while working with monitors. Metal and high voltage don't mix well.
 
also check the legs on them and traces around those transistors.

i have seen the legs broke on those transisters before causing your problem. i did not find that they were broke till i went to desoloder them and the leg fell out of the solder pad.

check your B+ and that will tell you if your VR is working corectly as it is a fixed regulated circuit

Peace
Buffett
 
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Waiting on some vertical output transistors to arrive, so when I have a chance to get to it, I'll keep everyone posted!

Thanks!
 
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