open leg on K7400 flyback?

pun84

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Hi, I have two non working K7400 chassis and both of them seem to have an open connection between one of the primary coils.

Between pin 1 and 3 I get connection, and also good rings with my ring tester, however between 1 and 2, or between 3 and 2 I don't get anything.. This doesn't seem normal, has the connection between pin 2 and the actual coil been broken? This is the first time I am working on a K7400 so, I am trying avoid buying a new fly-back just to see if pin2 is open on a new one. Anyone mind checking theirs for me?

thanks

 
i just pulled a k7500 fly(should be simular) and will be happy to test for you.

However, by the schems, yours tests bad.
 
ah ha! thanks man i really appreciate it. That is pretty weird that both fly backs failed in the exact same fashion!
 
Interestingly enough, the fly i have tests the same. No continuity on pin 2. 1+3 PERFECT.

Bieng this was a working fly that i happened to see a crack in and changed as a preventitive measure, im going to say thats the way its supposed to be and the fly is most likely not your issue.

these monitors in particular are known for smoking the color drive transistors on the neckboard (they run so hot they tend to desolder themselves over time) so if you have wacky colors, that would be the first place id check.
 
Hm, this is strange. I was hoping it was the flyback because I was getting just ticking, and now I am getting nothing. I checked all the diodes from the flowchart, and a bunch of the non-electrolytic caps as well and they all checked out good.

I haven't checked the transistors on the neck board though.. but I did do a little 'repair work' on the traces as they were pretty messed up. I will check them out next though, thanks!

PS: Is there any way to test if the LA7850 IC is good/bad ?
 
The ticking is usually caused by a blown HOT. If it won't even tick now, check the fuse, then the B+...
 
Hmm, HOT seems to be okay unless these readings are off..

Using diode test I get:

Base(+) Collector(-): 0.452V
Base(+) Emitter(-): 0.510

Using resistance:
Base (+) Collector(-): 6.5Megaohms
Base(+) Emitter(-): 7.2Megaohms

open with any other combination

Shouldn't the resistance be a bit lower?
 
you sure youre checking the hot? also a down a dirty test to look for a shorted hot..(works for alot of xustors but not all) is to simply ohm between the center and right leg(looking at it from the front) and if it measures low ohms, its shorted and bad.
 
Hm, yeah.. its not shorted

I measured B+ and was getting 170V.. hooked up a 25W bulb between tp202 and ground.. went back to 120V

I tested pretty much everything in the flowchart and no go (no ticking) :(

Should I try switchin out the flyback?

pin 4 and 5 reading as short with ring tester, and with ohm meter it reads 0.1ohm :eek:
 
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