K7900 blows fuse. Can someone smarter than me check my logic...

rmcelwee

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K7900 blows fuse. Can someone smarter than me check my logic...

My K7900 is blowing a fuse. I pulled R89, R95, R97, and R101. I think that means the power was only going to the flyback (it is brand new). I plugged in the monitor and the fuse still blew. I then put R89, R95, R97, and R101 back in and pulled the flyback (left neckboard on, left anode cap off). When I plugged in the monitor the fuse did NOT blow. I am not smart enough to know what to do next. Do I:

1) Reinstall the flyback/anode but leave the neckboard off?

2) Reinstall the flyback/anode but pull R92 and R95?

3) Something else?

Am I way off base on how to trouble shoot this? I put a cap kit on the monitor as soon as I got it (used and condition unknown) and changed out the flyback because the old one was cracked.
 
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First check for a short on the B+. You could have a bad horizontal out put transistor, C36, C38 or D15

I swapped out Q11 (HOT), C36 and C38 from my parts board. I didn't swap D15 yet but I just pulled D15 (and C37 since they kind of come out together) and they look the same as the D / C37 from my parts board as far as my ESR meter shows. I will swap them for the hell of it tonight.
 
make sure to test the parts instead of just swapping them and if your swapping from a parts board make sure to test the parts before installing the used parts since they could be bad too. i would also check the rest of the diodes in that section around d19-
 
make sure to test the parts instead of just swapping them and if your swapping from a parts board make sure to test the parts before installing the used parts since they could be bad too. i would also check the rest of the diodes in that section around d19-

I've been comparing the new parts to the old. So far I have not found any discrepancies. I tested the D19 - D22 diodes (I think I swapped them as well but I can't remember) where the AC turns to DC but didn't find anything. I'm thinking that pulling those parts that I listed on the first post in this thread proved that the Diodes were fine.

This stuff makes my head hurt!
 
I've been comparing the new parts to the old. So far I have not found any discrepancies. I tested the D19 - D22 diodes (I think I swapped them as well but I can't remember) where the AC turns to DC but didn't find anything. I'm thinking that pulling those parts that I listed on the first post in this thread proved that the Diodes were fine.

This stuff makes my head hurt!

when your testing diodes and resistors i suggest only desoldering and lifting one leg and than test them. this way you can't mix them up and its less soldering and desoldering which runs less risk to damaging pads or traces. if a part tests good then leave it and reinstall the one leg. each component will have a specific reading or a range (depending on tolerances) that a component should test at and if its out of range then replace the part. (i am mainly referencing resistors and diodes at this point but this information can be used on other components).
 
check all the parts in your Horizontal and power supply section.

one or more of them may be shorted.

they are the culprits most of the time.

test them out of circuit or lift 1 leg out off the chassis.

if you hit a brick wall your welcome to send it in.


Peace
Buffett
 
I think my IC4 was bad. It had continuity on many of the legs. I checked it against one on my parts board and they checked differently. I swapped it out (as well as pulling the new flyback I put in and putting another new one in) but have not powered up yet. I am worried that something else caused IC4 to die. Do you guys think I am safe to fire it up and see what happens or is a bad IC4 a normal cause of fuses blowing?

EDIT - I pulled Q11 HOT (I replaced it earlier with one from my parts board but did not test it) and I have continuity across all three pins. I think this is bad too so I will replace it. I am not sure if it was already but I think it is now.
 
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if you replaced ic4, flyback, and hot and checked those few diodes in the d19 area and gave it a good look over and reflow then most things are replaced so you then should try. with your fingers crossed....;)
 
if you replaced ic4, flyback, and hot and checked those few diodes in the d19 area and gave it a good look over and reflow then most things are replaced so you then should try. with your fingers crossed....;)

I will try it since you have given your blessing! I am trying to learn how these things work but I have yet to get all the fine points down (what causes what to blow, etc). I am slowly learning. Thanks!
 
Well, I fired it up and watched the cab fuse to see if it blew. Looked good but no picture. Found it blew the chassis fuse this time. Going to give up for the day and start again tomorrow. I really need to catch a break on this thing. I'm running out of parts to check.
 
I started looking at the chassis/schematic again and I have to go back to my original question. I opened a bunch of circuits up and the fuse still blew. When I put them back and removed the flyback the fuse no longer blew. It has to be related to that. I just need someone smart to look at the schematic and tell me what to look at. I did check the three color transistors on the neckboard since it looks like they have to do with that circuit. They were fine. I would think it can only be something on the right hand side of the flyback (on the schematic) since the left side is isolated.

The stuff I pulled from my original question:
I pulled R89, R95, R97, and R101. I think that means the power was only going to the flyback (it is brand new). I plugged in the monitor and the fuse still blew. I then put R89, R95, R97, and R101 back in and pulled the flyback (left neckboard on, left anode cap off). When I plugged in the monitor the fuse did NOT blow.
 
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without the flyback the fuse should never blow unless one of your PS diodes are shorted.

the fly connects many of the circuits that could blow the fuse.

without it in circuit, the chassis should be dead no matter what.

you have a short in the horizontal or PS circuit, measure ALL the poly caps H.O.T., VR and diodes.

you may have to pull one leg on the them out of circuit for a good test.

Peace
Buffett
 
I spent a few hours working on it today. Just throwing these pics out there (what I checked values on or replaced is in pink) so someone could tell me if I was on the right path or if there was something in particular I needed to look at next. BTW, you will see a red line in the first pic and three red lines in the second pic. This is where I broke the circuit and then fired it back up. It still blew the fuse. This was part of my troubleshooting and is where I need someone smart to tell me what is going on. I think this would narrow the problem down. I did check other things after doing this so some of the pink stuff is downstream of my lines. The degauss coil has never been hooked up and the flyback has been replaced (twice).

1.jpg



2.jpg



3.jpg
 
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