K7000 Hum.....

Kyrosfear

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Gentlemen,
Here is an interesting story. I took a (mostly) working K7000 chassis off of a monitor. It had a slight horizontal issue (as you may recognize from my Neo Geo thread) and I wanted to put a new C38 cap on it. I replaced the cap and hooked it all back up. Upon power-up, it blew the fuse. I took the chassis out and cut the leads closer down to the chassis and hooked it back up again. This time, upon power up, sparks started coming out from under the chassis. I took it back off and looked on the solder side. The area around the pin that the yellow wire from the yolk connects to was charcoaled. I cleaned it up and reflowed the solder and hooked it all back up. Now the chassis hums and that is all. This is not a typical high voltage hum, but more a "mechanical" sounding hum. I cannot specify further where the hum is coming from because I do not want to give it power for too long. Any ideas on where to start? If you need more information, just let me know. Thanks in advance.
 
wow that is a tale for sure.

pics of your repair on the solder side.
i would like to see what that looks like.

if i were to guess where to start.

i would start with a new fly and cap kit if it has not been done.
and re-flow the chassis.

check your VR to see if it is shorted.

other wise i would suggest sending it in to be looked at.
hope some other folks have some suggestions on this as i have not come across this yet.

Peace
Buffett
 
Humming huh... Did you replace the fuse with the right sized part? Something large or able to handle a good amount of current is shorted. Does the HOT or VR get warm really fast?
 
Yes, the fuse is the right amperage. I am attaching pictures, but my camera is terrible so I did the best I could. If they are inadequate, let me know and I will try to get some better ones. I have already done a flyback kit (fly, caps, hot) on this chassis (before all of this happened). Today, I reflowed the chassis and put a new flyback on there to see what would it would do. There was no change, however, not only do I hear a low hum, but I can also hear a clicking sound. I mentioned that I changed the cap a C38. I also changed the horizontal width coil because the ferrite was completely destroyed. All of the caps look fine. You guys mentioned the vr. If the vr was bad, would it still have power, or would it just be dead? I only ask because I am cautious about leaving the monitor on long enough to check the voltage at the vr (because of the original sparking episode) but you certainly know far more than I do so if I can leave it on to check, I will.
 

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in the pic circled in yellow i don't like that solder joint.

remove all the solder from it clean the pad and re-solder it back in.

that area is known for causing problems on the K7000.

we need to know the B+ on the chassis.

clean up that solder point and then see what your B+ is and if the humming stopped.

Peace
Buffett
 

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There are only a small handful of parts on that board that are capable of humming/buzzing. Flyback, VR, HOT and MAYBE the bridge rectifier diodes. How long are you leaving this thing on? Is the HOT or VR getting unusually warm really fast (DONT touch them with the power on).

Ticking sound?? Hmm.... Thats strange on this chassis.
 
Sorry guys, I had a couple other projects that had to get finished first, but now this chassis is getting 100% of my attention. A couple of updates.... I resoldered the area you told me to buffett. The B+ at resistor R301 is 6.3 volts. I have a lot of extra K7000 parts around my shop so I tried a new flyback, vr, and H.O.T. with no change. When I turned it on to check the B+, the clicking noise sounded as if it was coming from the flyback, or very close to it. I placed a finger on the flyback (carefully) and I could physically feel a tapping sensation in sync with the clicking. I appreciate your help, and await your suggestion.
 
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ok the reading at R103 is not your B+. sorry if i misled you about that.

measure the the tabs on the big wite resister on the left side hanging off the side of the chassis.

one closets to the fly should read 130vdc and the other should read 150 +/- vdc.

a K7000 should not tick like the newer chassis do. but you never know after 25+ years in service like theses chassis have endured.

we need to see if you are in HV shutdown.

when you put a new fly on the chassis did you turn up the screen control knob on the fly to see if you had raster.

Peace
Buffett
 
I still have not fixed this chassis yet, but I wanted to clarify something with Buffett here. I was checking the B+ at R301, not R103. I just wanted to make sure we are on the same page with this thread. I have not given up on this chassis and I appreciate all your help.
 
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this chassis has been fixed.

all the details are in the video.
ignore the snow that is my test station.

i wish i had a better camera for taking videos any recommendations please PM me.






Peace
Buffett
 
Buffett knows that I appreciate his expertise repairing this chassis, but I want to warn you relative newbies at monitor repair about what happened. The caps he is referring to in his video are from a cap kit I bought. As a newbie myself, I do not readily know much about replacing capacitors except the basic rules to make sure you do not drop in voltage and you keep the microfarads the same value. This situation was one where the cap kit had a couple of inaccurate caps but I ignorantly assumed that the person I bought the kit from did this intentionally so I used them to replace the existing caps anyway. My mistake. As is the case with any high volume service, errors will occur. Take the time to double check that the caps are exactly what they are supposed to be and do not assume anything. If you do jack it up, however, Buffett will fix it for you. Turn around on my chassis in particular was 5 hours. Thanks Buffett.
 
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