modessitt
Volunteer: Encyclopedia Submission Moderator
Help me talk thru this WG 49xx/55xx repair...FIXED!
I need to talk this through while I think about it. Perhaps someone else has an idea or has encountered something like this.
I'm working on a 5515, which is the 25" medium-res version of the K4900. Most of the chassis is the same, so repairs between them should be fairly similar.
Anyway, I got a chassis of undetermined status (I can't remember if it worked or not). I inspected it and found a crack in the neckboard that had been repaired well, a big epoxy blob on the flyback, and old caps. I installed a new cap kit and cleaned up some solder, checked the HOT and VR, fired it up and got HV static on the tube and neckglow, so I shut it down right away so I could hook up a video signal. I hooked up the video, turn it on, and about 2 seconds later flames came thru the epoxy. Okay. I had a new flyback, so I put that in. Found the fuse and HOT was blown, so I replaced them. Hooked it all back up, fired it up, got the HV static and neckglow - but no picture. I can't even get raster by turning up the screen control or black level pot. I did notice that it was missing a cap betweeen the +185 line and ground, and I installed it, but it's not helping. My +185 is measuring +175, and the voltages at the drive transistors is incorrect, which I've found can cause no picture (remember my red G07 thread?). +130 line and +15 line is spot on. I can hear the hum of the chassis running and I get a big spark when I discharge.
It just so happens that I have a 100% working K5515 right here, also. I've been doing some bench voltage checks, and everything seems to measure the same between the two while it's on the bench. But if I connect them to the tube (same tube for both), I get different voltages at the drive transistors - and I'm not sure why.
Just for kicks, I replaced the VR. It's an STR381. The parts listing says it should be STR380, but the schematic lists STR381, and the working chassis also has an STR381.
I also thought maybe the new flyback could have a problem, so I swapped in an old working flyback and still get the same result.
This is going to turn out to be some stupid resistor that is out of value like the G07 was.
Anyone got any bright ideas?
I need to talk this through while I think about it. Perhaps someone else has an idea or has encountered something like this.
I'm working on a 5515, which is the 25" medium-res version of the K4900. Most of the chassis is the same, so repairs between them should be fairly similar.
Anyway, I got a chassis of undetermined status (I can't remember if it worked or not). I inspected it and found a crack in the neckboard that had been repaired well, a big epoxy blob on the flyback, and old caps. I installed a new cap kit and cleaned up some solder, checked the HOT and VR, fired it up and got HV static on the tube and neckglow, so I shut it down right away so I could hook up a video signal. I hooked up the video, turn it on, and about 2 seconds later flames came thru the epoxy. Okay. I had a new flyback, so I put that in. Found the fuse and HOT was blown, so I replaced them. Hooked it all back up, fired it up, got the HV static and neckglow - but no picture. I can't even get raster by turning up the screen control or black level pot. I did notice that it was missing a cap betweeen the +185 line and ground, and I installed it, but it's not helping. My +185 is measuring +175, and the voltages at the drive transistors is incorrect, which I've found can cause no picture (remember my red G07 thread?). +130 line and +15 line is spot on. I can hear the hum of the chassis running and I get a big spark when I discharge.
It just so happens that I have a 100% working K5515 right here, also. I've been doing some bench voltage checks, and everything seems to measure the same between the two while it's on the bench. But if I connect them to the tube (same tube for both), I get different voltages at the drive transistors - and I'm not sure why.
Just for kicks, I replaced the VR. It's an STR381. The parts listing says it should be STR380, but the schematic lists STR381, and the working chassis also has an STR381.
I also thought maybe the new flyback could have a problem, so I swapped in an old working flyback and still get the same result.
This is going to turn out to be some stupid resistor that is out of value like the G07 was.
Anyone got any bright ideas?
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