modessitt
Volunteer: Encyclopedia Submission Moderator
You'd think that after all of these I've fixed, I wouldn't get stumped anymore.
Anyway, I was sent a chassis that had been rebuilt with new flyback - but wasn't working.
When I unpacked it, I saw it had a broken H-POS pot, so I replaced it. Checked the VR, HOT, C38, C36, fuse, R103, etc to make sure they were reading okay, then fired it up - or tried to anyway. B+ is stuck at +154vdc. Pulled one side of D10 with no change. I fixed a whole crapload of trace issues, and even jumpered a bunch that seemed okay but I wanted to make sure - nothing. Was getting some strange readings on Q10 so I swapped it - nothing. Swapped the flyback (again) - nothing. I saw a non-standard C36 in it with a .006 1600v value on it, and am in the process of swapping it with an original just to be sure it's not causing any problems. Then I guess I'll swap out IC4 to check that.
When I was working on the traces, I looked at some trace repair already done. I noticed that there was a solder bridge between the PS side of R101 and a nearby pad that went to pin 5 of U702. Uh-oh. that puts 154 volts on a trace that should only have 0.5 volts. Pin 5 is the input for the X-Ray Protector that feeds the horizontal oscillator. I replaced U702 with a couple different ones with no change in the B+. I found I do not have the AFC signal on pin 1 that is fed from the flyback pin 10 through some other parts which all test as good, and I do not have any +12 where I should have it.
I'm guessing that the solder bridge caused some damage somewhere, but I'm at a loss as to where. I've been following traces and I'm not finding any bad parts. I also can't find any other bad traces, and have found no cracks in the board.
This chassis is pissing me off. I'm beginning to think I may need to recap it again, even though I see no obviously blown caps. I've already replaced C57 again...
Anyway, I was sent a chassis that had been rebuilt with new flyback - but wasn't working.
When I unpacked it, I saw it had a broken H-POS pot, so I replaced it. Checked the VR, HOT, C38, C36, fuse, R103, etc to make sure they were reading okay, then fired it up - or tried to anyway. B+ is stuck at +154vdc. Pulled one side of D10 with no change. I fixed a whole crapload of trace issues, and even jumpered a bunch that seemed okay but I wanted to make sure - nothing. Was getting some strange readings on Q10 so I swapped it - nothing. Swapped the flyback (again) - nothing. I saw a non-standard C36 in it with a .006 1600v value on it, and am in the process of swapping it with an original just to be sure it's not causing any problems. Then I guess I'll swap out IC4 to check that.
When I was working on the traces, I looked at some trace repair already done. I noticed that there was a solder bridge between the PS side of R101 and a nearby pad that went to pin 5 of U702. Uh-oh. that puts 154 volts on a trace that should only have 0.5 volts. Pin 5 is the input for the X-Ray Protector that feeds the horizontal oscillator. I replaced U702 with a couple different ones with no change in the B+. I found I do not have the AFC signal on pin 1 that is fed from the flyback pin 10 through some other parts which all test as good, and I do not have any +12 where I should have it.
I'm guessing that the solder bridge caused some damage somewhere, but I'm at a loss as to where. I've been following traces and I'm not finding any bad parts. I also can't find any other bad traces, and have found no cracks in the board.
This chassis is pissing me off. I'm beginning to think I may need to recap it again, even though I see no obviously blown caps. I've already replaced C57 again...
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