Hi,
If QH01 is shorted again, then this is an indication that the FBT is already faulty causing QH01 to shorts right away and blew E907(self destruct when shorts occured on the line). 100VDC isn't causing E907 to blew, you've only got 100VDC because the FBT didn't oscillate yet. Before you replaced the FBT make sure to test all parts parallel on the B+ line like DH09, Q701, CH16, CH13, CH20. It is better to pull-out the FBT while testing the shorted/leaky parts on the board to make your reading more accurate. If you found the said parts to be in good condition, then you have to replaced the FBT.
Hope I helped you.
Have a nice day!
Thanks for using Fixya.
If you have a hard time unsoldering the FBT on the board, you may pull-out the said parts instead of the FBT then test them for shorts/leak using your multimeter set in resistance measurement.
Good timing on the response, I just came to the conclusion it might be something related to the flyback transformer also.
-I removed the new QH01 which appears to be okay, it doesn't not have a short from Drain to Source.
- I somewhat tested the other suspected components on the +115dvc line (Q701, Q777) transistors....checked for short.
- I checked CH13 and CH20 in-circuit... didn't measure actual capacitance but at least they weren't shorted to ground.
- I lifted the K255 jumper to isolate the Dynamic Focus circuit
- Then to confirm that the issue is somewhere around the FBT I lifted jumper K285 which leaves the +115vdc only connected to the FBT... and it still blows E907
SO, with only QH01 and jumper K294 removed, and a new E907, the TV turns on and I get sound and E907 does NOT blow.
The TV is currently sitting in my garage and the only test-equipment I'm using is my trusty Fluke 77 DMM.
Also, can you explain what goes bad within the FBT. It appears as though the +115vdc line just goes in through pin 9 of the FBT winding and out on pin 10, then through some caps to ground? So wouldn't that be sort a DC open?
The copy of the schematic diagram didn't shows the jumper you've mentioned so i can't exactly figured out where is the lifted circuit but based on you're description, I've understand that the E907 blew when the B+ line connected to the FBT, this is a clear indication that the FBT itself is already faulty. FBT has a very fine winding inside and has a thousand of turns for every pin-out. Pin no. 9 and 10 is the primary winding making the FBT to turn on when the H-out transistor turns on. Based on my experienced, the primary winding has a partial shorts inside making the B+ line gone to excessive current and made the E907 blew due to overload (current exceed from the allowable current can handle by E907). I'm pretty confident that replacing the FBT will resolve the issue.
Here in our country, there's some shop specialized on FBT testing, they only charged $0.5 for testing of FBT, if I've suspected fault on FBT on the set, I've brought it to them and none of my suspicious fail most especially this kind of symptoms.
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After a lil more researching I found that the Horizontal Output transistor QH01 was shorted to ground on the Deflection board. The +115v line from E907 fuse supplies +115v to the collector of this QH01. I figured that replacing QH01 and the E907 fuse would fix it but E907 still opens up when I power on the TV. I measured voltage on the fuse on the supply side and it measured only 100vdc.... I'm assuming this should be closer to 115vdc right? If so, could this cause E907 to blow?
sorry, I have not responded for a couple months. Im nearly giving up on this TV because after you said the FBT is probably bad, I searched all over the internet for a replacement and I can't find one anywhere. It seems like it is an obsolete part and there are no available replacements. Do you know of any websites where I can order a new FBT for this 57f500a?
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