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I’m working on a WS-55907 for a friend of mine. I’m an amateur at TV repair, but have pretty good experience with troubleshooting electronics. The problem is that the TV shuts down at power on. If I plug it in and check the diagnostic code before trying to turn it on I get the 1-2 code, indicating ‘no problem’. Once I try to turn it on and it shuts down it gives a 2-1 code for x-ray protect. I have the service manual and have done some testing and this is what I’ve found so far.
I believe the x-ray protect error is triggered by IC5A02, a dual comparator chip. I’ve tried to check the inputs to this chip with a scope and I believe they are good. On pin 3 I have 10.0 volts, on pin 2 I have 8.9 volts, and pin 6 reads 5.3 volts these are all good according to the schematic. The other input, pin 5 should be 8.3 volts according to the schematic, but on my scope it levels out at about 11 volts, before the TV shuts down. I would think that would still be all right since it is higher than the 5.3 volts on pin 6 it would not trip the comparator.
What I found next is my big question. The outputs from that IC pin 1 and pin 7 are tied together and go to the microprocessor (IC7A00 pin 59) to trigger the x-ray protect. The schematic says they should be at 3.4 volts. As best as I can tell they are at .9 volts, before I try to turn on the TV. When I turn it on it looks like it jumps up to about 2 volts before the TV shuts down. I traced this line on the schematic and it looks, to me, like this line should be at 3.3 volts even when the TV is off. I think it is derived from the 3.3 VS supply. I checked the 3.3 VS at pin 3 of IC7C07 with the TV off and it reads 3.29 volts. I think there is a pull-up resistor (R7B59 10K) from the x-ray line to the 3.3 VS supply. Is my thinking correct? Should the x-ray line and pins 1 and 7 of IC5A02 read 3.3 volts even when the TV is off?
If so I’ll have to do some more troubleshooting to find what’s dragging it down to .9 volts. Any suggestions or help would be greatly appreciated.
 

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Pins 1 & 7 should be high if inputs and references are correct, I cannot do research for every member who needs to troubleshoot a set, so you are on your own determining the proper levels on the beam current detect line, HV detect, and references. If they are correct and the output is low, you either have a bad chip or something on the other end pulling it low.
 

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Discussion Starter #3
Hi lcaillo,
Thank you for your reply. That is what I thought, but its good to have an expert reference.
I'm going over to his place tomorrow and I'm going to pull the comparator chip and install a socket, then I can check to see if the voltage pops back up to 3.3 volts where it should be. If so, I know I've got a bad comparator chip. If not, I'll troubleshoot further.

I agree with your signature! My son started Scouts as a Tiger Cub and just finished his Scoutmaster conference for his second Bronze Palm for his Eagle. He turns 18 next week and it has been the greatest experience for him.

Brian
 
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