Another K7000 mystery...

modessitt

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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...
 
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Str30130

I looked up the STR30130 power supply regulator chip and figured that your output at pin 4 should be 130 volts. Since it isn't I suggest checking first to see if the chip is getting a sufficient ground pin 1. You can also check to see if the large resistor that connects across pin 3 and pin 4 is good 220 Ohms. If this doesn't pan out then check the surrounding components connected to the chip.
Good Luck
Later
Brian
 
I looked up the STR30130 power supply regulator chip and figured that your output at pin 4 should be 130 volts. Since it isn't I suggest checking first to see if the chip is getting a sufficient ground pin 1. You can also check to see if the large resistor that connects across pin 3 and pin 4 is good 220 Ohms. If this doesn't pan out then check the surrounding components connected to the chip.
Good Luck
Later
Brian

Well, this model is a STR3130, but I'm sure it applies. I've just been told that the guy wants one with a remote board. Since this one doesn't have one, I'll be setting it aside for the moment and getting a different one running...
 
Now that I'm my PC instead of my phoine and can answer in more depth...


You're explanation is correct except for one point - the proper function of this part is dependent upon the load applied. This voltage regulator is supposed to supply our B+ operating voltage (130vdc for the P538), but if the applied load is not correct, then that output voltage will go up or down. It is very common on the K7000 to have the output voltage go up due to bad traces in the deflection circuitry.

I'm not saying the part isn't the problem. I'm just saying that you can't always determine that by the output voltage, nor the nearby parts. It's often something nowhere near that part...
 
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