K7000 VR replacement

obitus1990

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I'm about to replace the VR on a 19" K7000, due to getting HV shutdown activated which is in turn "fixed" by taking D10 out of circuit. Indeed, I'm getting over 160 V at the regulated side of the B+ resistor.

In the past, this VR had been replaced by an NTE equivalent 1741. Whomever replaced it used a bunch of thermal compound between it and the little plastic insulator and the chassis frame.

The insulator looks to be intact, but I am not so sure this compound should be here and going to clean it off, for fear of it being the electrically conductive type.

Should there be any of this stuff applied here at all? I haven't replaced a VR on a k700 in a few years now and can't remember.
 
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Yes, there should be thermally conductive paste there. Some don't run it however as home use isn't as hard on them.

Typically the "white" paste is good to use. I use this stuff on my K7000 chassis.
GC Electronics 10-8109 Heat Sink Compound

You need very little of this stuff, most folks over apply and it gets everywhere.
If you need to clean it up use some 99% isopropyl on a paper towel or q-tip. Be careful not to crack the mica insulator if you're cleaning it.
 
Ok, I changed it out (using a replacement from APAR), and I am still getting shutdown and high readings at the B+ resistor (over 165V regulated on one side and 200V unregulated, IIRC). I've tested every big resistor (following zenomorp's videos) the critical safety cap, width cap, recapped everything first, reflowed problem areas, etc.

I checked the AC outlet and it's 125V. So I unplugged the monitor and checked the power from the isolation transformer from this Taito cabinet power supply board I am using it with. It's outputting 165V!

So I used a different isolation transformer, which is outputting 129V and the monitor now works. So more than likely, the VR I swapped out was just fine.

Can isolation transformers go bad/drift out of spec?
 
Ok, I changed it out (using a replacement from APAR), and I am still getting shutdown and high readings at the B+ resistor (over 165V regulated on one side and 200V unregulated, IIRC). I've tested every big resistor (following zenomorp's videos) the critical safety cap, width cap, recapped everything first, reflowed problem areas, etc.

I checked the AC outlet and it's 125V. So I unplugged the monitor and checked the power from the isolation transformer from this Taito cabinet power supply board I am using it with. It's outputting 165V!

So I used a different isolation transformer, which is outputting 129V and the monitor now works. So more than likely, the VR I swapped out was just fine.

Can isolation transformers go bad/drift out of spec?
they can be jumpered wrong.
 
Could very well be the case. I've looked at various Taito manuals (this one came out of a Jungle Hunt) and they are kind of sparse with how the jumper block should be wired for a US cabinet, or I don't understand it. It looks like the only output options are 100V and 120V AC. Am I reading this wrong?

1759000050734.png
 
I went and changed the jumper plug on the power supply. It was set as above for 100V when I looked at the jumper, so I popped out the appropriate pins and switched them to 120V input. The monitor now receives 120V. It seems I have a long way to go with understanding these things. I am now assuming that the transformer will act as STEP UP isolation transformer when 100V AC input is selected, bringing it up to 120V, versus it behaving as a 1:1 isolation transformer when 120V input is selected. So applying 120V input when the 100V jumper is set will result in that 120V input being stepped up the higher output voltage that was making the K7000 shut down.

Now I've just got to figure out why I cannot get pure white color on the screen after doing the white balance procedure. White has a cyan cast to it, no matter how much adjustment I do, so I swapped out the neck board transistors (I swapped red with blue), with no change, so I guess I will now swap the red and blue transistors on the chassis and see if that changes anything. If white is appearing as a pale cyan, are those the correct ones to be swapping or should I try a different pair?
 
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