TECH: wavy K4600

[OSCAR]

I have a K4600 chassis that is doing the wave (or maybe the "hula") at me, but I can't seem to figure out the problem.  This K4600 is mounted vertically, and there is a black band the moves across the screen which causes a wave as it goes.  Here are a couple pics:





Everything I've read so far says this is a power problem, most likely a failure in the voltage regulation circuit, but I'm having trouble finding the problem.  Here is what I've done so far, and everything seems to check out and nothing has removed the wave:

1. New cap kit installed, including the VR201.
2. Blue lead of TR502 reads +145VDC.
3. TR502 tests okay (hi resistance on metal case).
4. Swapped known working interface & hor/vert cards.
5. New C608 cap.
6. Unplugged degaussing coil.
7. Checked and cleaned fuses.
8. Metal case on TR601 reads about +145VDC.
9. Checked for good grounds.
10. Resoldered the plug-in card headers.

One thing that intrigues me is that the VR501 +127VDC adjustment pot doesn't seem to do much.  I certainly can't adjust it enough to get +127VDC at the blue lead of TR502, as the manual says I should be able to do.  I have another K4600 chassis that reads +145VDC at TR502, too, but no wave on that one.

I'm sure I'm missing something simple here, any ideas?  

[Ken Layton]

Try swapping the plug-in power supply board on the monitor.

[OSCAR]

Thanks, Ken.  Next time I get a few minutes, I'll swap that out.  If that works, then I only have 1 more 4600 chassis to get working again!

[Ken Layton]

Remember that the power supply boards on the 4600 have a wire tie holding it in down by it's connector. You'll have to cut the tie before you can unplug the supply board.

[dhansen]

I had a simular problem and it turned out to be the isolation transformer...just a guess..

Doug

[OSCAR]

Ken - It turns out it was the TR502 transistor.  The blue lead voltage measured a steady +145VDC, but when I removed it from the circuit I found that I had a .45V junction drop both ways across the Base and Emitter, and a dead short between the emitter and collector.  I swapped out the big TR502 transistor and now it is perfect again.  I was just getting ready to swap the power board as you suggested when I figured I would pull the TR502 and do the diode test on it instead of relying on the +160VDC voltage test.  I guess when that transistor fails, it doesn't always produce a +160VDC voltage reading?

Thanks again for the help!  


Doug - I suspected the iso xformer at first, too.  But I swapped in a different chassis and it worked okay.  BTW, I haven't had a chance to get to that GO7 chassis, but that one is next after I fix one more 4600 chassis.  Thanks again for sending it to me!