Next monitor mystery - K7000 medium res

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modessitt

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Background - I rebuilt this thing a month or two ago and it was working fine. Installed new flyback, HOT, VR, caps, critical safety cap, and cleaned up a bunch of cold solder around the horizontal width coil. Game (not mine) has been in a commercial location, plugged into a surge protector, but I have no idea if it is turned off at night. I'm sure it runs at least 12 hours every day.

I got a call that the game wasn't working. Had them do some stuff over the phone and determined that it was playing blind, so I went and picked it up.

Here's where it gets mysterious.

I have power to the monitor. Fuse is good. HOT, VR and safety cap read as good. B+ is reading at +123vdc (which is spot on). But I have no HV at the anode cup and I have no neck glow. I pulled one side of D10, but it still won't work, so it's not in shutdown mode.

I was doing voltage checks via the schematic, and seem to have them where I should. I also checked R213 on the neckboard to see if it was keeping my heater voltage away, but it is fine.

I guess my next step is to check IC2 and C36, but does anybody else have an idea? What are the odds the brand new flyback is dead without blowing?


And no, this is not Shardian's chassis. I opened his up to find that there was no VR installed, and the two he sent along were blown, so I have to get some before I can check it out...
 
I had one of those where a brand new flyback only lasted a month. Replaced it again with another brand new flyback and it's been running fine for almost a year now.
 
I've got another new one here somewhere, so I may try a swap out to see if it makes a difference...
 
Update:

Replaced flyback with another new one - no change.
Checked R89. It measured good. Replaced it anyway - no change.

This chassis has had a bunch up trace repair/jumpers done round the width coil, but I'm getting continuity everywhere I should.

Here's where it get interesting. I noticed tonight that when I first power up, I have a perfect B+ and no high voltage or neckglow. After a minute or two, I'll get a quick second buzz of HV at the anode cup, then the B+ jumps to 160 and the HV shutdown activates.

Now, I can understand why the HV shutdown is kicking in, but I don't understand why the HV kicks in for a second after being powered up with proper B+ for a minute or two. I'd think it would work, then cut out when the B+ goes up.

I'm beginning to think it's heat-related. Perhaps some part isn't working properly until it gets hot, then it kicks on and then the B+ goes up because of it?

Ideas?
 
I replaced the HV shurdown pot - no change.

Replaced the HOT (just for the hell of it) - no change.


Did some more poking around tonight. I have one end of D10 pulled so I can see if the HV comes on if the B+ goes up.

I noticed that after it's been on for about a minute, I can make the HV come on by doing a voltage reading at the non-banded side of D18 (black lead on the side wall frame). The second I touch that pad with my red lead, the HV comes up. I can't make it do this until it's been on for a minute. Any readings before that just give me a normal +120vdc reading (approx). To make it even more interesting is the fact that there is no D18 there - just a couple component legs that appear to have been snipped off. I checked another K7000 medium-res chassis I have here to fix (Shardian's) and his is also missing D18 and has component legs sticking out. Probably a factory mod of a pre-made standard-res chassis into a medium-res chassis?

But this is getting me to thinking that it might be in that area - C37, C38, D15, D16, D17, R98, R99 - perhaps a grounding problem?

Thoughts?
 
Last edited:
Well, I did find and repair a bad contact around the width coil, but that didn't help.

I think it's definitely heat-related. Something is heating up, then once it gets to a certain level, it suddenly operates and the HV works. I can shorten the amount of time by doing a reading at any pad on that trace to Pin 1 of the flyback, but only after about a minute. Otherwise, the HV comes up on its own after a few minutes. If I turn it off and let it sit for a few minutes, I once again must wait a few minutes before it'll the HV kicks in. If I turn it off and on quickly, the HV usually kicks in after about 5-10 seconds.

Now I just have to figure out what is causing it. I'm thinking about swapping out IC2 and/or IC1 to see if it has any bearing on the situation....
 
Not that I can tell. I pulled each and replaced them again. Continuity everywhere it needs to be as far as I can tell...
 
Well, it's fixed. What fixed it? I have no idea.

First of all, it seems that even though it looked like my other new flyback that I swapped in, it was actually a bad flyback that I had for some reason not thrown away (but have now). It looks brand new, except for the lightning that shoots out the side.

Anyway, I was going through my theory, swapped out IC2, C36, C38, C34, R101, and Q11.

I've been testing this on my bench without a yoke hooked up, which apparently is necessary for the HV to come on (G07's, 4900's, etc don't require this so I didn't think it would matter in this case). I only figured that out when I decided I might as well pull the chassis from my working test-bench standard-res K7000 and swap the STR3123 VR to the medium-res just to make sure it wasn't the problem (as it test ok). For some reason I decided to check the B+ on it on the bench first, and found there to be no HV just like my bad chassis. So, I thought, "Does it need the tube?" and stuck the med-res chassis back in the Paperboy and fired it up - and saw the lightning shoot out the side. I immediately shut it off, pulled the chassis and realized my mistake. Put the original new flyback back in and it's working perfectly.

So - apparently one of the parts I swapped in earlier was the culprit.

Now I can check out Shardian's chassis before I have to take the Paperboy back....
 
I have been following this very closely, and am highly disappointed in the outcome. I am glad you got it fixed, but damn it I want to know what the bad component was.
 
Well, maybe you'll still get lucky. I have Shardian's here. After putting in a VR, the fuse blew. After replacing a shorted C38, I am now in HV shutdown. Pulling one side of D10 lets it run, but the VR, C36, HOT, etc are good.

What's interesting is that if I pull one side of D10, I get the normal voltage reading (12vdc) at the non-banded side per the manual. But if I reconnect D10, I get 0.5vdc at that side, and virtually nothing on the banded side. Obviously something is pulling the voltage to ground. I've already replaced D10, D12, the HV pot, IC2, R61, C20 and C21, thinking it might be close to D10, but nothing so far...
 
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