CurtPalme.com Home Theater Forum

[anqgiap]

I replaced the HVPS a couple of weeks ago, I noticed when I turn off the PJ, the light zoomed in/down to a tiny light dot in the center of the screen before completely went dark. Is this normal? I never notice that before replacing the HVPS. Appreciate the feed back.

[David_Web]

Do not run the pj until it is repaired.
The spotkill circuit is not working properly and you can spotburn the tubes!!!
Others will tell you what you should do I just tell you what I remember from reading around so you don't destroy the tubes.

[anqgiap]

Thanks David. Does anyone know what cause it and how to fix it?

[stefuel]

Absolutly DO NOT try it even one time more. You can ruin all three tubes in a heartbeat. You may have done that already. Carefully inspect the centers of the tube faces for tiny discolored dots. If they are there, it's to late.

[anqgiap]

Well, I looked at all three tube faces, I don't see any discolored dot yet - just solid surfaces. What do I need to replace to fix this problem?

[Nashou66]

The spot kill on a marquee has a few places the VIM and the VNB's and I think one more place I cant remember but maybe the HVPS.

Ok Just Happened to have my printed out service manual in front of me.

" The spotkill function consists of of two separate actions. on the VIM, the B-Limit signal is shorted to ground by activating a 2N7000 FET thereby reducing the CONTRAST and video signals to zero. On the VNB, Spotkill signal causes the Cathode(K) to go to its most positive value(+85Vdc) and the G1 to go to its most negative value(-85Vdc)."
Before this they explain the B-Limit:

" A B-Limit signal is generated for internal use on the VIM. By sensing the magnitude of the of each CRT beam current, the CONTRAST signal is limited to a value such that the avearage beam current does not exceed a safe limit."

So i would think it be a fault some here on the vim(maybe the 2N7000 FET) if you see it on all three tubes or it could be a pullup resistor in the HVPS not functioning also, that pull up resistor is part of the system for scan fail and shuts off the HVPS during bandswitching also if there is a failure somewher else in the system.

but i think its the Spotkill system not working, to be safe you'd have to swap out all the boards mentioned above so thay can be tested in a test machine where we dont care if tubes get killed.

Now to narrow it down was it on all three tubes or just one? if all three then it could just be the VIM or the HVPS, if on only one it be the VNB from the description of how it works above.


Another Point it looks like if you do have it one you should lower contrast to zero before powering off, not sure if its still safe but I would think so, anyone else know for sure?


Athanasios

[Tim in Phoenix]

Hello


If you have a meter, try measuring G2 on one of the neckboards at the tip of resistor R76. G2 in operation is 500 to 600 volts dc. Unplug the filament feed at mb P14 before measuring, to protect the tubes.





Spotkill should slam G2 to zero quickly at shutdown. If this is not happening, take the upright board from the back end of your failed hvps and try it in the newer one.



.

[Nashou66]

Tim can you explain it a bit more, for the HVPS to slam G2 to zero doesnt it need to be sent a signal to do so from somewhere? what happens if that circiut does not send the G@ to zero signal It wouldnt be the fault of the HVPS then. what i am asking is How would you know when it definitely is the HVPS if it could be some other part of the spotkill system malfunctioning?

Athanasios

[Tim in Phoenix]

[Nashou66]

Thanks Tim, I need to start thoroughly reading the post!!! How did I not see he just replaced the HVPS!!! Duh!!

thanks Tim,

Athanasios

[anqgiap]

Thank you all for your responses and advises. I have two VIM in this PJ and this happens to both VIMs; it's odd that both of them went bad at then same time. Again, this only happens after the replacement HVPS went in so I think it's the culprit more than anything else. I couldn't tell if that lighted dot formed by all three tubes or a single tube, it happened and went away in a couple of seconds. I have used the PJ for a few times and have not notice any dead spot on the screen or discolored dot on the tubes' faces; I thought it was OK but wanted to ask just to be safe. I do have a couple of questions:

If spotkill is not working properly, how long does it take to burn the tubes' surface?
Can I turn down the contrast as suggested here to 0 before turning the PJ off and still be safe?
Looks like I need to get another HVPS to test?

Thanks again.

[David_Web]

It usually take a hundredth of a second or so if not less. Can't know for sure. It happens once and the tubes are ruined beyond use.
I would not rely on turning the contrast down.
Maybe unplugging P14 would be safe but still.

You are lucky that the brightness and focus goes to zero faster then the scan yokes.

If any other fault would happen causing the pj to shut down I think the tubes would be ruined. If not already.

[Tim in Phoenix]

[anqgiap]

As others have said, I should have had spot burn by now - I looked closely at the dot at turn off, it's blue with a small halo surrounding. Could it be the VNB for blue tube? Before I though I saw a bright white spot but recently it has changed to blue ??? The spot burn has not happened, could it be something else? Thanks.

[stefuel]

[anqgiap]

I do not have the old HVPS to test the board; I'm getting a spare one from Curt and will try from there. I just noticed that when turned off, the blue and red went dark right away, the green stayed dim for a while (much lonnger than the other two) before went dark; does anyone know why???

[garyfritz]

If you were looking at the tube faces, that's normal. The green phosphor has much longer persistence, and will keep glowing for several seconds after the beam is removed. You shouldn't normally see it on the screen, though.

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