Using Marquee 8" VNBs in a 9" machine

He I do not have a direct answer to your question but I have LCP tubes in my Marquee and some of my neckboards are 8500 some 9500 I do not even know. I run contrast at 78 or so and with 72 Hz is is bright enough for my taste. Never had a 8500 neckboard go into protection or cut off allthough the pots must be for 8 inch tubes. So I do not know if this is really a problem. If it is just adjust the one that fires to fast? It could be a problem if you use 9500 neckboards on a 8500?

Hi Julian

This has been a lingering question for many years now and has never ended with a solution. But I may be able to clarify some things for you. I spent some time looking over the VNB and VIM schematics and any other documents on these boards. Here is what I found.

There are 2 current detection circuits on the VNB, I-OVER and I-SENSE, both feeding info to the VIM for interpretation and control.

The I-OVER circuit monitors the cathode current and if it is determined there is to much current will initiate a shutdown. To be more specific, it initiates an EHT-INHIBIT, SPOTKILL, G2-INHIBIT and sending contrast signal to zero. This can be seen on page 5 of the 00-260335-02P manual. This part is for protection only.

The I-SENSE circuit also monitors the cathode current but this signal when sent back to the VIM will adjust global contrast, there by reducing cathode current if to high a reading is determined in reference to the +5v rail. This is the control circuit. Pretty much just a loop circuit. Contrast signals are sent, I-SENSE monitors it, sends signal back to VIM, contrast is adjusted globally and sent again.

Having said all this we now come back to the 8500/9500 VNB's. If you now look at the schematics for these 2 boards you see that the pot resistor R86 (R64 on the newer VNB)is on the I-OVER circuit. The I-SENSE circuits are the same. This means that the contrast control on both 8500/9500 are the same and are not adjustable.

So the adjustment would be for I-OVER which is for protection shutdown. Naturally the 9" tube can handle more current so it could run a little higher before needing shutdown. That is "IF" they require a different setting. They may not.

I have not found any differences in any parts on the 2 different VNB's even though they carry a different board number. So it has been assumed that this adjustment may be what the difference is. I spoke with Scott at VDC about this some time ago and he indicates he knew of no documents on the adjustments of this pot and they were most likely set during manufacture.

So in my opinion, there is no difference in the beam current control (I-SENSE) circuit between the 8500/9500 VNB's since the circuits are identical. So using either VNB would be fine.
"If" there is a different threshold on the 2 boards I-OVER signal then it would make sense that the 9500 would be set higher and therefore make the 8500 VNB acceptable for the 9500. The only questionability would be a 9500 VNB board on an 8500. But, there is no confirmation there is even a difference.

But no adjustment of that pot is either required or recommended. It will change the shutdown threshold and not the current control.

I think I just said that, with an explanation.

Ah. i always hated those guys who are new to a forum, ask something and never show up again.
Seems i've become one of them. 
Let me explain something: I'm still interested in this. But I've seen another problem that is currently more important to me.
We'll figure this out, Gabor, i'm glad you wrote about it over on Curts forum. 
So thank you all for your answers, i'll come back to it.

One of my FGMs does misbehave in terms of geometry and vertical convergence.
I does not do horizontal convergence, pincushion and keystone. Each on all three colors.
Everything else works with the other FGM.
Both (the working and problematic) are 03-270354-02P, both with the strange black box on it.
I don't have a schematic for that board number,the only schematic i can find is 03-260354-02P.

Does anyone have the right schematic or can tell me the differences?
I stared at the 03-26... schematic for a long time and tried to interpret it.
Measured some resistors (all ok) that seemed fusibles to me.
And since that board consists of three amps (or six), each for every color /direction, i taught something common for all of them
is broken. But i cannot find it. 
If someone could explain the shortcuts in the schematics and where i should look..

As far as i understand this, the signal goes from the CLM to the FGM and then to the convergence amp which is connected
to the convergence coils. I cannot imagine how the FGM does affect convergence, but it does.
Regards, Julian

www.xymox1.com/Projector/Ultras/Focus/00-260354-02P.pdf

Did you want to link a schematic?

It appears to be the same as the schematic I linked.
I don't know for sure. Measured resistors r1 to r20.
Some of them seemed to be fusible's. But no, they're all ok.
Regards, Julian

Thanks, but I do not need a link.

You need to check a couple voltages on the FGM while in the projector and running. Hopefully you can access the board when installed but if not then you will have to attach some fly wires.
Looking at page 6 of the schematic you can determine where best to check for these voltages if the locations I give you are to difficult.

+ and - 15V_G at U9, pins 4 and 11
+ and - 5VG at U10, pins 5 and 14
+12v at U7, pin 1
-2V5REF at R93 or any of the other resistors using this supply

Let me know your results.

I am assuming you just have the pin numbers mixed up on U9 and U10 since the + and - should be the opposite on both.

How about U7, pin 1?