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17" Orion CRT G2 voltage

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First post, by aitotat

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I have this 17" Orion 7E750BT9 that has too dim picture just like most old CRTs. But this one does not have G2 adjustment on the flyback transformer.

I found schematics only for some closely related model, 7A750 and those look very similar. Schematics show that there should be some kind of G2 adjustment, likely from service menu. I managed quite quickly to discover how to enter service menu: when power is off, hold down MENU and SELECT buttons when pressing power button. There won't be any picture until releasing MENU and SELECT but once they are released, picture appears and when entering menu, there are more controls than usual.

And it also has G2 ADJUST in there! Unfortunately it did not work. It cannot be changed. Since nothing else helped, I selected EI (EEPROM initialize) and that was a wrong move. It did change it, from 61 to 48 so to wrong direction. In the process all colors need to be re calibrated since those were reset just like everything else. But I still can't change the G2 voltage.

This is one of the cheaper monitors but looks nice and has sharp picture (after focus adjustments from flyback) and no noticeable geometry errors.

Any idea why G2 cannot be adjusted when it is in the service menu? Maybe the monitor needs some factory test bench signal or something?

But if service menu won't work, then maybe modify the G2_ADJ signal with resistors? I'm not expert at this and I don't fully understand how the adjustment works but in the schematics near flyback there are R916 then there are R911/R922 and R914. I would start by modifying one of those a little? Am I on the right track?

Here are the schematics and couple of pictures.

Reply 1 of 2, by Deunan

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I've never tried this myself but it might be as simple as a resistor mod: http://www.electronicrepairguide.com/modify-m … en-voltage.html
My last CRT monitor, before I've moved on to LCDs, had either great tube or some sort of automatic adjustment - it got darker but never so much I'd want to mess with it. Only when the cathode lost so much emission that focus voltage was no longer able to provide sharp picture in corners (well not without HV going above limit) that I figured it's time to scrap it.
That page also mention higher heater voltage, this can be effective but also very short lived. You might also try to incrase the heater voltage for a short time, a few minutes, then go back to standard. This can help clear any dead spots on cathode, this "fix" can actually last for months to years. But if it goes to current dark levels again afer, it means the tube is just spent.

Reply 2 of 2, by aitotat

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I started to experiment. I first tried to change resistance of R252 (on the schematics. It was actually R265 on this CRT neck board). It is between G2 wire and CRT socket on the CRT neck board. The neck board was glued well and I didn't want to risk breaking it so I soldered new resistors in parallel on back side of the neck board. I started with 1Kohm and went down to 390 ohm (so halving the resistance since it was 390 ohm resistor on the neck board) but no visible change.

Next I tried to change R914 on the main PCB. It is connected to ground just like on that resistor mod link above, although the circuit is not the same. R914 is 1.8M ohm resistor and I started to increase it step by step. But even at 3M ohm there was no improvement. So I tried to go opposite direction. I replaced the 1.8M ohm with 1M but no visible difference. I was almost about to give up at that point but then I tried 470K ohm. And surprise, it did improve things. In fact I think it is very close to optimal so I leaved it there.

I'm not done yet since colors are not bright enough because of the EEPROM reset. So next I'll have to adjust 6500K and 9300K color presets with some good CRT monitor as reference.

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