CurtPalme.com Home Theater Forum

deronmoped · Joined: 03 Nov 2006 Posts: 1154 Location: San Diego

I installed a spare gain control board in my 10PG to try to track down a problem. This board did not fix the initial problem I was trying to fix (that problem is in my missing scan line post). But this spare board has a problem all it's own. It shows very faint rolling waves going across the screen at a 45 degree angle. You can only make it out in uniform areas of the image. Now I'm betting this is a adjustment to one of the pots on the board. Kinda reminds of old TV sets that had the roll control adjustment.

There are six pots accessible for adjustment when the board is installed. They are:

1) BKG.
2) WHT.
3) CONT.
4) B-WHT.
5) G-WHT.
6) R-WHT.

I guessing 4, 5, and 6 are for blue, green and red gain.

3 looks like a contrast control.

2 looks like a white gain.

1 I have no clue.

Anyone ever try adjusting these pots? Know what they do? Would they have anything to do with the rolling wave problem with this board?

Thanks, Deron.

gjaky · Joined: 05 Jun 2010 Posts: 2802 Location: Budapest, Hungary

If the rolling wave only appers with this particular board then I guess the problem is there.

The BKG. pot. sets the blanking level (in other words the black level) -I think... So this is what I would try to adjust first, try to decrease it a little. If that doesn't affect the problem then you should try the CONT. pot, decrease again.

It is a good idea if you sign with a marker how the pots were set before you adjust them.

CasetheCorvetteman · Joined: 09 Nov 2008 Posts: 6326 Location: Australia

Hmmm... I wouldnt be moving any of those pots if i were you, get your ESR meter out and test all those capacitors before you do anything.

**Curt Palme** · Posted: Sat Nov 30, 2013 1:50 am Post subject:

you will ignore me, but if you adjust any of those pots, you'll never get a white balance. the tubes, neck boards, and two video boards and I think the system board are all calibrated as a unit, and neither myself nor Doug Baisey could ever understand the NEC service manual's explanation as to white balance. I spent a whole week on it about 12 years ago, called Doug and he shrugged his shoulders saying he couldn't do it either.

Change one board, and usually the white balance goes to hell, so I always ended up repairing the bad board rather than trying to get proper white balance. As I said, tweak a pot, and you're screwed.

gjaky · Joined: 05 Jun 2010 Posts: 2802 Location: Budapest, Hungary

Well, the GAIN CTL board is not that bad from this point of view. Last year or so I've managed to replace the RGB amplifier IC to an almost completely other type IC (the IC was on a small add-on board), and I was able to recalibrate that board to perform similarly to my original one. However I admit the VIDEO OUT board has much more adjustments and therfore much harder to readjust, I didn't go far in to that (except for AKB readjustment...)

As I said above, before making any adjustments it is wise to mark the original standing of the pots, or even better to measure with a multimeter how is the resistance divided between tha tap and the two ends (and note them), to be able to restore the original settings.
The output of the GAIN CTL is nearly a unity gain signal, but the input selecting, and test pattern injection also happening there, along with some basic brightness/contrast adjustments, and beam limiting also happening (not to mention the sync processing part)
In further details the output signal is biased from the ground level (0V).
Now the above mentioned pots setting three things:
-setting the blanking level during retrace (affecting the brightness, eg black level in a way)
-setting the black level respect to ground (affecting black level on an other way)
-setting gain /white level

If you have a caliobrated more or less working GAIN board, with an oscilloscope and with a full black and full white test pattern you can find out what the controlls are exctly doing, and will be able to readjust a non calibrated GAIN CTL to the projector, just note down the original setup (also pay attention to the menu contrast brightness settings)

I'm not saying this is the only way to save a machine, just wanted to share my findings.

deronmoped · Joined: 03 Nov 2006 Posts: 1154 Location: San Diego

gjaky

That is what I was thinking, mark the pots so I know where to return them to. That's also a good idea, measure the resistance and note that, as a secondary way of getting them back to the original settings.

Curt

Your right, I will ignore you, I'm the type of person that can't leave things alone. Heck, I wouldn't want to use this gain board anyways with this issue, so basically it would sit in the, not to be used pile of boards.

White balance, who needs white balance. I don't think any of the boards in this 10PG are original to the machine. The picture still looks awesome though. I always tell people as long as flesh tones look correct, then your good to go. Being able to tell if a color is not perfect, you would need something to reference it to. A calibrated chart of colors. You couldn't even use white paper, it's probably has a lot of gray in it. Flesh tones are pretty much the only thing a person can relate to, everything else is subjective.

Corvetteman

Caps are a crap shoot, from all the testing I have done, they can measure bad and still not effect anything. I have boards with all kinds of caps leaking all over, yet the board still does it's job. (clean the board up stuff it back in the machine). I used to replace them thinking they would fix problems, more often then not, they didn't effect anything.

**Curt Palme** · Posted: Sat Nov 30, 2013 5:25 pm Post subject:

CasetheCorvetteman · Joined: 09 Nov 2008 Posts: 6326 Location: Australia

Well that last section just spelled out your issue clear as crystal.... Youve got leaking and rooted caps and yet you dont see this is an issue? Laughing

What you describe sounds to me like interference, which is probably not going to be adjustable pots anyway. Its more likely to be ripples in a DC circuit, which is reduced by what type of component?