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Author Topic: Sanyo 14" Monitor - Color switches to full at halfway point  (Read 644 times)

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nexusmtz

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Sanyo 14" Monitor - Color switches to full at halfway point
« on: August 18, 2018, 05:55:45 am »
I recently picked up a game with a Sanyo 14" color monitor which closely matches Atari Monitor TM151. The problem with the monitor is that a gradient test pattern of 16 levels only displays 8, with the rest of the gradient being the maximum color.

I know that's not the greatest description, so I've attached a picture of the Red test screen. Blue and Green are the same. If I adjust the bias pots, the lower half of the gradient fades/disappears, but the upper half stays constant - no gradient appears in the right hand part of the screen. The brightness at the front of the monitor and screen control on the neck only change the overall picture (as would be expected.)

If have tested the board (JROK Mylstar) on another monitor and confirmed that the gradient covers the entire screen, left to right, as shown on page 23 of the MSF manual.

The monitor had been capped a couple of years ago (according to a sticker on the frame), but I don't know if the problem had presented before or after that point.

Any suggestions on where to look? The RGB path seems to keep the colors separate (3 of just about everything) so I can't determine what is shared between them that would have the same effect on all 3.

Thanks.

big10p

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Re: Sanyo 14" Monitor - Color switches to full at halfway point
« Reply #1 on: August 18, 2018, 06:29:38 am »
When you say "bias pots", are you talking about the RGB cutoff and gain pots which are usually located on the neck board? Adjusting these pots would normally fix the problem.

There's a pretty good guide to calibrate monitors, here:
https://wiki.arcadeotaku.com/w/How_to_Correctly_Set_Up_Monitor_Colours_and_Brightness

nexusmtz

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Re: Sanyo 14" Monitor - Color switches to full at halfway point
« Reply #2 on: August 18, 2018, 05:14:14 pm »
Yes, the cutoff pots are labeled 'bias' on the neck board. The manual refers to them as "Bias (Cut-off)", so we're talking about the same ones. The R+B gain pots don't affect the picture much (if at all), but I confirmed that the value on the meter changes when they're turned.

The one thing I noticed in regard to the cutoff pots is that the Sanyo schematic shows B as 10K, but R+G as 200. All 3 of mine are 10K, which seems to make more sense. The only other similar neck board I've seen pot sets for is the Sanyo 20EZ, and that uses 10K for all three. Also, since the problem is present on Blue anyway, that discrepancy doesn't appear to be the source of the issue.

Thanks for the Color/Brightness procedure. I ran through that, but I can't get the top half of the gradient to vary.

big10p

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Re: Sanyo 14" Monitor - Color switches to full at halfway point
« Reply #3 on: August 19, 2018, 05:26:59 am »
Sounds like you have a component failure, somewhere. Can't really help with that, sorry. Maybe a monitor expert here can point you in the right direction?

nexusmtz

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Re: Sanyo 14" Monitor - Color switches to full at halfway point
« Reply #4 on: September 04, 2018, 04:02:48 am »
Short update:
I noticed that the isolation transformer wasn't a stepdown unit and had been feeding 124V to the monitor instead of 100V. I put in a 100V transformer, and tried B+ at 105 per the manual, but it didn't make a difference other than introducing some flagging that went away around 109.

Any guesses as to what might not have tolerated the higher voltage that could explain the symptom?