makvision 2427 b+

[lilshawn]

anybody know what the makvision 2427 b+ is supposed to be. i'm having some issues with the screen going brown when bright white gets displayed, i have a feeling the B+ is not set right...

(i sent this chassis off to get fixed about 6 months ago (bad SMPS tranformer)) been sitting in the shop ever since today.

[grantspain]

any idea of the true make/model-all makvision are rebranded from rodotron or wei ya i believe

[lilshawn]

CH-288 looks like the last of the pentranic/first of makvision line where the chassis where the same/similar.

running in 15k mode...

The B+ right now is about 104 to 106.5 depending, measured from the output side of L501

[grantspain]

I do not have any info on that chassis but I think Rick Nieman could help as I am sure he used to use these.

[lilshawn]

yeah, I e-mail'd him a few days ago, but got

turn the contrast down and try again. Sounds like there is too much beam current causing the shadow mask of the tube to heat up and “Dome”

 i did ask about the value of the B+ in my original request but was not answered. i replied telling im i adjusted it but it didn't seem to help. and mentioned the image seemed to be breathing a little.

i took a look at the main filter cap and it seems to be okay. (ESR metered) the voltage of the B+ doesn't seem to be varying any more than i think I've seen others...but i don't want to start cranking on the b+ cause i don't particularly feel like replacing the HOT.

[Rickn]

Yes around 108 at the cathode of D501.
No this is not an issue of the B+.
What it is is that the high beam current hits the shadow mask of the picture tube. The shadow mask heats up and "bends" this is called doming. Depebding on the tube and the material used for the shadow mask it may apppear worse than others. For example an invar shadow mask will absorb the heat better and dome less.
Only fix here is to reduce the brightness and or contrast.

Good luck.

Rick

[lilshawn]

Well, I turned down the brightness to 50% and contrast to 0%. i turned down the screen control and it's slightly improved. I had to turn up the gains on the RGB to get a decent pic. i ended up having to turn the brightness up some more to maybe 60 or 75%...left the contrast at 0%

the whites are staying okay now. but when it gets displayed it's almost draining the colors out or getting darker. seems to take some time for the picture to settle when the image changes.

example:

sega logo (blue) on white background. starts out nice then drains the blue to black as the white settles to final brightness. (about 1-1.5 seconds)