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Crumbtrail TV Forums: TV Equipment: Projection TV Forum:
DIM Picture

 
 


Larrybia
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Jun 13, 2005, 2:25 PM

Post #1 of 2 (1443 views)
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DIM Picture Can't Post
I have a 1992 Mitsubishi Projection TV with a very dim picture. Green is a little better than the other two colors. I have turned up the drives on the CRT using the control at the base of the CRTs and can get a little better picture but still dim. I guess the CRTs are bad? Is there any reason to suspect low HV or some other problem with the brightness circuits? I say this because all colors are dim and I am hoping that there may be some other fix than replacing the CRTs.

Has anyone considered increasing the CRT filament voltage like they use to do in the olden days? I measured the filament voltage and it was 7 VAC. I am considering putting in a variac in series with the filaments and increasing the voltage to see if I can increase the CRT brightness’s. I may also try a separate DC supply. The filaments are not very bright as they are now. Any idea how much the filaments can take? Worst case is that I burn out a CRT and I just junk the whole set. With no other suggestions I will do the experiment. Let you know the results.

Larry



Larrybia
New User

Jun 14, 2005, 1:26 PM

Post #2 of 2 (1429 views)
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Re: [Larrybia] DIM Picture [In reply to] Can't Post
Provided this information just interested experimenter.



I disconnected the filaments and used a DC power supply to drive the filaments separate from the TV. I increased the voltage to 10 VDC. (It was 7 VAC in the TV.). At this DC value the current draw was 1 amp total for the three CRTs. Cold DC resistance is about 8 ohms each. I got a significant increase in the brightness of the TV and was able to adjust the drives to get a reasonable picture. This is a temporary fix for bad CRTs that will not cost $$$$. All you need is a constant current supply that is set for 1 amp. Be careful about just driving with 10 VDC because when cold the filaments are at a lower resistance and will draw more current to the extent that you may burn out a CRT. Do they make picture tube rejuvenators for projection TV CRTs?



Larry

 
 
 


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