Build Your Own Arcade Controls Forum

Main => Monitor/Video Forum => Topic started by: Vilepickle on October 08, 2015, 12:36:32 am

Title: K7000 monitor too dim even after adjustment
Post by: Vilepickle on October 08, 2015, 12:36:32 am

I recently got my K7000 25" working again. Previously when I had it working, it was bright enough. Now the screen is very dim, even after turning the Flyback up all the way and adjusting the black/contrast pots.

What are other reasons the screen might be too dim? I assume bad tube, but would bad caps do this?

Title: Re: K7000 monitor too dim even after adjustment
Post by: Ken Layton on October 08, 2015, 01:23:27 am

Yes, brightness problems can be caused by bad electrolytic capacitors.

Other causes can be bad solder joints or a weak picture tube.

Title: Re: K7000 monitor too dim even after adjustment
Post by: baritonomarchetto on October 08, 2015, 07:45:52 am

If the monitor went dim slowly in time (years), the problem could be the Crt, but if you had a dimmer image abruptly (or within a limited time period) the crt it's very unlikely to be the prob

Title: Re: K7000 monitor too dim even after adjustment
Post by: Vilepickle on October 08, 2015, 02:46:14 pm

Cool, thanks! I'll try to re-cap this and see if it fixes it. This happened after a cap kit and me having blown out some other parts when I tube mismatched this chassis. I imagine some of the caps may have been affected too.

Title: Re: K7000 monitor too dim even after adjustment
Post by: baritonomarchetto on October 09, 2015, 12:45:31 am

Check components between the flyback transformer and G2 too

Title: Re: K7000 monitor too dim even after adjustment
Post by: Vilepickle on October 11, 2015, 09:56:00 pm

Tried the re-cap with no luck. Here's an image for reference on the brightness. Flyback adjustment and brightness/contrast don't have much effect.

At this point I do think it's the tube, but what components between the flyback and G2 would be suspect?

Title: Re: K7000 monitor too dim even after adjustment
Post by: baritonomarchetto on October 12, 2015, 12:40:27 am

There are only few components between G2 and EHT, so... anyway, if both focus and G2 (screen) have only small effect it must be something before the fly... check your B+.
Is the neckboard well inserted in the back of the tube?

The image is of no help: take a closer pic of the screen...

Title: Re: K7000 monitor too dim even after adjustment
Post by: Vilepickle on October 12, 2015, 07:54:57 pm

One side of the B+ is 160vdc, the other is 111vdc. Is the 111 too low? Maybe the cause of the brightness being so low?

Title: Re: K7000 monitor too dim even after adjustment
Post by: grantspain on October 12, 2015, 10:50:15 pm

is it a k7000 or k7400?

Title: Re: K7000 monitor too dim even after adjustment
Post by: Vilepickle on October 17, 2015, 12:05:56 pm

It's a k7000, the 25" version so k7191 I think is the designation.

I'm waiting for my tube rejuvenator to give that a try. I'd prefer not to call it quits on this setup since the chassis is rebuilt at this point, but finding a replacement tube for a yoke swap seems like a potential option if that route fails.

Title: Re: K7000 monitor too dim even after adjustment
Post by: grantspain on October 17, 2015, 07:09:09 pm

where are you reading the b+?
it should be 130v(pretty sure 123v is for 19") at c57,111v at that point would certainly cause it

Title: Re: K7000 monitor too dim even after adjustment
Post by: baritonomarchetto on October 18, 2015, 04:33:43 am

Avoid rejuvenating a possibly "good enought" tube: there's the risk of killing it by opening of the filament.
Rejuvenation is recommended only if you are 100% sure the tube is weak or in the case of a shorted to ground filament.

Title: Re: K7000 monitor too dim even after adjustment
Post by: Vilepickle on October 19, 2015, 09:28:25 am

Quote from: grantspain on October 17, 2015, 07:09:09 pm

where are you reading the b+?
it should be 130v(pretty sure 123v is for 19") at c57,111v at that point would certainly cause it

At the large white resistor on the side of the chassis (both sides mentioned here), pretty sure this is R301. I didn't measure right at C57.  If that is too low perhaps that resistor is just bad?

The tube is also missing red and I've seen red using this chassis on another tube, so I know that at least is an issue which is tube-related.