Zenith with jaundice

[bgadow]

I decided it was about time to tear into my Zenith 25MC33 roundie. This set was given to me by a used furniture store in WV back a few years ago. It only needed a focus rectifier & a tweak of the height control to give a good picture. The recent talk of tuner rebuilders set me to action because neither this nor the 25MC30 I have would work right on UHF. I pulled them both apart & I don't think the 33 was anything more than loose connections because now its behaving.

But, it has another problem that maybe somebody can help me with. The raster has a yellowish cast. I cannot adjust it out using any controls. Its not real bad but I would rather it was gone. I read in the RCA Color Pict-o-Guide that this can be caused by a bad cap between the color difference amp & b/y amp but that is for RCA & this circuit is different. I did change out a cap in a similiar part of the Zenith's circuit (a ceramic disc that was fat & cracked) but that didn't help. I also swapped the demodulators & the 2nd chroma/bandpass/killer tubes.

Has anyone run into this before on a Zenith? Other than that the set works well; great sound, power tuning, plus a nice cabinet.

[wa2ise]

There should be inside trimpots for adjusting individually the red contrast and brightness (gain, bias), for green and for blue. Sounds like the blue is a little low. The cathodes inside the CRT probably have aged some. You can either turn up the blue or turn down both green and red. Adjust the biases so the blacks look grey (this is with the user brightness control turned up higer than normal), and then adjust the gains so whites look white and not color tinted.

[bgadow]

I haven't been able to correct this with the screen & gain controls. I worked on this quite a bit last night & I'm afraid this is a crt problem. After reading up on it I measure the G1's. Blue was reading about 20+ volts below the other 2. I then unplugged the crt & retested; they were all three equal @ 225v. I have several crt testers but only one works right now & its a basic & tired Sencore. It shows strong readings from red & green but blue being basically dead. I took this with a grain of salt, hoping the gun switch on the tester was bad. Unless someone has other ideas, I guess I'll just set it up for the best compromise I can make & live with it against the day that I can find another crt. The one in there is a rebuild from '74, a Zenith with a Sylvania EIA #. If the blue was awake it would be great.

[kx250rider]

Have you tried doing the white balance setup from step 1? My old method is:

1) Color, Brightness, Contrast all at Minimum

2) G2 (aka SCREEN) pots Min

3) G1 (aka DRIVE) pots centered

4) CRT Bias (if it has one; either a switch or a pot) Middle

5) Service switch to Service

5 1/2) If there is a line now with no G2 turned up, you have the BIAS too high. Turn it down to have no line at this time.

6) Turn each G2 (aka Screen) up, one at a time, and see that it makes its color line. return each to Minimum as you go. If one or more does NOT, then turn the Bias pot or switch up, and try again.

7) Assuming that each of the G2 pots will give you a line, now turn each G2 carefully to where you get a BARELY visible WHITE line (all 3 colors the same brightness so it makes white)

8) Set the Service switch back to normal, turn the Bright, Contrast up to normal and see how it looks. If in the DARK areas of the picture it is too tinted, VERY SLIGHTLY adjust the G2 pots for best gray in the dark areas. If the brighter areas (whites) are tinted, then use the G1 (DRIVE) pots to correct that.

Anyway, that's my method to get it in the ballpark. The OFFICIAL procedure for white balance slighly varies from mfr to mfr.

This procedure does NOT apply to newer inline TVs, as many (especially Sony TVs and all Projection TVs) can get CRT damage if the setup is done differently from the exact book procedure for that TV!

Charles

[Chad Hauris]

Also try the boosted heater voltage trick on the crt filament for a little while and it should help it. This works best when one of the guns is just starting to go weak. The effect should last for a while once the normal fila voltage is restored.

[bgadow]

I tried the procedure Charles outlined, which varies a little from what I normally do. But...no luck. I can get a pic that is too green or too red but the only way to get a blank raster to show close to proper grayscale is with the bias up over halfway & both red & green all the way down. Then if you display video its all screwed up. The blue drive control has almost no effect. I swapped the taps for red & blue (no control for red on this chassis, it is full-on) but no luck there either. So I just dialed it in the best I could. Chad, I'll try the 8 volt trick & see what happens. I'll leave the tester hooked up overnight & try it tomorrow.

[kc8adu]

resolder the crt pins in case its just a bad joint
does your tester show any shorts?
last resort is to rejuvenate.

[bgadow]

Well, I ran it for 18 hours @ 8 volts and the needle for the blue had budged just a little. I was able to confirm the tester was okay by hooking it up to another 21FJP. I then tried brief periods of 10 volts but that gun is just too weak. After all that the balance was a little better but it's still yellowish. I would think that it wouldn't be a pin problem at this point? Anyway, this will give me a reason to keep looking for another crt. I've never stopped since having a spare is a good feeling. I really don't want to drop the money on a rebuild.

[andy]

---

[bgadow]

I'm not sure how well the rejuvenate function on this Sencore works. It has, as I recall, 3 levels & I chose the first & maybe the second but I didn't really see any improvement. The next better unit I have is a B&K-I found a bad cap in it so maybe after replacing that I could put it to use. My best unit stopped working after I doused it with contact cleaner trying to remedy a dirty switch.