65 maggie roundie

[timmy]

hey all, i got this 65 maggie roundie and its always had a dark picture as long as i had the set but i recently replaced the boost diode which was infact bad and it brightened it up very little but now im able to see the color lines on setup however its still very dark. all tubes are ok and sometimes i turn it on only to have the color bands that sometimes indicate a bad crystal but i changed that also and still is not right again tested tubes and subbed a few in the circuits that may effect this. any thoughts on the dark pic and the color bands, which go away after warmup. thanks also the crt is fine and the bias has to be maxed clockwise to get any brightness worth watching.

[Tom S]

What's the boost voltage?? 600 to 800 be good. Filter caps in boost circuit? Just throwing that out there

[timmy]

changed all the caps when i first got it. boost voltage i dont know yet i didnt check it but the diode was bad and i changed it.

[rca2000]

What does the cathode voltage on the three CRT cathodes look like? If it is real high-- and not varying much with the video content--that means the tube is NOT being driven enough. Tube color chassis ALWAYS had a seperate video out tube--IIRC. You might just be seeing the COLOR---without video behind it.

[rca2000]

Originally Posted by timmy

changed all the caps when i first got it. boost voltage i dont know yet i didnt check it but the diode was bad and i changed it.

Also--MEASURE the screen voltage to all 3 guns. There may STILL be a resistor way up in value or something. It MUST be at least 500-800 volts available. If you only see about 200 or so--this is likely the problem. Screen resistors went out a LOT on SS sets--so am sure a tube set could suffer the same problem.

[Bill R]

You need to check the video amp and video output circuits. Some chassis used a 6GH8 as a sync and video amp before the video output. Look for bad coupling caps and out of tolerance resistors. The color burst is taken after the video output. So, if the video is low causing dark picture you will also have problems with color sync. These problems often make it look like a bad CRT.

[zeno]

Originally Posted by Bill R

You need to check the video amp and video output circuits. Some chassis used a 6GH8 as a sync and video amp before the video output. Look for bad coupling caps and out of tolerance resistors. The color burst is taken after the video output. So, if the video is low causing dark picture you will also have problems with color sync. These problems often make it look like a bad CRT.

And dont forget the blanking tube, easy to forget. Testers only do so
much so use a known good tube when trying tubes.

73 Zeno

[timmy]

ok ill have to check these voltages and ill get back to you guys. thanks

[Zenith26kc20]

My T-933 looked like a bad CRT when I got it. Horizontal blanker was causing it. Check that first!