Interesting hack repair on a CTC-5, your thoughts?

[drh4683]

Today I wanted to go after a problem that I never addressed on my 21-CT-7857, CTC-5 westcott. This is the same TV I posted on youtube last year. I noticed I never was able to cut off the brightness on this set, but the picture was OK so I left it alone. I recapped the entire set with exception of the lytics back in October of 2007. Today I decided it was time to fix it and make it right. I checked voltages in the video output circuit, they looked pretty good, boost voltages on the screen grids at the CRT looked good too. Voltage at the control grids of the CRT were fine so I don't really understand why the CRT isn't cutting off. The CRT tests perfect, so I pulled the chassis to look into this a bit further. Just to rule out CRT defects, I slid the chassis into that CTC-5 I picked up earlier this month as that also has a good CRT. Same problems, so I can rule out CRT issues. I took resistance tests on PW-400 and found a raised value 220 ohm, 1/2watt. It went up in value to about 420 ohms, nothing dramatic but still way off. Everything else looked good until I looked at the contrast circuit. I found this bizarre looking combination of components just hacked in, and soldered around the contrast control. I have no idea what this was all about. My only guess is that it was some tech who tried to fix the problem by sticking different parts in there.
In the pictures, you see a 50uF@50v mallory lytic, a 68 ohm paralled to a 3300pF disk cap and then a 1k from the contrast pot to ground. None of this belongs here according to my schmatics. I'm working out of a 1955-1966 RCA Field Service guide. This particular set has a CTC-5D chassis. I havn't touched a thing yet as far as making it original again per the CTC-5 schematic. Im curious if there are any chassis revisions for the CTC-5 that I'm not aware of? This can't be factory by the sloppy installation, but I want to make sure if I go back to the schematic as shown in my service guide that its indeed the correct wiring with component values for this particular chassis as I know alot of times schmatics don't always reflect what was done at the factory due to chassis improvements. Im curious if anyone else has a CTC-5 chassis can take a look and see if the contrast pot is wired like how I drew it up in the original circuit. I hope that when I put this all back together the right way, my brightness irregularities are solved.

[Steve K]

Doug:

That modification is how the contrast control in the CTC-7 chassis is wired.

Steve

[drh4683]

I just looked at the CTC-7 schematics and noticed the same thing and came to report back. Must have been a field modification to enhance contrast on the 5. I just looked on PW-400 and C410 is missing, a 22pf from pin 7 to 1 on the 12BY7. This cap should have been there per the original schmatic. There isn't even a place for that capacitor on the board in my set. This cap is not used on the CTC-7, so can I assume this wiring job was a modification at the factory?

[bgadow]

Maybe a year ago we had a discussion about this, it might be in old_tv_nut's thread ("I've been blessed"). This mod was put out to correct the odd interaction of contrast/brightness in that set. I added it to mine and found that it does make the set "normal" compared to the way the factory did it. I wonder if you have a bad cap that is giving the brightness trouble?

[drh4683]

ok. I left it alone, I just reworked the lead dress so it looks better and installed a new 50uF lytic by the pot as well. As far as possible bad cap, that shouldn't be a problem at this stage as I replaced all those wax caps back in 2007. For the record, I did test each of the new caps before they get installed. In fact, I specifically remember the brightness not cutting off when I first got the set before I did the recap. The TV had a great picture but little to no control of the brightness. Basically the brightness would go from "normal picture brightness" at fully CCW to "slightly brighter" fully CW. Brightness control tests fine.
I'll have to be on the lookout for out of tolerance resistors as thats basically all it can be at this stage.

[roundscreen]

Check r 601, c601 and the horz drive voltage going to the brightness control.
I take it the horz drive voltage must be there cause you have a raster.
I took the part location numbers from the rca field service guide so you should be able to find them.
I have seen a write up on this problem but never had the problem with my set's.
Ed

[drh4683]

Interesting. I'll have to keep checking around. If I turn down the brightness control, the contrast and turn all the screens and background controls fully CCW, the raster is still present, a drab green image is what appears, slightly below average viewing brightness.
I worked on a CTC-19 a couple years ago that had little control of brightness that I couldn't cut off as well. That set turned out to have a completely hacked up B-boost circuit with parts that didn't reflect any schematics. I changed it all back to factory and it worked perfect again.

[roundscreen]

Originally Posted by drh4683

Interesting. I'll have to keep checking around. If I turn down the brightness control, the contrast and turn all the screens and background controls fully CCW, the raster is still present, a drab green image is what appears, slightly below average viewing brightness.
I worked on a CTC-19 a couple years ago that had little control of brightness that I couldn't cut off as well. That set turned out to have a completely hacked up B-boost circuit with parts that didn't reflect any schematics. I changed it all back to factory and it worked perfect again.

This sounds like a problem with the grid voltages going to the crt. OR you may be right about the screen voltage being to high
Snoop around the 12AT7's and check voltages,Caps, Resistors and the coils.
The green gun seems to be the brightest color on the 21axp crt for some reason so that may be why you see the green stand out with everything cut off.
Still, The drab green image may be a clue as to what is going on too.
My favorite hack job is when they would turn the high voltage all the way up when the set had a weak crt or there was not enough horz width.
Of course I would see there kids watching the dam thing to boot
The next thing would be when they put in a 10 amp fuse in a circuit that takes a 1 amp fuse. . Man , Techs did some strange things back then.
Ed

[grimer]

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[Ampico-kid]

Originally Posted by grimer

I have a question,does the hack/mod actually improve the picture?

A little over a year ago I was working on my RCA CTC5 Aldrich and was having a heck of a time with the brightness and contrast controls. They seemed to interact with each other and neither worked as you would expect it should. bgadow (Bryan) came up with an article discussing a factory modification to correct this problem. I performed the fairly simple modification and presto....the brightness and contrast worked 100% normal. Yes...it improved the picture greatly!!

I don't know what the RCA engineers were thinking when they designed the set? It just didn't work right. But with the modification all is well.
Bob

[reeferman]

Just a thought, haven't had time to look. How does the -5N circuit compare to your -5D??

[old_tv_nut]

This was a recommended change, and when I tried it, it really helped. The original circuit makes the brightness vary with the contrast setting because it varies the DC in the video output stage. If you find my posting in "I've been blessed" you will see that my only mistake was wiring the contrast control backwards when I modified it. Otherwise, it is much better than the original circuit, as reported above. I had no problem getting cutoff, so I'd guess you have a problem somewhere with bad R values.

[John Folsom]

Wayne, could you post that circuit change again, I tried going through the 10+ pages of stuff in your I've been blesse topic, but must have missed it somewhere. Or just send me an email with attachment. Thanks!

[old_tv_nut]

it was in a post on 11-15-08, but i see it has disappeared -
[stupid system says I already uploaded it even though it's not there - here it is as a pdf]

[old_tv_nut]

note on left (ctc5) min contrast is wiper at top; opposite on right (ctc-7)