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.

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Doug:

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

Steve

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?

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?

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.

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

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.

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. :scratch2:. Man , Techs did some strange things back then.
Ed

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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

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

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.

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!

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]

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

Thanks Wayne.

It looks like Wayne and I are both continuing with an on-going approach to CTC-5 troubleshooting.

I took voltage and resistance measurements around PW400 (luminance board) and found a few out of tolerance resistors, but nothing significant. They got replaced but I did not expect to see a cure to the lack of brightness cut off.

Needless to say, nothing changed. I'm really at a loss here as I'm showing proper voltages at the cathodes, grids and screens at the CRT. I noticed I had no control on the green background control but then discovered I have a loose tube socket for the G-Y demodulator. This may have been the cause of the green background control failing to work. I noticed I didn't have much green in the picture anyway. The chassis was on the bench when I made this discovery, so I've yet to test it, but I do read proper voltages on the grid leads when turning the background controls back and fourth. I switched demodulator tubes and a new tube made much better contact, almost like the pin diameters of the previous tube were slightly too small. Since I pulled the sheild off the chroma board, I took some resistance measurements for fun, everything seemed OK trying not to be fooled by resistors paralled over peaking coils.

This TV has been a challange to say the least. The contrast control has been left per the modification/change of the CTC-7 as it was when I got the set.
What else could I be missing here? Im so used to working on 60's sets like a CTC-38 chassis. The CTC-5 is a whole new situation! I thought hobbies were supposed to be relaxing :) (I wouldn't have it any other way I guess, I like a good challange)

it will be that much sweeter when (not if) you get it figured out.

Try pulling out the video output tube when you put the chassis back in to see if the problem goes away. I am sure you checked it, But is the high voltage and horz output tube current set properly? Double check the replacement caps you put in. If you have a working ctc5 set, Put them next to each other and check voltages on both chassis to see if they are the same.
Ed

Yes, in these scenarios, its nice having the "good working chassis" to compare voltages. Unfortunately, I gave the other CTC-5 I had to friend and I know the brightness circuit was working properly on that one too. This was actually a problem before I recapped the set, so capacitors are basically eliminated as a cause at this point. I used nothing but the best in the recap (spragues) and I tested them for capacitance and leakage prior installation as reassurance. If I remember correctly (not at home at the moment) the brightness did not go away after pulling the 12BY7 video output. That should be a give away that the problem is elsewhere. I'll find it...eventually.

Doug, are you measuring CRT voltages right on the pins? Any possibility of a bad solder joint on the pins? (just trying to check off random possibilities)

Yes, in these scenarios, its nice having the "good working chassis" to compare voltages. Unfortunately, I gave the other CTC-5 I had to friend and I know the brightness circuit was working properly on that one too. This was actually a problem before I recapped the set, so capacitors are basically eliminated as a cause at this point. I used nothing but the best in the recap (spragues) and I tested them for capacitance and leakage prior installation as reassurance. If I remember correctly (not at home at the moment) the brightness did not go away after pulling the 12BY7 video output. That should be a give away that the problem is elsewhere. I'll find it...eventually.

When you run into hard to find problems like this it is time get out the oscilloscope. Put the chassis back in, Connect her up and check waveforms.
I would do it with the crt cut off and the brightness problem should stand out. If the problem was there before you did the recap is it possible you missed one? I have done that many times.
One other thing to check, What does the brightness do when you turn the color up and down, Do you see a change in the brightness when you adjust the color level?
I am just throwing ideas at ya and it will be interesting to find out what was wrong with your set.
Ed

Can't offer much help, except to mention that on mine there were issues on the bottom side of the video board. As I recall, that one has a shield underneath. I had a terminal strip/cap/resistor all turn to crispy critters.

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

Perhaps they were thinking....

Let's see, how can we get the price below $ 500....cut wideband I demodulation......cheapen the cabinet...Hmmm, electrolytics aint cheap, any way we can get that one out of the contrast circuit???

I wonder of this omission was common to all CTC-5s, just the less expensive "Super" models, or unique to the least-expensive Aldrich

I think it was across the board, mine is a Deluxe and had the odd circuit.

I started looking into this chassis again. One thing that is really holding me back is the ability to set this chassis up on a test jig, the repeated pulling of the chassis in and out of the cabinet is very annoying. I noticed one thing, from a cold start up, all guns cut off and after about 3 minutes, the green slowly starts to brighten up, even with all screen, background and brightness controls fully ccw. However, within that 3 minute warmup period when things seem to be working correctly, I can get the controls to properly light up the screen and then extinguish.
I think the only thing I can do now is reverse the boost leads on two guns of the crt to see if the green still lights up. Should green light up if say blue and green boost leads reversed, then I know the crt is at fault and Im out of luck. Should the other gun light up, then I know the trouble is in the green circuit. The boost circuit is simple though, since all other guns are working right, the correct voltages are there. The only thing I can think of is that perhaps the resistance of the background or sreen pot changes value as the set warms up.
This is a real fun one to say the least...

You should be able to diagnose what is going on my simply measuring the DC voltages on the 3 G2 (screen), G1 (grid) and cathodes. Compare the behavior of the reg, green an blue gun bias to figure out what is going on.

One thing that is really holding me back is the ability to set this chassis up on a test jig, the repeated pulling of the chassis in and out of the cabinet is very annoying.

It might also help if you make some extension cables so the chassis can sit on your bench while hooked to the CRT in the cabinet. Just a thought, it might be handy in the future when working on similar sets.