Interesting hack repair on a CTC-5, your thoughts? - Page 2 - Videokarma.org TV - Video

John Folsom · #16 12-02-2009, 03:31 PM

Thanks Wayne.

drh4683 · #17 12-15-2009, 09:08 PM

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)

DaveWM · #18 12-15-2009, 10:19 PM

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

roundscreen · #19 12-16-2009, 07:59 AM

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

drh4683 · #20 12-16-2009, 10:13 AM

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.

old_tv_nut · #21 12-16-2009, 11:03 AM

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)

roundscreen · #22 12-16-2009, 08:16 PM

Quote:

Originally Posted by drh4683

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

bgadow · #23 12-16-2009, 10:27 PM

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.

Robert Grant · #24 12-16-2009, 10:35 PM

Quote:

Originally Posted by Ampico-kid

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

bgadow · #25 12-16-2009, 10:47 PM

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

drh4683 · #26 01-03-2010, 01:03 PM

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...

John Folsom · #27 01-03-2010, 02:05 PM

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.

*zenithfan1* · #28 01-03-2010, 02:47 PM

Quote:

Originally Posted by drh4683

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.