Another RCA CT-100 discovery B8001599

[Penthode]

Originally Posted by dtvmcdonald

I commend you on those plots. They are truly top notch.

Thanks.

Want to tackle the chroma demodulators. But did purity convergence and gray scale next.

I found it difficult to get a consistent grey scale. I find when I advance the brightness and contrast beyond a certain point, a high luminance level (towards white) causes the picture to go out of focus and the raster expands suggest poor HV regulation. Need to look at the HV rectifier...

What is the technique to get a good consistent grayscale?

[dtvmcdonald]

Good grayscale???

Answer: CONSERVATISM and a dark room.

You need a dual probe scope that can subtract. I have a pile
of Tektronixs that do fine.

Attach probes to the CRT red grid and any CRT cathode (i.e. either side
of Sam's R284.) Scope in subtract mode so you measure the grid to cathode voltage.

Adjust for a good picture, not very bright, well below blooming or other "obvious"
problems. The image you are looking at needs lots of both full black and
full white, and preferably full red too.

When you turn up either contrast or brightness you will note double clipping of the red.
and thus compression.

The DC restorer clips one extreme and the CRT itself clips the other. Turn up
the contrast too much and they BOTH clip. You want to avoid this like the plague,
its bad for the CRT and makes the picture turn cyan.

So what I did was to make up little arrows out of red and blue masking tape.
I set the contrast and brightness at the middle of the "usual" range and attached
a blue arrow to both knobs, pointing up. I then turned up the contrast to just
below the double clipping point and attached a red arrow to it.
i returned the contrast to the blue arrow and turned the brightness to the double
clipping point and attached a red arrow to its knob.

So, when adjusting the set, if you DO set one knob above the red arrow, you
need to set the other one below the blue arrow by an appropriate amount.

I also added blue arrows to hue, color, and tone.


You could also attach the scope across R284 and set the arrows so you never
get (dc) current flow (you will of course get spikes at sharp edges.)

One other note: at constant line voltage, my settings for these have remained
correct for three years now! Never Twice the Same Color is simply wrong.

[Penthode]

Thanks for the wealth of information. I was earlier pondering why the focus and HV supplies were drooping on video high average luma level scenes. I suppose it was driving the tube to hard. Everything settles down if you back off the contrast.

The set otherwise appears healthy apart from one odd anomaly: when powering up, after about 40 seconds the sounds comes up and you hear the characteristic HV crackle. The picture is initially low contrast. About after two minutes the contrast comes up. It is as if one tube in the video chain is extra slow warming. When testing the tubes, the emission was good and I have replaced many (most) of the tubes.

Any thoughts?

[Electronic M]

You could do the 7 second test, pull one tube in the video chain while it's warmed up, plug it back in 7 seconds later and see if contrast returns fast or slow and repeat till you find your culprit or through all possible tubes that could cause it.



I have to commend you for actually listing the parts you used for the peaking coils...Other brilliant minds here have gone through the same effort without listing part numbers, and only giving a vague notion of the brand and the process with their specific test equipment, so someone without the right gear can't duicate their results.
It's good to teach the process, but it's also important to explain the results of the process so others can check the work, and benefit from it.

[kvflyer]

I guess that these sets were used in a rather dim room and that was normal. When I worked for E.J. Korvette, (1966-1967) color sets were in a special room and the light was dim. I have a CTC-9 in queue and I must remember this when I finish restoring it. I do have a raster so far!

[Penthode]

Originally Posted by Electronic M

You could do the 7 second test, pull one tube in the video chain while it's warmed up, plug it back in 7 seconds later and see if contrast returns fast or slow and repeat till you find your culprit or through all possible tubes that could cause it.

After starting pulling tubes and plugging them in and not finding the culprit, I decided to try this: I measured the AGC line bias when it was operating okay. It measured about minus 15 volts. I put my bias box across the AGC line, set it to -15 volts and then switched on the set. The picture popped up normally as I would have expected as the high voltage appeared. I swapped the 6AN8 AGC keyer tube and no change. Something is amiss with the AGC.

I will put the scope to the circuit tomorrow.

[Penthode]

Spent much of yesterday determined to solve the AGC problem. AGC was wonky:picture would appear about a minute late and if you advanced the AGC control and returned it, you would enter picture overload and it would not recover unless you detuned.

Checked the waveforms (courtesy of the Sams notes) fror AGC keyer in and keyer plate and was correct.

I had changed the paper capacitors and checking resistors, the 4.7 Mohm resistor (used for agc delay) was open. After replacing the resistor, the AGC system slowly oscillated between overload and positive video.

As it is a closed loop system, tried checking tubes. After about four hours, it occurred to me to check the AGC line for wiring. Recall the IF section was badly rusted due the mouse inhabitants. I finally discovered a little choke feeding AGC voltage to the 2nd IF stage open! It was affected by the mouse urine and had earlier tested okay after I cleaned it and reinstalled it. The corroded leads broke of as I removed it a second time. So I performed an autopy on it and in the process found it was simply a winding of a single layer of AWG 44 wire. I took a 1 Mohm 1/2w resistor and made replacement holding the resistor in a variable speed Ryobi power drill.

Installed the choke and AGC worked properly and as it should.

There was a happy side benefit to all this: I checked again the IF response due to the added choke. Went through the full sweep alignment in an hour as outlined in the RCA instructions. Recall from my earlier postI had to make addition tweaks to stages to minimize a bump in the response around the video carrier? Wekl that annoying bump disappeared after the choke fix. The RCA instructions said you only need to slightly tweak T109, T110 and T111 (third fourth and fifth IF stage bifilar transformers) to get the proper response. Sure enough, the final oberall sweep was very close and as RCA prescribed, only those three transformers needed minor tweaks to get a good response!

I have included pictures of the chokes and the set as it is now performing.

[Penthode]

Some off air pictures.

[kvflyer]

Originally Posted by Penthode

Some off air pictures.

It doesn't get much better than that. I would submit to you that only a few left the factory displaying a picture that good. Maybe I am wrong.

[old_tv_nut]

Great detective work!

Those pics look super! White balance and purity look about perfect. Can you capture some pics from an old favorite source like The Wizard of Oz?

[etype2]

Looks much improved from the previous shots. You’re well on your way. Congrats on bringing another CT100 back to life.

[Penthode]

Spent a bit of time on the set tonight. It is working well. Issues are fairly minor. Here is a rundown of the good vs bad points.

The Good:
1. Repeated the full IF sweep alignment. This was to ensure each preliminary stage of the IF alignment fell into place so that the final overall sweep alignment only required minor adjustments. Succeeded with only having to make minor tweaks to get the proper response. With the entire RF - IF alignment, chroma bandpass and demodulators complete and successful peaking coil replacements for the luma, I and Q channels, the picture is nice and sharp with no ringing.
2. Convergence very good. Better than I had expected with the electrostatic convergence.

The Bad:
1. Cannot get as good purity as I think it should. Could not get rid of a green contamination in the lower right raster. I removed the chassis to demagnetized as best I could with a degaussing coil to no avail. Was able to conceal it with compromises.
2. AGC problem. Although I fixed the open choke, the AGC still provides "bi-stable" operation. Set starts up okay. However, if you interrupt the signal briefly and reapply, the the AGC voltage drops to zero and the video overloads. By detuning with the fine tuning control and retuning the picture returns. The problem disappears if I remove the AGC delay 4.7Mohm resistor (between AGC line and B plus). Checked all components and wiring. (Recall I had to disassemble the IF chassis to remove the heavy rusting).
3. 15GP22 socket intermittent. The red dropped out this evening. Checked okay the voltages for the red gun at the tube socket. Flexed slightly the socket and the red returned. Suspect it is the Red Screen pin 4 connection which is iffy. I earlier cleaned a rebuilt the CRT socket on the set. The 20 pin plug on the tube it looks like a large chunk of the socket previously broke away and the previous set minder reconstructed it with an epoxy resin. Appears they used a mould for the resin and was well done. However it would be nice to put a good original socket on it. Does anyone know if there are any available?

I hooked up the DVD player and played the prerequisite Wizard of Oz...

[old_tv_nut]

Coming along nicely!

[dtvmcdonald]

Quote: Spent a bit of time on the set tonight. It is working well. Issues are fairly minor. Here is a rundown of the good vs bad points.

comments interspersed:

The Good:
1.
2. Convergence very good. Better than I had expected with the electrostatic convergence.

IF you adjust adjustments 82-88 (of 88!) carefully you can get almost perfect convergence. These are the tilts and translations of the yoke.


The Bad:
1. Cannot get as good purity as I think it should.

My first CRT, now gassy, had great convergence. My current one has
a bit of red in two places that I just can't get rid of. Its really very minor,
even on a B&W picture.


2. AGC problem.

I had the exact same thing. Simple cure: since you are running on a
local modulator, signal is too strong. Install a pad right at the set terminals. I have a cheapie balun followed by a balanced 300 ohm pi-pad made from 1/2 watt resistors. That instantly cured this infuriating problem. Signal to noise is still essentially perfect. I recall that my uncle, in 1954, had the exact
same problem ... he lived 3/4 mile from the tower!
But it was already known, since his huge B&W set had the same pad necessary. So does my Hallicrafter's 7 inch set. Other sets work fine with the huge signal.

3. 15GP22 socket intermittent.

I had the same problem on a filament terminal. I got a replacement socket from the ETF but did not use it ... I just carefully bent the contact a tiny bit
in the original socket. The replacement would work, but its not the same constrution and would need some machine shop work to be reliable.

I hooked up the DVD player and played the prerequisite Wizard of Oz...

[etype2]

Very nice images! My opinion only, looks to lack a bit of contrast, can you adjust? I know this is subjective.

Try contacting Steve McVoy at the Early Television Foundation. I received one for my 15GP22.

EDIT: I’m sure the images are much better than the camera captured.