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  #361  
Old 07-17-2015, 11:12 PM
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old_tv_nut old_tv_nut is offline
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I have to comment that I can't think of a good reason to include 4.5 MHz traps in a video input circuit, unless the input was from a separate RF receiver. Why would anyone deliberately limit the bandwidth?

Edit: I can think of one reason - to create the same awful chroma delay and quadrature distortion that an IF amplifier produces, so that it matches the delay lines in the TV chassis.
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Last edited by old_tv_nut; 07-17-2015 at 11:16 PM.
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  #362  
Old 07-18-2015, 07:44 PM
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Quote:
Originally Posted by old_tv_nut View Post
I have to comment that I can't think of a good reason to include 4.5 MHz traps in a video input circuit, unless the input was from a separate RF receiver. Why would anyone deliberately limit the bandwidth?

Edit: I can think of one reason - to create the same awful chroma delay and quadrature distortion that an IF amplifier produces, so that it matches the delay lines in the TV chassis.
Wayne, I figured it was their cheap and dirty way of matching the 20-dB down slope of the CTC4 i.f. (I've never worked on a CTC4 and only am going by how I interpret what the 1956 article mentioned about the i.f. response.)

Pete
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  #363  
Old 07-19-2015, 01:47 PM
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Phil, could you please measure the L and C of the traps when you get them?
Sure I can measure. The parts are supposed to arrive in a few days.

Phil Nelson
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  #364  
Old 07-21-2015, 12:02 AM
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Okay, here are the key video preamp parts. From left to right, we have three Miller TV-151 4.5-MHz sound traps, a Miller 6195, and a Miller 4566:



There are minor differences among the three TV-151s, but presumably they work the same. The last two are adjustable across the target value of 400 muh given for L3 in the schematic; I'm hoping that one or the other will work.

Tomorrow I'll gather the other components and start building.

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  #365  
Old 07-21-2015, 12:35 AM
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ChrisW6ATV ChrisW6ATV is offline
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Phil, you are a "trooper". I am impressed by the tenacity you put into your projects-and the results you get, which are well-deserved. Your CTC-4 is going to be a fine performer at some point.
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  #366  
Old 07-21-2015, 01:55 AM
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Thanks for the encouragement. The jury's still out on this set, but at least I haven't run out of things to try.

Phil Nelson
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  #367  
Old 07-28-2015, 12:57 PM
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Originally Posted by Phil Nelson View Post
Sure I can measure. The parts are supposed to arrive in a few days.
Here's the data sheet for the 4.5-MHz trap:



The sheet identifies that as a 47-mmf cap. Two of my traps have a 47-mmf cap and the third has a 45-mmf cap. They all measure 2.2 ohms across the coil:



I put one of them on a scope with a 4.5-MHz signal and was able to peak the amplitude by turning the adjuster. I don't know if that's where you want the adjustment to be in the preamp, but I was curious.



Haven't done much beyond that inspection, except to collect the needed parts. Perhaps I will create a new thread about building the preamp.

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Last edited by Phil Nelson; 07-28-2015 at 01:15 PM.
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  #368  
Old 08-08-2015, 02:04 PM
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Back to the video preamp plan, with a question about the heater connection. I posted the original article at:

http://antiqueradio.org/art/RCA_Broa...eo_Preamps.pdf

The plan I'd like to build is on the last page of the article, which says, "The heater connection is made to pin 4 of the adaptor socket."

The heater connection is to service the 12AT7 tube on the preamp. Here's the 12AT7 basing diagram, showing that its heater is center-tapped at pin 9.



The article tells you to connect the heater connection to pin 4 of the adaptor socket. That's clear, but I want to make sure I understand the way they drew the heater connections in the preamp schematic:



The drawing seems to show pin 4 of the adaptor connecting to pin 9 (the center tap) of the 12AT7. And pin 5 appears to be jumpered to pin 4.

Or am I reading the schematic incorrectly?

(The ground connection for the heater is made through pin 5 of the 6CL6 1st video amp tube, whose socket the adaptor plugs into.)

Phil Nelson
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  #369  
Old 08-08-2015, 04:33 PM
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Quote:
Originally Posted by Phil Nelson View Post
The article tells you to connect the heater connection to pin 4 of the adaptor socket. That's clear, but I want to make sure I understand the way they drew the heater connections in the preamp schematic:



The drawing seems to show pin 4 of the adaptor connecting to pin 9 (the center tap) of the 12AT7. And pin 5 appears to be jumpered to pin 4.

Or am I reading the schematic incorrectly?

(The ground connection for the heater is made through pin 5 of the 6CL6 1st video amp tube, whose socket the adaptor plugs into.
Looks ok to me... remember that the 12AT7 actually has two 6.3V heaters (one for each triode). They are wired in series between pins 4 and 5. If you used just those two pins, you would need to apply 12.6 V to light up the heaters. Pin 9 is the center tap between the two heaters... by connecting pins 4 and 5 together and applying voltage to that connection and pin 9, only 6.3 V is required, as the two heaters are then wired in parallel.
What could be missing on the schematic is the connection from ground of the TV to the ground of the added preamp... IMHO, the wire from pin 5 of the 6CL6 socket should also be connected to ground(s) of the preamp.

jr
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  #370  
Old 08-08-2015, 09:56 PM
old_coot88 old_coot88 is offline
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Yup, the whole family of 12V twin triodes (12AT7, 'AU7 'AX7, 'AY7, 'AZ7 etc) have the centertap for 6V heater operation.
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  #371  
Old 02-04-2016, 07:06 PM
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OK, after pushing my CTC-4 project aside for a long winter's nap, I hauled it back out.

Last year, I had decided to build a video preamp in hopes of injecting a usable video signal at the 1st video amp, so that I could bypass the set's RF/IF stages and debug my obvious color problem in a "clean" environment.

Long story short -- Yes, I did eventually build a video preamp; No, it didn't work as expected (owing to the inevitable leetle wiring mistake by me); and then I got disgusted and worked on other projects.

Recently, I got a helpful email from ChrisW6ATV, who asked whether I had ever tried injecting a video signal from my Sencore VA62A, using the Drive Output jacks at the lower right of the front panel. Chris mentioned that you can set the Drive Range level to 3VPP and basically accomplish what I wanted to build a video preamp for (that is, inject a somewhat stronger-than-standard video signal). He reported getting good color on his CTC-4 when injecting video in this manner.

Duh!

So, here we go. The VA62A makes good color bars, using the 10-bar pattern referenced in the RCA service manual ( http://antiqueradio.org/art/RCA CTC-...ice Manual.pdf ):



Here is that video signal scoped at the injection point (terminal J on the IF board; the junction of L105/R126/R324 in the RCA schematic):



Here's the screen image when that signal is injected (after tweaking L127, reactance coil, to slow down the rolling enough for a photo). Yes, we have seen this barberpole effect many times before:



Chris noted that you can pipe an external video source through the VA62 using its video jack in the rear. To try that out, I set a second generator to display NTSC color bars and sent that signal through the VA62:



Of course, you can also inject a DVD video signal through the VA62 in that manner, but I'll spare you any more ghastly-looking scenes from the Wizard of Oz. Trust me, they look awful.

Now that I can inject a known-good video signal at the right level, I'll back up and:

-- Run through color setup procedures as best I can
-- Check voltages throughout the color sections
-- Scope signals throughout the color sections

These things have been done (or sort-of done) before, but now that I can inject a nice strong 10-bar color signal, it should be possible to work through the demodulator phase adjustment procedure on page 49 of the RCA service manual. Other parts of the manual that reference that pattern may also make more sense.

As always, I'm open to other ideas. And thanks again to Chris for this bit of advice.

Regards,

Phil Nelson
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  #372  
Old 02-05-2016, 11:19 PM
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Penthode Penthode is offline
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Phil,

Are you connecting the Sencore from terminal J to ground? The detector circuit output is across J and L. I looked at the CTC4 schematic and the connection of the generator from J to ground may not work because L is floating. Note that L is the sync separator feed. L also attaches to the cathode of the first video stage. V110 is a cathode follower.
The first cathode follower video stage does not invert the signal nor does it provide any voltage gain. It only provides an impedance match to the delay line.
So why not try and attach the generator from L to ground and see if that works? (You might want to DC isolate the connection to L with a 2uF 50v electrolytic with "+" to the cathode of v110).

Cheers,

Terry
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  #373  
Old 02-06-2016, 01:51 PM
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Phil Nelson Phil Nelson is offline
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So why not try and attach the generator from L to ground and see if that works?
I tried connecting the generator from L to ground and the picture became completely unstable: horizontal wouldn't really lock, and I couldn't tell whether vertical locked or not. When I connect to J, the image is stable apart from rolling colors.

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(You might want to DC isolate the connection to L with a 2uF 50v electrolytic with "+" to the cathode of v110).
Not sure what this means. Can you explain what is connected to what?

Phil Nelson
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  #374  
Old 02-06-2016, 09:04 PM
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ChrisW6ATV ChrisW6ATV is offline
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Quote:
Originally Posted by Phil Nelson View Post
And thanks again to Chris for this bit of advice.
Phil, after all of the great advice and tips I have learned from many of your restoration pages, I am glad to be able to offer even a little tidbit back in return.

Quote:
Originally Posted by Penthode View Post
Phil, Are you connecting the Sencore from terminal J to ground?
Terry, I also connected my VA62's video between point J on the RCA CTC-4 schematic and ground, and in my case, I got near-perfect video right from the start. The signal was, if I remember right, inverted video at about 3 volts peak-to-peak. Feeding it at point J seems to also put the needed signal to point L, through the nearby components. I also tried between point L and ground and did not get a usable signal; I did not try between points J and L (but did not need to, as noted above).
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  #375  
Old 02-11-2016, 08:19 PM
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Okay Phil you've been invaluable to my own CTC-4 restoration SO let me tell you some things you likely already know I've learned in case they are of help.

It's important to remember the CTC-4 chassis is one of the only color sets ever made where the local oscillator *must* be running for correct black and white setup. RCA's Color TV Troubleshooting Guide (by John R Meagher, chief engineer of the RCA service division, 1964, RCA Publishing) specifically mentions unlike practically every other set ever made the cathode bias balance of the three guns will be upset unless they are fed a 3.579545 MHz signal even during black and white setup because of the high level demodulation almost never used elsewhere. And indeed in my set I saw this to be true. I was glad I had the book because no one else had this information and assured me what I was seeing was impossible.

John H.
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