Ok. Let’s try this again…RCA 9TC240 Alignment

[Penthode]

The last thing about the 21.25 MHz sound signal. Why do to think it is to be nulled out of the video channel?

[Chris K]

Originally Posted by Penthode

Look at it this way. This set is designed to include a two stage DC coupled video amplifier. Let's go back to basics: why is there a two stage DC coupled amplifier? Next to provide the two stage DC amplifier, the two stages have to be DC stacked. With regards to the cathode and control grid, the plate is roughly at +120vdc. So to DC couple the two stages, the first stage grid/cathode is at about -120vdc, the plate of the first stage is DC coupled to the second stage grid/ cathose at around 0vdc and the plate of the second stage is coupled to the grid of the picture tube at around +120vdc. This is to couple the DC component. Do you understand this?

Can we take this description a concept at a time. The 12AU7 contains the 2 stages of video amplification. This 2 stage, DC coupled amplification happens here? I don't know what coupled refers to here...the 2 stacked DC voltages?

[Chris K]

Originally Posted by Penthode

The last thing about the 21.25 MHz sound signal. Why do to think it is to be nulled out of the video channel?

It would leak into the video signal and create unwanted interference?

[Chris K]

Originally Posted by Penthode

The video signal at the video detector is only about 1v p-p across R119. But the R119 return is connected to the -120vdc supply to facilitate the DC coupled amplifier.

The DC coupled amplifier is to maintain a consistent black level. Do you understand about black level clamping?

No sir I do not. I just know the basics of the production of a scanned black and white video image based on varying intensity of phosphor luminescence from the intensity and position of the electron beam at any one time on the CRT screen. Is that correct? This scanning from top to bottom horizontally goes back to the top of the screen and creates a new set of beam intensity driven fluorescent intensities on the phosphor...do enough of these scans over a second and you get the illusion of motion. Is that correct?

[Chris K]

And just a practical question for the setup in this step...I have the ground lead from the VTVM connected to one side of R119. The DC voltage probe from the VTVM is connected to the other side of the resistor R119. I have a 21.25 MHz signal going into pin one of the 6AG5 converter through a 1500pf ceramic cap. The VTVM is on the 5 volt scale. What do you think the RF signal level should be? They don't specify anything in the instructions.

[Chris K]

And when I have this setup and turn the TV on (with the potentiometer installed and adjusted to the -4.5V AGC voltage) the needle deflects on the VTVM off scale. Doesn't matter if I'm reading DC+ or DC-. I cannot use the zero adjust to get it on scale so I can see any sort of "minimizing". The needle is blasted to one side or the other. The VTVM works...I can read the -120 volts in the circuit if I just do a basic measurement but in the either side of the resistor setup, I can't get the needle on scale. It just fully deflects to one side or the other depending on if it's set to read + or - DC volts.

[Chris K]

If I were to connect my DVM across R119 that is supposed to have -120V DC on both sides, what should I see on the meter? I think I'm confused as to what the meter is measuring in this configuration. If I connect the ground lead to chassis ground, I would see -120V DC if I put the voltage probe on either side but what exactly am I looking at on the meter when It's connected as part of the DC line?

[Penthode]

Originally Posted by Chris K

No sir I do not. I just know the basics of the production of a scanned black and white video image based on varying intensity of phosphor luminescence from the intensity and position of the electron beam at any one time on the CRT screen. Is that correct? This scanning from top to bottom horizontally goes back to the top of the screen and creates a new set of beam intensity driven fluorescent intensities on the phosphor...do enough of these scans over a second and you get the illusion of motion. Is that correct?

The setting of a reference black in a display is a long discussion that is as old as television itself. It is the need to define a black in the display and maintain the black with changes in video content. DC coupling provides this and is the reason that the designers of this set we t to the trouble of providing DC coupling between stages. Notice there are no coupling capacitors between the stages which would block the passing of DC changes .

The video amplifier of this set maintains the black level very well by passing the DC component which sets the black reference. Another method employed by the RCA 630TS which preceded this set was the use of a DC Restorer. Note all modern displays employ black level clamping. Interestingly, thru the1950s and 1960s, sets dispensed with the DC clamping. This meant that the black level drifted upward in dark scenes and blacks were darker in a bright scene.

The topic of DC restoration or lack of in later sets is seen as a problem by many. Just be aware that the strange design of the set with the -120v DC supply and the stacked stages was to provide the proper consistent black reference that later sets did not have.

It is a topic well discussed in this forum.
https://www.videokarma.org/showthread.php?t=248278

[Chris K]

Originally Posted by Penthode

The setting of a reference black in a display is a long discussion that is as old as television itself. It is the need to define a black in the display and maintain the black with changes in video content. DC coupling provides this and is the reason that the designers of this set we t to the trouble of providing DC coupling between stages. Notice there are no coupling capacitors between the stages which would block the passing of DC changes .

The video amplifier of this set maintains the black level very well by passing the DC component which sets the black reference. Another method employed by the RCA 630TS which preceded this set was the use of a DC Restorer. Note all modern displays employ black level clamping. Interestingly, thru the1950s and 1960s, sets dispensed with the DC clamping. This meant that the black level drifted upward in dark scenes and blacks were darker in a bright scene.

The topic of DC restoration or lack of in later sets is seen as a problem by many. Just be aware that the strange design of the set with the -120v DC supply and the stacked stages was to provide the proper consistent black reference that later sets did not have.

It is a topic well discussed in this forum.
https://www.videokarma.org/showthread.php?t=248278

I will check it out...Thank you P

[Chris K]

OK...I got the top adjustments done for the traps and transformers. The issue was with the signal strength. The ideal was around 75mV. I was able to minimize the meter output on the VTVM at the 5V level and was able to switch to the 1.5V scale to make very fine adjustments down to an absolute minimum. I'm going to start the maximum adjustments on the bottom cores tomorrow. I may need to apply some mineral spirits to the trap bottom cores to be certain they are loose. All of the VTVM off scale issues had to do with the signal strength being much higher than needed. I found the signal strength sweet spot and the core adjustments were easy...like they're supposed to be

[Penthode]

Looks like you are getting the knack.

[Chris K]

Well my friend and part time mentor, you and the others who said this TV would be the hardest thing I’ll ever do in this hobby were absolutely right! I’m nowhere near finished with it yet but I think (I hope) I may be getting close to the other side of feeling lost. I believe I can probably deal with the next series of transformer and trap adjustments on the bottom cores but please, nobody abandon this ship just yet!!!!!! The next step after that will be looking at the overall response curve using the 330 ohm shunts I made and there are some steps in that protocol I’ll need help with. Thank you again for your help!

[Chris K]

All bottom cores have been adjusted passing a signal indicated in the service notes. I want to move on to the next step of the picture IF transformer adjustments. This involves shunting the primary coils of T-102, 103, 104 and 106 with a 330 ohm resistor. I have the resistors soldered to mini alligator clips and can do this easily. That is, If I can figure out what bottom terminals are for the primary coil in each. Here is the schematic section with the transformers and the letters of the tabs shown.





Are the secondary coils the ones that are standing alone without being connected to the circuit, except for T-103 where the secondary (I think) is where the sound takeoff goes to the 6AU6 grid in the sound IF circuit?

[Chris K]

Here is what I have setup:
1. Tuner tuned between channels 13 and 2.
2. Potentiometer adjusted so the voltage at R135/136 is -2.0V DC
3. Sweep generator at a 25MHz center frequency with a symmetrical sweep from 20
to 30 MHz.
4. Sweep output at 300mV
5. Generator RF out connected to pin 1 grid of the converter (V2) through a 1500pf
capacitor.
6. Oscilloscope connected to Pin 1 of the 12AU7 Video amp (V106).
7. Transformers shunted with a 330 ohm resistor in the following way
T-102 Tabs A and B
T-103 Tabs A and D
T-104 Tabs A and B
T-106 Tabs C and D
8. HP 8600A Digital Marker Generator set for 25MHz center frequency marker and
22.05 and 24.75 MHz additional markers.

With this setup and the scope in XY mode, I get a trace that isn't flat but the amplitude is very low where it does deviate and changing the signal strength through the available voltages doesn't affect it much. I do not want to touch the T-1 and T-101 transformers used to adjust this until I get some feedback I have all of this correct.

[old_tv_nut]

"With this setup and the scope in XY mode, I get a trace that isn't flat but the amplitude is very low where it does deviate and changing the signal strength through the available voltages doesn't affect it much. I do not want to touch the T-1 and T-101 transformers used to adjust this until I get some feedback I have all of this correct."

The voltage you set with the potentiometer is the IF gain control. The resulting gain will depend on the exact gain control characteristics of the particular tubes you have. You can try adjusting the potientiometer slightly to get the output signal strength you expect/need.