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

[Yamamaya42]

As I mentioned early on, this is one of the most challenging sets to align even for the most seasoned techs who have done alignments, so it's understandable you are having trouble, perhaps you can sharpen your teeth on a newer less complicated set first?

[Chris K]

Originally Posted by Yamamaya42

As I mentioned early on, this is one of the most challenging sets to align even for the most seasoned techs who have done alignments, so it's understandable you are having trouble, perhaps you can sharpen your teeth on a newer less complicated set first?

Hard for me to give up on a project even for a short while once I've started. I think I'll learn more not just from the difficulty but also from the engineering chronology of how these RCA TVs were designed, modified and redesigned over the first few years after the war. All that being said, one or two projects are currently on the shelf waiting for an "Ah Ha...That's what's wrong with that one!" moment

[Penthode]

Having worked on RCA televisions prior to and after this chassis, you chose the most compluicated early receiver to re-align. You are making progress and have highlighted the pitfalls in the process along the way.

I think you should feel confident once you are through this set you should feel confident attacking any other set.

[Chris K]

Originally Posted by Penthode

Having worked on RCA televisions prior to and after this chassis, you chose the most compluicated early receiver to re-align. You are making progress and have highlighted the pitfalls in the process along the way.

I think you should feel confident once you are through this set you should feel confident attacking any other set.

I agree...It's not the first time I've chosen the steepest learning curve in my life and I do intend to stick with it as long as the good will of yourself and all the other contributors to this endeavor continues. Compared to where I was a couple of months ago, I feel like I've come a long way in understanding how these early commercial TVs were designed and how this first generation was subsequently modified. I've also acquired some impressive equipment for the Starship bridge!

Speaking of equipment, I bought a serviced and calibrated new VTVM off eBay from a seller with a long history of electronic sales. To be honest, I don't trust that my present VTVM is giving me accurate information and consistent sensitivity. I'll plug along with it for now but the replacement should be here shortly.

[Chris K]

Learning curve just got significantly steeper. I broke the T-103 73572 bottom slug. Looks like the slot sides broke out. How do you curse profusely on VK without being banned? Can't believe it. I've got a couple of 630 parts chassis but they don't have this 21.5/22.0mc trap coil. For the cursing, here's the best I can do

I was warned about this. Crap. I was being gentle but happened anyway. I looked on eBay and the RCA transformer stock at the ETF but no joy.

[old_tv_nut]

"Standard IF" is not a useful designation.
The "opposite" of staggered tuning is synchronous tuning. Both of these generally can apply to any IF where each stage has a single resonant circuit (coil and capacitor).
Synchronous tuing means every stage is tuned to the same frequency, and the overall response has the single peak frequency with a bandwidth narrower than any one stage. With stagger tuning, the peaks of the individual stages are different, so one stage may have maximum response at center, while others have maximum at higher or lower frequency. Stagger tuning produces less gain at the center freuency, but boosts the gain somewhat at higher and lower frequency, giving a wider bandwidth.

Then there are double-tuned stages, as illustrated above. A double-tuned amplifier has two tuned circuits between stages that are coupled (usually magnetically, but could be with a capacitor). Even though each tuned circuit is (usually) tuned to the same frequency individually, when they are coupled the response bandwidth becomes broader, and if the coupling is strong, will actually develope a double-peaked response, with a dip in the middle.

Fun fact: coupled oscillator theory applies to any sort of coupled resonators including mechanical objects. It is used on those strange dumbell type weights (vibration dampers) you may see on high tension wires, to reduce wind vibrations, and also in "mass dampers" in some tall skyscrapers to reduce wind sway.

[Penthode]

This is an interesting discussion! With stagger tuning you are not adjusting the four stages to one single frequency like in the FM stages or in an AM Radio. The stages are tuned to different frequencies or stagger tuned.

Simply put, stagger tuning is to stagger the frequencies each stage is tuned to. This is to provide the wideband needed to accommodate the wide bandwith video signal.

The stages are tuned to different frequencies so that the combination of the whole provides the over 4MHz bandwidth required.

Another point to understand and appreciate is the use of vestigial transmission for the video. For standard amplitude modulation (AM radio broadcast for example) both sidebands are transmitted. For television requiring 4MHz for the signal would require a 8MHz wide channel for both the AM sidebands. In the late 1930's it was discovered that if most of one of the sidebands was removed, a full resolution video could be delivered at the expense of slightly worsening the signal to noise performance of the transmission. So Vestigial Sideband is the sending only a "vestige" or portion of one of the sidebands.

How this relates to your alignment is the importance of placing the video carrier at exactly the 45% point of what is termed the "Nyquist Slope". Setting the carrier too high on the slope will emphasize the lower video frequencies in relation to the higher video frequencies. The higher video frequencies contain the fine detail in the displayed video. So improper carrier placement too high on the curve will make the picture less sharp and fuzzy. Conversely, the carrier too low on the curve will make the picture too sharp and edgy and make the picture noisy. Watching NTSC color signals on with the carrier too low will emphasize the subcarrier dots and destroy the image.

This you have to keep in mind when doing the sweep alignment.

We can discuss more closer to the time. Let's first get back to where we were and establish a baseline from which to begin the stagger tuned alignment.

[Penthode]

One other thing. The coupling between the tuner and the first video IF is not stagger tuned or part of the overall stagger tuning in this design. Instead the stage includes an overcoupled transformer link. That is the stage cannot be adjusted to one single frequency. Instead it tunes very broadly.

To align this stage separately, you must follow the instructions and sweep align it before doing an overall sweep alignment.

[Chris K]

Thank you everyone. Let me digest this for a bit. Getting a little bit “behind the aircraft” as they say in aviation accident investigations! I need to put all this information together in my head and ask some questions. Standby!

[Chris K]

The core did not split and I can see the top of it from the top of the chassis side. Should I unscrew it up and out of the top of the coil or screw it down through the bottom?

[Penthode]

I always use a hard plastic tool to adjust the cores. Try and refrain from using a metal tip because unless the tip fits snugly, the uneven sape will imose high stress to the brittle slug slot wall. I carved a hard plastic rod to the shape of the slot.

You can see the slot the other end? I would gently try and screw it downward and out. Do not force it if it jams.

If the core only has the slot damaged one end and is otherwise okay, I would simply flip it 180 degreses and put it back and continue the alignment.

[Penthode]

correct that, I didn't recall the brass ring obstruction. Remove it from the top seems the only option.

[Chris K]

Originally Posted by Penthode

correct that, I didn't recall the brass ring obstruction. Remove it from the top seems the only option.

I agree...slowly turning it out with a plastic flat blade tool.

[Chris K]

I don't want to remove the coil from the chassis if I can help it. I can see the top and bottom of the core. The top of it has a slot, the bottom dies not and there's a little debris I cleaned out from what broke off. Very difficult to photograph but the core is intact...I just have to get it out and screw it in from the bottom

[Chris K]

Here is a photo of the bottom with it run all the way down. The only think that broke was the parts of the core on the sides of the slot.