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

[Penthode]

Originally Posted by Chris K

Ok so once I’m confident I have the primary of the transformer peaked, I then zero the VTVM in the center and take the reading from the new spot (not in front of it right now) adjusting the secondary from the bottom for zero volts. Then go back to the primary setup and maximize it again…then to the bottom again and so forth until everything is stable with no additional adjustments?

Yes. Typically just go back once and confirm. Then move on the the sound IF amplifier.

[Chris K]

But not yet right? I need to do the 1mc sweep around 21.25 (symmetrical) and make sure the response is linear between 21.17 and 21.32 with 21.25 right in the middle correct? I do have that right now on the scope as per alignment instructions. Figure 13

[Penthode]

Originally Posted by Chris K

But not yet right? I need to do the 1mc sweep around 21.25 (symmetrical) and make sure the response is linear between 21.17 and 21.32 with 21.25 right in the middle correct? I do have that right now on the scope as per alignment instructions. Figure 13

Yes that is correct! yes you want to ensure it is symmetrical around 21.25MHz

[Chris K]

Symmetrical it is but it does not look exactly like the trace in the service information. Don't want to mislead anyone. I don't have all of the signal trace in figure 13 but what's missing from it is the same from the positive end to the negative end. I don't get the first or last 20% of the trace but what I do get on the scope is centered and symmetrical. Putting 21.25mc on the origin (X=0, Y=0) and the appropriate markers on the sweep, they are all symmetrical with respect to zero and almost straight between 21.17 and 21.32 with 21.25mc at the midpoint. IDK if that makes any sense but I'm pretty sure this is OK given what you've said about the relative swings of voltage above and below the midpoint (21.25)

[Chris K]

Not to keep pounding this but I figure the more I know the better I'll be able to do this properly. This is from an article by Stuart Seeley and Jack Avins in a 1947 RCA review publication titled "The Ratio Detector" This snip from the paper shows the 350kc sweep around the CW frequency. This is exactly the trace I have on the scope at 100mV.

[Penthode]

Originally Posted by Chris K

Symmetrical it is but it does not look exactly like the trace in the service information. Don't want to mislead anyone. I don't have all of the signal trace in figure 13 but what's missing from it is the same from the positive end to the negative end. I don't get the first or last 20% of the trace but what I do get on the scope is centered and symmetrical. Putting 21.25mc on the origin (X=0, Y=0) and the appropriate markers on the sweep, they are all symmetrical with respect to zero and almost straight between 21.17 and 21.32 with 21.25mc at the midpoint. IDK if that makes any sense but I'm pretty sure this is OK given what you've said about the relative swings of voltage above and below the midpoint (21.25)

Did you try increasing the sweep width on the 8601a generator? It appears that the sweep width is too narrow. Once you have the sweep width wide enough to see the s curve, you can use the adjustable markers to ensure you have the 350kHz between peaks. This is very important.

[Penthode]

Originally Posted by Chris K

Not to keep pounding this but I figure the more I know the better I'll be able to do this properly. This is from an article by Stuart Seeley and Jack Avins in a 1947 RCA review publication titled "The Ratio Detector" This snip from the paper shows the 350kc sweep around the CW frequency. This is exactly the trace I have on the scope at 100mV.

FYI: This is a discussion of the Ratio Detector. That is not what is used in the RCA receiver. The TV uses the Foster-Seeley Discriminator. It lacks the AM limiting function of the Ratio Detector.

You can tell the difference at a glance by looking at the diode polarities in the schematic. The Foster Seely are in the same direction and the Ratio Detector have the pair opposite facing.

[Chris K]

Originally Posted by Penthode

Did you try increasing the sweep width on the 8601a generator? It appears that tghe sweep width is too narrow. Once you have the sweep width wide enpough to see the s curve, you can use the adjustible markers to ensure you have the 350kHz between peaks. This is very important.

Yes I did and I could see the reciprocal downturn and upturn of the curves with a broader sweep but this scope is so fuzzy, any wider on the sweep and the trace just trails off. I'm pretty confident in my assessment that the curve peaks are correct at 21.07mc and 21.43mc and equidistant from the 21.25mc frequency. The curve is pretty straight as it gets close to the carrier frequency on both the + and - sides. You know, this is the best assessment I'm capable of doing at this point on my learning curve. Other than having either you or Bob sitting with me and twirling the knobs and moving the slugs, this may have to do. On the practical side, looking at these adjustment screws on the top and bottom of T-113, I can tell they're pretty close to where they were for years looking at the oxidized surface of the screw outside the can and the shiny surface that's just popped through the "nut" part of the can. It's not a nut really. It's metal pressed on the screw threads from all sides. I know this step is very critical to the rest of the process and I will be certain to do more testing with the scope and sweep and research on this type of circuit to have more confidence in saying it's adjusted correctly.

[Chris K]

Yes I see that now but it looked to me like this preliminary discussion about maximizing the primary voltage, setting the second voltage to zero and sweeping 350mc symmetrically across 21.25mc at 100mV was close to what the instructions said to do. When I looked up the FS discriminator, several folks pointed to this article as a good read to get a baseline understanding. Anyway, I'll keep at it. Thanks again and again!

[bandersen]

I'm concerned about the trace on your scope being fuzzy.
What do you have your volt/div set at?

It should be very sharp and defined. I've never measured the level of the signal I'm putting into a set. I just use the lowest signal that gives me a clear trace.

Even on a super basic, old scope, you should get a clear trace.

[Penthode]

Originally Posted by bandersen

I'm concerned about the trace on your scope being fuzzy.
What do you have your volt/div set at?

It should be very sharp and defined. I've never measured the level of the signal I'm putting into a set. I just use the lowest signal that gives me a clear trace.

Even on a super basic, old scope, you should get a clear trace.

A super old scope will provide a clearer trace on the basis of lower bandwidth. A lo pass filter feeding the scope should ameliorate the noise.

The other causes of noise are bad grounds, lack of proper shield and electromagnetic/ electrostatic noise in the environment. A test to determine the noise in the environment is to simply remove the scope probe see how much the hanging probe noise is. Then touch the probe hot lead with your finger.

With the probe disconnected and the sensitivity set to 0.1v/div anda x10 probe, I see a 3vp-p sinusoid when I touch the hot lead. when nothing is connected to the probe at the same vertical sensitivity of 0.1/div, I see a straight line.

[Chris K]

Originally Posted by Penthode

A super old scope will provide a clearer trace on the basis of lower bandwidth. A lo pass filter feeding the scope should ameliorate the noise.

The other causes of noise are bad grounds, lack of proper shield and electromagnetic/ electrostatic noise in the environment. A test to determine the noise in the environment is to simply remove the scope probe see how much the hanging probe noise is. Then touch the probe hot lead with your finger.

With the probe disconnected and the sensitivity set to 0.1v/div anda x10 probe, I see a 3vp-p sinusoid when I touch the hot lead. when nothing is connected to the probe at the same vertical sensitivity of 0.1/div, I see a straight line.

OK guys...thank you so much. I'm off to teach tonight so I won't be back on this until tomorrow. I think we're getting close.

[Chris K]

200mV input of 21.25mc signal. This is swept symmetrically through 1.0mc as the alignment instructions say. The shape of the right side of the curve curve doesn't look like Fig. 13 in the service data but it is sorta symmetrical and the 3 markers are where they are supposed to be I think. The trace should begin ascending back up after the third marker (21.43mc) but it isn't and I don't know if that's important or not. I have the scope probe grounded through a 1000pF capacitor. The fuzzy smearing doesn't appear to be an issue anymore. In person, the line is tighter than it appears in the photo.

[bandersen]

The scope probe must be grounded directly to the chassis. You have it going through a cap?

[Chris K]

Originally Posted by bandersen

No, he's saying 102 cap from the scope input to ground. You should really have the end of your RF out terminated. Without it, the RF output level meter is meaningless.

Do not use a demod probe. Use a 10X scope probe. You will rarely if ever use a scope probe in anything but X10 mode. It loads down circuits in X1 mode.

No I do not have the capacitor between the scope probe and the place where I'm supposed to get the scope output. I wouldn't get a signal if I did that right? I grounded the scope input by attaching the scope input to the same point. Is this correct? See below.



The scope probe lead is grounded to the chassis as well