RCA 630TS sound problems, Round 2

[Phil Nelson]

I'm having a devil of a time getting a useful display on the oscilloscope, in hopes of aligning the sound IF on my RCA 630TS TV.

As background, I have recapped my EICO 324 signal generator and EICO 360 sweep generator (and fixed an output problem on the sweeper). Both work, as far as I can tell. At least they put out signals that I can display on the scope.

The alignment instructions tell you to connect the sweep generator output to the 2nd sound IF amplifier (V105) grid and to connect the scope to the 3rd sound IF grid return (terminal A T112) in series with a 33K ohm isolating resistor.

Here's part of the schematic for those who are eagerly following along :-)

http://antiqueradio.org/art/temp/RCA630TSSchematic1.gif

Here's the setup page from the EICO 360 sweeper manual:

http://antiqueradio.org/art/temp/sweepmanual.jpg

I am using this setup.

- The output of the signal generator (set to 21.25 mhz sound IF frequency) is fed to the external marker input of the sweep generator.

- The 60hz sweep output of the sweep generator is fed to the horizontal input of the oscilloscope.

- The RF output of the sweep generator is fed to the 1st TV connection point given in the instructions.

- The oscilloscope vertical input is connected to 2nd TV connection point given in the instructions.

According to the manual, I should see a nice textbook waveform with a little marker pip. What I'm actually seeing is a pair of fuzzy horizontal lines on the scope. Twiddling the scope controls doesn't change the display. Twiddling the generator (either one) controls doesn't change the display, except that turning the 60hz phasing control on the sweep generator can either bring the horizontal lines together vertically, or move them back apart.

I am not a scope expert. No doubt I'm doing something stupid . . . but what?

[blue_lateral]

Are you using a demodulator probe on the scope? I'm pretty sure you would need to here.

John

[Phil Nelson]

Just using a regular probe. The instructions say nothing about a special probe, although I guess there are lots of places in old instructions where they assume you know how to do things not spelled out.

[blue_lateral]

I'm sure you need a demodulator probe here. In this type of a sweep, you use the same ramp (sawtooth) generator to sweep the sweep generator and the scope beam. In the diagram you posted, this is the ramp generator in the sweep generator, and is jacked into the "external sweep" or "x axis" on the scope. This is how it is usually done.

As the frequency of the sweep generator goes up, the beam goes right, they're synchronized. So far so good. A point further right on the scope corresponds to a higher frequency.

Now you need a response curve. As the sweep generator passes through the passband of the IF strip, the signal level goes up and down. If there is any AVC or AGC or anything like that, it must be disabled, because it will try to compensate for the level going up and down. The alignment instructions should tell you how.

As the level goes up and down, this becomes an AM signal, modulated on the sweep. To recover it, you need an AM detector. If you were aligning an AM radio, or a video IF strip, there would be an AM detector already there in the set, and you could use a regular scope probe after the detector.

Since this is the sound strip, I suppose the detector is FM (and useless for this procedure), so that is why I think they have you picking up the signal just before the detector. You'll need a detector probe.

Any demodulator or detector probe as found on an old scope should work. I made my own for aligning chroma in color sets. Here's the schematic for the one I made. It's the one on the bottom.

John

[John Folsom]

Use the RCA sound alignment procedure, see Sams or RCA for details. Since the 3rd sound IF grid is at IF frequencies, 21.25 mhz, you must use a demodulator probe (diode) to develop a signal for the vertical input of your scope.

[blue_lateral]

If you connect the sweep generator output directly to the demod probe, you will get a flat line. This is because the response of the direct connection is flat. It's hard to tell if the equipment is working.

You have to run the sweep through something with a response curve to get anything on the scope.

Here is a sweep I did of the video IF on a CTC15 color set. This was done using the video detector in the set as a detector probe. There are several markers on in the picture, and they are turned up really high. In this case, the markers were injected after the IF strip, so the fact that they are too high doesnt matter much.

I only mention this because if the marker is going through the IF strip you are trying to tune, you need to keep the marker output as low as possible, because it will affect the response curve.

Good luck!

John

P.S. If the pattern is upside down, you can just reverse the polarity on the y-axis or vertical input on the scope. If it is backwards, (higher frequencies to the left) you can reverse the polarity on the x-axis or external sweep input.

[Phil Nelson]

Thanks, that gives me something new to try, and the schematic for the probe looks straightforward, although I don't have that type of diode on hand (1N4007 is all I have in the parts bin, if memory serves).

While I'm waiting for the next parts order to arrive, I have plenty of other things to work on :-)

[blue_lateral]

For what it's worth, probably any small signal diode would work fine I suspect, like a 1n34 or something. I'll bet even a silicon one would be fine. A 1n60 is just a very low capacitance germanium diode, selected for good output I think. They are used for the video detector in some color TV sets.

[Phil Nelson]

I could have sworn I had a little pack of 1N34 diodes floating around, but they must have fallen into the crack of some parts box. Best I can come up with is a 1N4148 silicon diode. Think that's worth a try?

[John Folsom]

Phil,
Why not move on to the overall sound if alignment steps, where you are probing after the discriminator. That way, you don't need a demodulator probe. For sound if alignment, you can generally just peak the if transformers for maximum response, there is no stagger tuning to obtain a complex bandpass response as in video IF alignment. This will likely suffice. If not you can always go back to aligning each stage separately using a demodulator probe. You can also do most sound alignment using a DC voltmeter instead of an 'scope (probing after the discriminator), as shown in the RCA alignment procedure. Of course, once you have aligned the sound IF, you may still have to do a video IF alignment to get sound and video to align properly.

[Phil Nelson]

That's not a bad idea. Maybe, for practice, I'll try aligning the FM on my Hallicrafters SX-42 first. The manual has instructions for aligning with a voltmeter as well as a scope. And the FM on that set never worked that well, anyway.