I have some questions about I-F alignment that I'm sure are remedial enough to make some people smack their foreheads. Nevertheless I don't know the answers and perhaps asking in public will help some one else also. The set I am working on is a 621ts and the alignment is badly off. The set was a mouse house for part of its life and the little critters decided to chew through two of the IF transformers. I've replaced the transformers with ones from a donor 721ts. I am secretly glad the alignment is off as I have wanted to learn how to do I-F alignment and this is a perfect excuse to get started.

Here are the instructions according to Riders:

PICTURE I-F TRAP ADJUSTMENT- Set the voltage on the i-f bias bus to approximately -3 volts. Set the station selector switch to channel 9. Connect the "VoltOhmyst" across the picture second-detector load resistor R118. Connect the output of the signal generator through a 1000 mmf. capacitor directly to the converter grid (either end of R5 to ground). Set the generator to 21.25 mc and set the specified adjustment for minimum indication on the "VoltOhmyst". The generator should be checked against a crystal calibrator to ensure it is exactly on frequency.

21.25 mc - T2 (top)
21.25 mc - T101 (top of chassis)

PICTURE I-F TRANSFORMER ADJUSTMENTS - Set the frequency of the signal generator to each of the following frequencies and peak the specified adjustment for maximum indication on the VoltOhmyst.

22.8 mc - T2 (bottom)
23.9 mc - L101 (top of chassis)
26.0 mc - L102 (top of chassis)
24.5 mc - L103 (top of chassis)

Picture i-f oscillation - If the receiver is badly misaligned and two or more of the i-f plate transformers are tuned to the same frequency, the receiver may fall into i-f oscillation and alignment by the usual method becomes difficult. I-f oscillation shows up as a voltage in excess of 3 volts at the picture detector load resistor. This voltage is unaffected by r-f signal input and sometimes is independent of picture control setting. If such a condition is encountered, a cure may sometimes be effected by adjusting the plate coils approximately to frequency by setting the adjustment stud extensions of T2, L101, L102, and L103 to be approximately equal to those of another receiver known to be in proper alignment. If this does not have the desired effect, oscillation may be possibly stopped by increasing the grid bias with the picture control. If so, the i-f stages may then be aligned by the usual method. Once aligned in this manner, the i-f amplifier will be stable with reduced bias.

If the oscillation cannot be stopped in the above manner, shunt the grids of the first two i-f amplifiers to ground with 1000 mmf. capacitors. Connect the signal generator to the third i-f tube grid and adjust L103 to frequency. Next remove the shunting capacitor from the second i-f grid, connect the signal generator to this grid and align L102. Then remove the shunting capacitor from the first i-f grid, connect signal generator to this grid and align L101. Then connect the signal generator to either end of R5 and align T2 to frequency.

Starting at the very beginning here are some questions and clarifications.


Set the voltage on the i-f bias bus to approximately -3 volts.

Adjust the picture control until the grid of the 1st i-f amplifier tube is -3 volts?


Set the generator to 21.25 mc and set the specified adjustment for minimum indication on the "VoltOhmyst".

Does this mean as close to zero volts as possible?


Set the frequency of the signal generator to each of the following frequencies and peak the specified adjustment for maximum indication on the VoltOhmyst.

What is maximum indication? Does this mean the highest voltage possible?


If the oscillation cannot be stopped in the above manner, shunt the grids of the first two i-f amplifiers to ground with 1000 mmf. capacitors. Connect the signal generator to the third i-f tube grid and adjust L103 to frequency....

It doesn't say this, but I'm assuming you would connect the generator to the third i-f tube grid again through a 1000 mmf. capacitor as was done on R5?

My interpretation.

1. Set IF amp grids to -3 by use of picture control or external bias supply if needed.

2,3. Max indication = highest (in this case voltage) reading the circuit will give your meter while making the adjustment, and Minimum indication is lowest indication circuit will give. (I'm surprised this is confusing.)

Just a thought or two.. The Riders procedure pre-dates the conventional practice of using a sweep generator and scope for alignment. Is there a Sams of this set giving sweep alignment instructions? If so, learning sweep alignment would be far more beneficial long-term than trying to do a one-off using the obsolete procedure.

If nothing was disturbed other than those two IF xfmrs., I would first try tweaking them, looking for best fine detail in the pic consistent with no ringing or smearing of detail.

If alignment is still necessary, and if sweep alignment instruction for the set is available, try to get someone with a genny and scope to mentor you. Or if you already have genny and scope, the collective wisdom here could walk you through it. (Of course the genny has to be able to do the old 24 mc. IF.)

While the old method may work OK, the scope gives you a visual graphic of exactly what the IF strip is doing.

BTW, the two things you replaced were IF transformers and not traps, right?

The RCA manual has similar alignment instructions and a overall response
for sweep generator alignment. There is no stage by stage sweep
procedure.

Thus, one must simply follow instructions for the signal generator response
and then check using the sweep generator. If the curve is off,
one must tweek and pray.

My interpretation.

1. Set IF amp grids to -3 by use of picture control or external bias supply if needed.

2,3. Max indication = highest (in this case voltage) reading the circuit will give your meter while making the adjustment, and Minimum indication is lowest indication circuit will give. (I'm surprised this is confusing.)

It's not so much that it's confusing, more that I've never done this before and I want to make sure. It's always better to ask questions from experts than to just assume you know what you're doing. The thing that really made me ask was the fact that the instructions were seemingly saying to maximize voltage readings at various points, but then saying that if voltage got too high it was a bad thing.

I-f oscillation shows up as a voltage in excess of 3 volts at the picture detector load resistor.

As to my last question, should I always use a capacitor to couple the signal generator to the i-f amplifier grids?

...
BTW, the two things you replaced were IF transformers and not traps, right?

That is correct, they were the second and first video i-f transformers.

I set out to align the set using the "If the receiver is badly misaligned" method and I got a puzzling result. When I shunt the first two i-f amplifier grids and peak the third I get exactly the response that was expected. Adjusting the i-f transformer (L103) has direct effect on the voltage at the second detector load resistor (R118). I can turn the slug on the transformer and the voltage will raise to a point and then lower...exactly what it should be doing. If I adjust the signal output of the generator the voltage also rises and drops. So we're doing great and I move on to the second i-f stage. I remove the shunting capacitor from the grid of the second i-f tube and inject signal there. This gives me a completely different result though. Adjusting the transformer (L102) gives me no change in the voltage at the second detector except at the two ends of the the adjustment. When I adjust to almost the limit on the transformer the voltage jumps way up and the generator signal injection has zero effect on it. This is described in the service notes as i-f oscillation. It seems like there must be a problem between the grid of the second i-f amplifier tube and the grid of the third i-f amplifier tube. The only thing I found so far that was out of the ordinary was the plate voltage on the second i-f amplifier tube (V102) was high at 130V or so.

http://i1075.photobucket.com/albums/w438/vts1134/RCA%20621ts/621%20Picture%20I-F_zpsfup8pncj.png

The plate of V102 is essentially at B+ which means it's not drawing any current (through R113). So, you need to find out what is wrong with that stage, could be a bad tube or could be bad components / connections / bias for that tube.

The cathode resistor on the second i-f amplifier tube had gone high. After replacing that resistor I was able to align that stage as well as the two adjustments before it. I've gone from a barely perceptible picture to a very nice (not at all perfect) picture. I'll have to align the audio next, and I'm going to make more attempts at the video, but my first ever attempt at i-f alignment was pretty successful.