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

[Penthode]

Now you are making me confused.

The pot is adjusted for setting the -4.5v which is the AGC voltage measured at the junction of R135 and R136 wrt ground.

The RF CW signal is detected or rectified by the 2nd detector. You vary the 8601a signal until you get roughly 2 to 3 volts ACROSS the detector load resistor R119. The -4.5v should not be changing much: how much is it changing?

[Penthode]

Originally Posted by Chris K

Got it. I think my VTVM is a bit wonky. One more question...the channel selector is set between channels 2 and 13 but that's not a firm detent correct? I just have it between the two channels.

That is correct. There is no detent.

Historical note: This, the KRK5 RCA tuner replaced the KRK2 tuner used in RCA sets from 1946-48. The KRK2 included Channel 1 and the KRK5 in 1948 was the first RCA tuner to drop channel 1. The blank space is where channel 1 would have gone.

[Penthode]

with regards to the core positioning for maximum. You said the core was unscrewed to get the peak. Unscrewing the core reduces inductance which means you are tuning it higher in frequency. What I think you have done is tune it to the same frequency as another stage and it has introduced oscillation! I mentioned the danger earlier of it breaking into oscillation. I suggest you must fix this before adjusting the traps!

Put the core back close to the original position. The circuit tunes broadly and you should detect a smaller peak near the original position. Also if you do get into a state of oscillation and cannot get out of it, what you do is increase the AGC voltage until the oscillation ceases and try increasing the 8601a input until you see the signal.

Certainly if you are having to adjust any opf the circuits that far away, you are doing something wrong. Becareful and stop and think and ask before making assumptions.

[Chris K]

Originally Posted by Penthode

Now you are making me confused.

The pot is adjusted for setting the -4.5v which is the AGC voltage measured at the junction of R135 and R136 wrt ground.

The RF CW signal is detected or rectified by the 2nd detector. You vary the 8601a signal until you get roughly 2 to 3 volts ACROSS the detector load resistor R119. The -4.5v should not be changing much: how much is it changing?

It's not...I was varying the output RF voltage way too high messing with it when I had a bad VTVM lead connection. With the latest min/max adjustments it didn't vary much at all

[Chris K]

Originally Posted by Penthode

with regards to the core positioning for maximum. You said the core was unscrewed to get the peak. Unscrewing the core reduces inductance which means you are tuning it higher in frequency. What I think you have done is tune it to the same frequency as another stage and it has introduced oscillation! I mentioned the danger earlier of it breaking into oscillation. I suggest you must fix this before adjusting the traps!

Put the core back close to the original position. The circuit tunes broadly and you should detect a smaller peak near the original position. Also if you do get into a state of oscillation and cannot get out of it, what you do is increase the AGC voltage until the oscillation ceases and try increasing the 8601a input until you see the signal.

Certainly if you are having to adjust any opf the circuits that far away, you are doing something wrong. Becareful and stop and think and ask before making assumptions.

OK I will go back and turn the core back in on T-106 and look for the smaller peak.

[Chris K]

Oh darn darn darn darn darn...I think I may have done something wrong! Now this is how inexperienced and electrostupid I am...but it's good because I'm making mistakes I'm learning from...

I was not paying attention to the VTVM DC voltage position...positive or negative. Obviously, if it was set to DC-, the peak is going to be deflection of the meter to the left correct? I can't account for the position when I did the min/max adjustments so it's back to step 6 once again!!!!!!! Besides getting valuable lessons, I sure am learning this particular chassis and the audio and video circuits!

[Penthode]

Maximum is deflection to the right and minimum is the dip to the left.

The problem here is the careful staggered tuning in order to get the full 4MHz bandwidth. There is a lot of gain in the four stages. You can assume unless someone before has tinkered with it, the stages were close to being correct.

So let's go back. Make sure it is not oscillating. To confirm it is not oscillating, switch the HP8601a off and ensure there is no voltage across the 2nd detector load. By no voltage, I mean less than 100mV. Confirm the traps are correct and then set the preliminary staggered tuning setup prescribed in the manual. The spot frequencies are to get the alignment close before the sweep alignment.

[Chris K]

Originally Posted by Penthode

Maximum is deflection to the right and minimum is the dip to the left.

The problem here is the careful staggered tuning in order to get the full 4MHz bandwidth. There is a lot of gain in the four stages. You can assume unless someone before has tinkered with it, the stages were close to being correct.

So let's go back. Make sure it is not oscillating. To confirm it is not oscillating, switch the HP8601a off and ensure there is no voltage across the 2nd detector load. By no voltage, I mean less than 100mV. Confirm the traps are correct and then set the preliminary staggered tuning setup prescribed in the manual. The spot frequencies are to get the alignment close before the sweep alignment.

Ok I will do that. Gotta shut it down tonight. Be back on it tomorrow evening

[Chris K]

This snip is from a different RCA model alignment set of instructions but the chassis has the same traps and the same frequencies. There's something in here I don't understand...


I see the warning about getting false, resonant peaks and nulls when "running the cores completely through" . I don't understand the next statement..."The correct position is with the cores in the outside ends of the coils". What do they mean by outside ends?

[old_tv_nut]

It means positioned near the top or bottom rather than the center of the coil.
In other words, start with the coil mistuned by having the core too near the end and move towards the center to get the correct tuning, rather than starting with the core near the center and turning to go towards the end.

[old_tv_nut]

Note in both cases the upper and lower coils are each tuned correctly. The only difference is the amount of coupling. Too much coupling will produce a wide double-humped response with low center frequency response.

[Chris K]

Originally Posted by old_tv_nut

Note in both cases the upper and lower coils are each tuned correctly. The only difference is the amount of coupling. Too much coupling will produce a wide double-humped response with low center frequency response.

Wow...thank you so much. Resonance...harmonics etc...these concepts have always been the most challenging things for me. I can understand a vibration stimulating something else at rest to vibrate but at the same frequency. I don't entirely understand the production of other frequencies from the primary one.

So by coupling here you mean frequency or vibrational coupling...one coil and slug stimulated electrically transferring this frequency based energy to an adjacent coil? These traps are tuned to eliminate specific frequencies from the signal circuit resulting in a broader bandwidth. At least that's my understanding from an earlier part of this thread. How does this primary/secondary tuned pair do that? I have an idea as to how but I'm not very eager to put it out there!

[Yamamaya42]

It's a magic spell, and you are a mystical witch casting a spell on the set, mixing all the ingredients in your cauldron one at a time VERY carefully, get one wrong or out of order, it's all messed up, and you have to start ALL OVER AGAIN!

[Penthode]

I also wish to point out the coil you were earlier adjust from the bottom was not a trap but a stagger tuned stage. Take heed of the attached warning from the manual. This unfortunately happens when one begins making changes without carefully reasoning through.

[Penthode]

So for the traps, you are to find the null with the core on the outer most postion. The stagger tuned stages should be in the relative position you started with. If it falls into oscillation, as I am sure others have had on the model, then tuning each stage to the correct physical (original) position should eliminate the oscillation. I have only had to resort to increasing the bias to fix stubborn cases and never having to shunt stages. But if you are not careful you may have to resort to that in order to get back where you started.