Zenith Color Roundie Alignment with B&K 415

[old_tv_nut]

You can also try adjusting the center frequency, as long as that doesn't clip off one end or the other (left or right) of the waveform.

[bhegges]

Continued 41.25 Trap Alignment:

• Connect from the "Direct" output to test point C2
  Adjust T4 top and bottom slugs to match Figure 11, posted below
• These did take a very small amount of adjustment, hopefully I make it better, see below, sorry looks like the x axis on the before pic is not the same as the after,

[bhegges]

L1, L2, L3 alignment:

• Grounded test points E, B, F and tuner in-between channels
• Connect from the "Direct" output to test point C2
• Connect from the "RF-IF-Video" output to test point A on the tuner and set the function to CH 4, I kept the probe set to 75 ohm
• Directions note if needed to apply a positive voltage to test point E, I did not

The direction call to adjust for minimum response as you adjust each coil and there may be some interaction, I was not sure on how to do this as it relates to the scope response, I just tried to get the response cure to match the picture, zooming in on the scope was some help. SAMS directs making these adjustments and using a VTVM to indicate. I tried this too but not easy to adjust to the minimum, note I cheated and used test point C2 (accessible on the chassis) which is in front of R49, 330 ohm, where SAMS has test point E on pin 7 of the sound & sync amp tube.

Adjustment to L1, L2, and L3 got tricky real quick and trying to keep track of the adjustments in case I need to go back went out the window. I did find adjustments in the subsequent step helps with getting a matching response curve. I likely will repeat all of these steps as I zero in on the alignment.







Starting point with the same 7 markers turned on as indicated in Fig 12, note Zenith uses a 41.75 while the B&K 415 has a 41.67, close enough

[bhegges]

Tuner converter plate coil & IF Transformers T1, T2, T3 alignment:

• Grounded test points E, B, F and tuner in-between channels
• Connect from the "Direct" output to test point C1 and later C2
• Connect from the "RF-IF-Video" output to test point A on the tuner and set the function to CH 4, I kept the probe set to 75 ohm

Adjustments are described below. Question: the directions often state adjust for maximum or minimum response at the specified marker frequency, I assume I should adjust the specified coil to get the marker to fall on the pictured response curve in the correct place, but I can also adjust to simply get the highest or lowest scope response for the marker, doing this completely messes up the curve.

I will follow up with some pics of where I land for the final IF & Trap alignment.

[old_tv_nut]

Originally Posted by bhegges

... I kept the probe set to 75 ohm[/LIST]... I assume I should adjust the specified coil to get the marker to fall on the pictured response curve in the correct place, but I can also adjust to simply get the highest or lowest scope response for the marker, doing this completely messes up the curve.

...

1) Exactly what probe are you setting to 75 ohms? Is this a scope input setting?

2) What do you mean by "fall in the correct place?" If the procedure says min or max at the marker, it means only that - the vertical position of the marker should be min or max. If the curve ends up looking wrong *at the end after all adjustments have been made*, there is something wrong with your setup or you made a mistake earlier in the procedure.

It is possible the curve won't look right when you are only part way done.

[old_tv_nut]

The spec for 41.25 attenuation at C1 is 24 dB or greater, which means it's Ok if it's higher on the scope (closer to the top, which equals zero signal) than shown in the illustration.

At C2: a 1 dB reduction in signal from the peak is the same as a 20% [correction - 10%] reduction. So the distance shown as 1 dB in the illustration should be 20% [correction - 10%] of the max height. (the sketch looks poorly drawn - more like 10%.) [Correction - the sketch looks correct]You are getting over 30% attenuation at the markers, so this indicates the curve is too narrow.

[bhegges]

Originally Posted by old_tv_nut

The spec for 41.25 attenuation at C1 is 24 dB or greater, which means it's Ok if it's higher on the scope (closer to the top, which equals zero signal) than shown in the illustration.

At C2: a 1 dB reduction in signal from the peak is the same as a 20% [correction - 10%] reduction. So the distance shown as 1 dB in the illustration should be 20% [correction - 10%] of the max height. (the sketch looks poorly drawn - more like 10%.) [Correction - the sketch looks correct]You are getting over 30% attenuation at the markers, so this indicates the curve is too narrow.

old_tv_nut, thank you for the ongoing advice and help. I will follow back up hopefully with an improved alignment. Two quick items, first I transposed labeling C1 and C2 against the last pictures I posted, I wont edit my earlier post as your reply align with what I originally had labeled. Second I have Ch1 and Ch2 set to DC. Here are my scope settings, if something seem incorrect please advise:

[bhegges]

I have tried to get the 43 to 44 mc at 1db (10%) of the 42.75 and 45 mc markers. I got it down to 20% but this comes at a cost of moving other markers out of proportion. Thoughts on which markers are most important to have correct? Note currently I am trying to get the curve shape to match and to keep the 42.75 and 45 mc parallel.

[old_tv_nut]

Please see correction to previous post - 1 dB should have been 10%, not 20%

[old_tv_nut]

Curious that the drawings show 42.75 and 45 equal for both test points, but your measurements don't. Not sure what to think about that.

[old_tv_nut]

OK, I went back through the whole thread from the beginning, and I see you are not using the RC network from figure 6, and have the sweep generator output termination set to 75 ohms.

Try this - keep the sweep generator output termination set to 75 ohms, but add onto it the RC network from Figure 6. My guess is that the 75 ohm impedance is loading whatever grid circuit you connect to and that is causing the incorrect responses.

[old_tv_nut]

I think I read too fast earlier and mistook the RC network in figure 6 for an external marker adding arrangement - apologies!

[old_tv_nut]

Oh! Revelation! You are using the RF cable, 75/300 ohm switchable, which should only be used for antenna input. Use a plain unterminated cable as shown in the Zenith illustrations, with the RC network of figure 6.

[bhegges]

Originally Posted by old_tv_nut

Oh! Revelation! You are using the RF cable, 75/300 ohm switchable, which should only be used for antenna input. Use a plain unterminated cable as shown in the Zenith illustrations, with the RC network of figure 6.

I am using the Zenith service manual directions coupled with trying to use the B&K 415 and supplied signal cable which is the RF cable with built-in 75 or 300 ohm termination. There are no other signal cables described in their manual, see below. Also the snippet below had me believing I could the supplied cables and use the manufacture's directions, the output switch does have options to output video, IF, ch4, & ch 10, but it is odd to use a RF cable to feed into a IF circuit.




The Zenith service manual directions for the IF alignment steps in question state to connect the sweep and marker signals to the converter control grid (Test Point A on the tuner). No reference is made to any coupling circuit, which matches your note to use a plain unterminated cable. Zenith info below:

[old_tv_nut]

In that case, the 75 ohm setting may work when used with the Zenith RC circuit added. The 75 ohm setting may actually just be a direct feed through.