Zenith Color Roundie Alignment with B&K 415

[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.

[bhegges]

Originally Posted by 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.

I found some info on the TP-41 cable here: https://www.youtube.com/watch?v=kRaYG-dmCPw and have drawn it below. Note when using this cable connected to the tuner mixer grid input I have the B&K 415 set to ch4 and am using the 75 ohm setting, I tried the 300 ohm setting which works and seems to provide the same curve but I have to turn down the vertical resolution to fit it on the screen. For the earlier 41.25 Trap Alignment I connected the cable to the 3rd IF grid (test point G), set to 75 ohms and the B&K 415 output set to IF.



Zenith Service Manual Sweep and Marker generator circuit drawn below.



The directions note to use the above circuit on the first step which is 41.25 Trap Alignment, it does not mention it's use for the subsequent steps, either you don't need it or it is assumed you use for the remainder of the steps. Below is all that is said for connecting to the tuner. If the circuit is to be used the B&K 415 circuit is close, if you are not to use this circuit than the B&K 415 supplied cables are lacking as it did not come with a BNC pass through cable, which I hope means you don't need it.

[old_tv_nut]

OK, the 75 Ohm setting of the B&K termination is closest to the Zenith drawing, and has a capacitor of reasonable size in series.

At this point, I'm not sure if the difference is enough to cause a problem, but the best way to find out is to use a direct cable with the Zenith RC network and see.

Another thought: is it obvious which of the B&K clip leads is ground and which is the 750 pF? Could they be reversed?

[bhegges]

Take a look, below are 3 scope images using different cables to feed signal into the tuner converter control grid (Test Point A). Note I had to turn down the vertical scope by one setting for the direct connection cable as the signal came in stronger. I tried to overlay all three images in photoshop, each was slightly different in horizontal or vertical size but I would have to argue the curve and marker locations and marker attenuations all look very close.

Maybe antidotal but while waiting for the set to warm up I was watching some content via the converter box and the picture to me looked better, so at least I am not making it worse.

[bhegges]

I may try to further tweak the IF alignment but its maybe about as good I can do. Would there be any components that are suspect? See below for all of my in-circuit measurements, I did replace the 2 film capacitors also using NOS tubes that test very good, I guess I could swap just to see what happens.

[old_tv_nut]

I don't have enough direct experience with this particular chassis to point at a particular component and say "look at that." The curves are close to
the manual except more peaked in the center, so actually may be within normal range, especially since you see differences depending on the cable.

One experiment you could try: Since A9, A10, and A11 have their tuned frequency specified on the schematic, simply tune each one back and forth a bit to see if each is really producing maximum response at the specified frequency. If they are, I would say there's no more to be done.

[bhegges]

Here is an interesting finding, I swapped out the three IF tubes with spare tubes. No real change for the first and second IF tubes but the third IF tube (6EJ7) did yield a much different response curve, see below. Note the tubes were tested with a B&K 747 tube tester where good starts at 65 on a 0 - 120 scale.

Sylvania 6EJ7 testing at 77, this is the tube I have been using


Alternate tube, Zenith 6EJ7 testing at 80, note I still need let this tube run for a good amount of time, but interesting how much the tube affects the response. Also note about 1 turn on the alignment transformer and I can get the curve about correct but not as good as the first tube.

[bhegges]

A thought: while each tube type tests differently the good 6EH7 tubes I have all test around 90 to 100 while the 6EJ7 tubes I have only test around 75 to 80. Again different tube type but maybe the 6EH7 tubes I have are a bit weaker.

[old_tv_nut]

Your tube tester may be telling you whether the tube's emission is normal or weak, but it will tell nothing about the frequency response.

What do you mean - "need to let it run a good amount of time?"

The difference in response with different tubes is way too much. It looks like the last waveform you posted is grossly mistuned, the dip at the 4th major division from the left should not be there, just a smooth slope like the next to last waveform.

From your chart it looks like you replaced the few caps that were grossly out of tolerance., so unless you made a mistake resoldering them, there's no clue there.

Do you have more 6EJ7s to try?

Could we be missing something that can't be seen in your posting, like a dirty socket? What happens if you wiggle the tube?
Is the tube shielded? Does the shield have good ground contact?

[bhegges]

Thanks for the next round of advice, see below in red my response. I will work through the below and report back.

What do you mean - "need to let it run a good amount of time?"

I snapped the picture right after the set came on with the tube substitute, I thought maybe letting the set run for 30min might make a difference as the tube warms up.

The difference in response with different tubes is way too much. It looks like the last waveform you posted is grossly mistuned, the dip at the 4th major division from the left should not be there, just a smooth slope like the next to last waveform.

I agree this is odd, when I substituted the alternate 6EJ7 I got the large dip, I could get a smooth slope but had to adjust the coil (L6) by approx 1 turn. I would think a bad socket as you noted could be a possible culprit. I will do some investigation here.

From your chart it looks like you replaced the few caps that were grossly out of tolerance., so unless you made a mistake resoldering them, there's no clue there.

the chart I posted was what I measured with the parts in circuit, no parts in the IF section with the exception to the 2 film capacitors were replaced. Everything I read says leave the IF section alone unless you are sure they are bad. As for the out of spec parts they are ceramic capacitors and I will guess/hope that if I lift a lead and measure they will be ok too, but I may have to come back and check. C19, C20, and C21 are listed as 10% and I ,measured them at 20%, so maybe worth pulling a lead to better test

Do you have more 6EJ7s to try?

Yes I will try some more swapping?

Could we be missing something that can't be seen in your posting, like a dirty socket? What happens if you wiggle the tube?
Is the tube shielded? Does the shield have good ground contact?

the chassis by design has no shields on the IF tubes, the chassis is also in very good clean shape, thus the tube sockets all look clean, that said I tried substituting with a 3rd 6EJ7 that I did wiggle and got the scope curve to bounce around, I will go back and check this out more closely

[old_tv_nut]

I didn't understand the cap readings were in-circuit. Never mind - probably all OK.

I will be intersting to see how much variation you get with multiple tubes.

[old_tv_nut]

The happiest result would be to find one tube is the odd one out and then you could trash that one.

[old_coot88]

Back in the day, ringing of fine detail in the luma was cured by replacing the 3rd IF/6EJ7, even though the tubes showed 'good' on the tester. Ran into this right after the 6EH7/EJ7 lineage started appearing.