Zenith Color Roundie Alignment with B&K 415

[bhegges]

Curiosity got to me so I pulled the chassis out and measured the capacitors around the 3rd IF that my earlier in-circuit testing showed way high/low. See below, what does everyone think, would tighter tolerance capacitors improve the response? C19 looks the most critical and is spot on.

• C18 - .001uf 10% 1kv ceramic capacitor, measures out of circuit 1090pf which is 9% high
• C19 - 100pf 10% 500v N750 temp code ceramic capacitor and measures out of circuit 99.54pf
• C20 - .0015uf 10% 500v ceramic capacitor but measures out of circuit 1194pf which is 20% low (Zenith # 22-12)
• C21 - .001uf 10% 1kv ceramic capacitor but measures out of circuit 785pf which is 22% low (Zenith #22-17)

If I do look to replace C20 & C21 they look to be a generic temp code, searching for the SAMS replacements part numbers returns a temps code of X5F

[old_tv_nut]

All those .001 caps are just bypasses, so value is not critical unless lower than spec'd tolerance.
.0015 is also a bypass, but I would replace it since it's out of 10% tolerance. Note that a larger value with a wider tolerance could have been spec'd here but probably was not because the next widely available larger value would have been more expensive even with the wider tolerance.

The 100 pF is a tuning cap, so should be within spec (as it is). Correction: hard to say the 100 pF is the main tuning, but it may affect it.

[old_tv_nut]

More thoughts:
1) I just re-read C21 is low - replace it.
2) the input capacitance of V3 may be affecting things
3) I hope you don't have anything connected to point D while alighning the IF

[bhegges]

I will plan to replace C20 and C21 with the below parts from DigiKey which have a temp code of XF5 and align with what SAMS specifies for replacements:

• https://www.digikey.com/en/products/...0TSD15/2825257
• https://www.digikey.com/en/products/...0TSD10/2345250

As for the SAMS test point D, that correlates to the Zenith Service Manual test point G. This test point is only used to feed the signal generator in as part of 41.25 trap alignment, for the remining IF alignment steps this test point is unused with nothing connecting to it. Before I order I will double check parts in the IF circuit. I also gambled and purchased on Ebay two claimed NOS 6EJ7 tubes. Thank you, Brad.

[bhegges]

Time for an update. I replaced C16, C20, and C21 which all measured 20+% low. I also spent some time swapping tube and seem to have found the right mix, note I did have a poor performing 6EJ7 tube. I might be getting closer, thoughts on what to try next?

• Does one of the 2 below attempts look ok?
• Are there components I should go back and check?
• It's possible a fresh attempt might yield 100% success.
• I can try again and maybe find a happy middle ground?

Attempt 1
After going through all of the IF alignment steps I was able get the markers and attenuation to match the Service Manual overall response curve at test point C2. But when going back and remeasuring at test point C1 the 41.25mc marker is not at minimum and the response in-between the 39.75mc and 41.25mc markers seems large, not sure if this is concerning. See the image below:



Attempt 2
Next I went back to test point C1 and found adjusting the 41.25mc trap was my best option to minimize the 41.25mc response while not messing up the overall response to bad, note the response in between the 39.75 and 41.25 mc markers is still large. Going back to test point C2 shows the 41.25 marker is not correct. See below:

[bhegges]

For reference here is the SAMS IF circuit schematic with the Zenith Service Manual test points and coil/transformer part numbers. Also here are the abbreviated alignment steps:

1. L4 41.25mc (Bottom Tuned) - set to minimum response
2. L4 41.25mc (Top Null) - set to minimum response
3. Directions say not to go back and adjust L4 (this is what I did to get the option 1 response curve, the response curve in option 2 is closer to correct, but that means other adjustments are needed)
4. T4 Primary & Secondary - adjust to match response curve
5. L1 41.25mc - set to minimum response
6. L2 47.25mc (Bottom Null) - set to minimum response
7. L2 47.25mc (Top Tuned) - adjust to 47.25mc (some interaction may exist)
8. L3 39.75mc - set to minimum response
9. Recheck L1, L2, L3
10. T1 45.75mc - adjust to match response curve
11. T2 45mc - set to maximum response
12. T3 43mc - set to maximum response
13. Recheck to ensure marker locations and attenuations

[old_tv_nut]

Attempt 1 looks better than atempt 2, except - is it possible you have swapped C1 and C2? The scope response to the right of the Figure 12 C2 drawing looks like a reasonable video response.

The scope picture to the right of the Figure 13 C1 drawing has a peak at the sound subcarrier, 41.25, which it seems might make sense to feed the Sound-Sync amp, although the drawing doesn't show such a huge peak.

Did you replace any capacitors in the sound-sync detector area? Particularly note C22 is a very small value, 0.82 pF.

[bhegges]

IF capacitors that could be in question:

• C22 and C23 are original and untouched
• C18, C19, C24 previously measured good after lifting a lead
• C13, C14 replaced film capacitors, re-measured good in circuit 12/27
• C16, C20, C21 all measured 20% low, replaced today and measured good in circuit 12/27
• No other IF capacitors replaced

Doubled checked C1 and C2 test points on underside, all is correct

Maybe an issue: I retested R52 which is a 5% (gold band) 180 ohm that in and out of circuit measures 219 which is 22% high. I will replace, note I only have metal film resistors which some argue against using in the IF circuit.

[old_tv_nut]

R52 looks like it could affect the width of the 41.25 trap.

Looking at your two attempts again, I don't quite understand how readjusting L4 doesn't minimize the 41.25 response at both C1 and C2 simultaneously, resulting in the desired curves, but obviously it's not doing that, and is making one or the other worse.
So, I'm puzzled and waiting to see the result of replacing R52.

[bhegges]

For the alignment directions I have been using the Zenith CM-106 Service Manual which covers the 25MC33 chassis, nearly identical is the CM-105 Manual which covers just the 25MC30 chassis, but there are a few small changes.

Alignment for T4 is slightly different, maybe...

• I just measured the output at test Point C2 and I believe I was overdriving the signal from the B&K 415, reducing the signal does have an affect on the response, I now have it set to exactly 2 volts via my DMM from Test Point C2 to ground.
• I was hesitant to make any adjustments on the tuner but with the above detail I rocked the converter plate coil on the tuner and believe the adjustment may help.

My chassis is a 25MC33, note SAMS lists both chassis under the one folder. These two chassis are very similar with only minor circuit changes I could see in the schematic. Interestingly the IF section on my chassis is wired like a 25MC30.

• C15B on the 25MC30 is a 0.001uf vs a 470pf to ground on the 25MC33
• C17B on the 25MC30 is a 0.001uf vs a 470pf to ground on the 25MC33
• An added 100pf capacitor is present from Pin 6 to Pin 8 on the sound sync amp tube for the 25MC33

[old_tv_nut]

Aha! These curves do show a peak between 41.25 and 41.75 at C2. It also looks pretty obvious that you swapped the C1 and C2 sweep traces in your previous posts.

Based on what you just posted, I would say the "attempt 1" traces look correct or very close.

[bhegges]

I am getting closer. I now am ensuring the sound detector (C2) voltage is at 2 volts and the video detector (C1) is at 6 volts. Over/under driving these voltages does affect the signal response. I also labeled the scope each time I snapped a picture so not to mix them up (label includes signal generator input and scope output). See below for where I am at:

The one concern I have is on the sound detector C2 response curve there is no real response dip like shown, my curve just starts to slope down continuously until it gets to the 41.75mc marker. Is it possible that I am still way off and need to push the 41.25, 41.75, and 42.75 further right on the wave form, that would put the 41.25 on the top of the first peak but currently is at 25% vs the spec of less than 5%.

[bhegges]

Out of curiosity I adjusted L1 (41.25mc trap) and L3 (39.75 trap) to their max and I was able to get the below curve, all other coils and transformers I left as-is. It seems like if I had a bit more adjustment in the coils I could get the response curve to dip between the 39.75mc and 41.25mc markers. Also not sure then why such a response around the 41.75mc marker, while the drawing shows a bit of a curve here my set exaggerates the response around this marker.

Would there be a particular capacitor in these trap circuits I should look at? All of these capacitors are precision low values that are not easy to measure, I would have to remove the capacitors and either try to measure or just replace.

I also could just move on. Maybe unrelated but after going through the power supply, horizontal and vertical circuits there was low sound and serious buzzing/noise in the sound circuit, the Quadrature, Intercarrier, and Sound Take Off coil all needed a good bit of adjustment. I still get buzzing/noise during screen changes such as when going to commercials.

[old_tv_nut]

"I am getting closer. I now am ensuring the video detector (C2) voltage is at 2 volts and the sound detector (C1) is at 6 volts. "

I think you mean "sound detector C2 and video detector C1" ?

[bhegges]

Originally Posted by old_tv_nut

"I am getting closer. I now am ensuring the video detector (C2) voltage is at 2 volts and the sound detector (C1) is at 6 volts. "

I think you mean "sound detector C2 and video detector C1" ?

Yes, you are correct, I will fix my typo.