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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? |
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 |
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. |
The happiest result would be to find one tube is the odd one out and then you could trash that one.
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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.
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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.
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 https://i.imgur.com/2HAc7ZX.png |
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. |
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 |
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:
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. |
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?
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: https://i.imgur.com/8UXuUT8.jpg 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: https://i.imgur.com/FcXuzga.jpg |
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:
https://i.imgur.com/R9kqx9u.jpg |
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. |
IF capacitors that could be in question:
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. |
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. |
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... https://i.imgur.com/33leKSj.png https://i.imgur.com/0Fpe1U3.png
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.
https://i.imgur.com/ivXhv1j.png |
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. |
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%. https://i.imgur.com/3L7yKuK.jpg https://i.imgur.com/rfD4r4p.jpg https://i.imgur.com/Ty8ZWCr.jpg https://i.imgur.com/ps4OXEv.jpg |
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. https://i.imgur.com/Af39TQY.jpg |
"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" ? |
Quote:
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"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."
When you say dip do you mean lower on the scope screen (actually an increase in signal)? Matching that blip is not important. What is important is that the response at 42.75 and 45.0 are equal, 45.75 is at 50%, 39.75 is practically zero (high on the scope trace) and 41.24 is high on the trace, at -24 dB or greater, which means less than 6% of the peak response. |
Also, 45.75 should be at 25 or 30% per the drawing, not 15% as in your last trace.
Correction: I meant 41.75, whihc I was reminded by later post is actually 41.67 in the B&K. |
I think I got it. Note the B&K 415 outputs a 41.67mc vs 41.75mc marker. I maybe could try to reduce the 47.25mc marker as it is not quite at zero. The peak to peak signals at each test point is still an interesting element as changing the amount of signal from the B&k 415 going into the tuner does change the response curves. I might try to measure the tv when it is normally running to see what I get.
C1 https://i.imgur.com/Yi3MCP2.jpg C2 https://i.imgur.com/s7ccnzw.jpg |
I think this looks excellent.
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Next up: Sound and 4.5mc trap, these went well. See below for directions:
Sound
4.5 Trap
Chassis test point and component locations: https://i.imgur.com/aWwl92O.png https://i.imgur.com/oKXWk2O.png |
Moving right along to the Color Amplifier alignment, below are the steps I took. Also posted are the Zenith Service manual and SAMS directions.
Here is my starting point, mostly a mess. The 3.0mc marker needs a lot of gain, and the either the 3.58mc has too much gain or the 4.1mc needs more gain. https://i.imgur.com/nYFT040.jpg Zenith Service Manual CM-105 (25MC33 Chassis) https://i.imgur.com/vNUGYCL.png SAMS directions (Folder 757, Set 4) https://i.imgur.com/DTkwQAi.png |
For now I skipped the Zenith direction around replacing the 8.2k damping resistors and simply followed SAMS, unfortunately I could not get the proper response curve by adjusting L17 and L19. first up I will try some different 6KT8 tubes before pulling the chassis out to further investigate.
See the following post for the color circuit schematic from the Zenith Service manual. I have marked in red the relevant Zenith and SAMS coils and test points. Highlighted in green are out of spec parts I earlier replaced, note A3 is an integrator (Zenith part number 87-9), I edited the schematic to show the parts used to replace. |
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I tried swapping out the two 6KT8 color amp tubes and seen some improvement from changing the first color amp tube (V6) but still not getting enough response at the 3.0 marker.
https://i.imgur.com/VxxXnFC.jpg |
I’m confused... since the scope photo is posted upside down, what is the marker directly above the Tektronix “x”? :scratch2:
jr |
The lower right marker is the 4.5mc marker. I flipped the scope picture so to match the Zenith Service manual figure 14 reference. See my above post for the picture.
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How much does the 3.08 MHz marker move when you adjust L17/A26?
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Here are the curves when adjusting L17 (1st Color Amp Plate Coil) for min and max 3.08mc response:
https://i.imgur.com/pvGghdS.png Also for reference below are the responses when adjusting L19 (2nd Color Amp Plate Coil) which is used to minimize the 3.58,mc response, pics showing min, middle, and max adjustments: https://i.imgur.com/TCWQGOC.png In looking over the full color alignment directions the Zenith manual notes to perform the color amplifier alignment first via adjusting L17 and L19 as the color sync alignment will be affected. Also interesting that SAMS alignment directions are the opposite and have you do the color amplifier alignment last. Question how much affect will the remaining alignment affect the color amplifier response? I am curious if there is an issue with the color amplifier circuit or if the issue is further misalignment. I might pull the chassis and double check to ensure I didn't make any repair mistakes. The earlier posted schematics highlights replaced parts, mostly way out of spec resistors and then film capacitors. Note there are a couple of suspect ceramic capacitors that in-circuit measure over the 10% spec:
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C54, C59, c61 are bypass caps not likely to be a problem unless far too small.
C55 looks like it could load T9, maybe check it and the resistors on the other end of it in the burst amp circuit? I forgot - did you check the 8200 ohm damping resistor connected to T19? How about C58? |
Have you also checked all T19 windings to make sure nothing is open?
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I went in and measured all of the parts in the color amp and burst circuits, pulled leads where needed. All of the coils and transformers seem to measure ok via resistance and inductance measurements. All resistors and capacitors are in spec except the following:
The only other thing I noticed is the 2nd color amp tube socket appears in poor condition, its the wafer style socket and its bowed down. I will do some further continuity tests while wiggling and also will try to wiggle the tube once I get the chassis back in place. I will replace the above capacitors and report back. Also getting at the 8.2K damping resistor is a real pain, I'm a bit surprised replacing this with a 100K test resistor was part of the Zenith service manual alignment process. Note SAMS simply skips this step. https://i.imgur.com/DnOgp5b.jpg |
I am studying the schematic and noticed that the earlier 25MC30 chassis used a 270K resistor on grid 1 (pin 1) of the 6EW6 burst amp tube and the later 25MC33 chassis switched to a 150k resistor. Thoughts on the design reason for the slight change?
For reference these two chassis are probably 99% identical, my chassis is stamped 25MC33 but must have been an early one as most of the changes for the later chassis are not present on my chassis to include it using the early design 270k resistor. |
No idea why they decided to change the resistor value.
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By the way, you said the 8200 ohm is hard to get to. Were you able to check the coil winding it is attached to?
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I removed the coil from the chassis and then was able to remove and measure the 8.2k resistor. The resistor measured 8.24k. Also the 36pf capacitor measures 36.14pf. I have a used pull spare transformer so I could measures against each other measure, they are very close in resistance and inductance so I assume good vs both bad.
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