Zenith Color Roundie Alignment with B&K 415

[bhegges]

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:

• C54 - .0047uf measures 21% low
• C59 .001uf 1kv - measures 16% low
• C61 .001uf 1kv - measures 18% low

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.

[bhegges]

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.

[old_tv_nut]

No idea why they decided to change the resistor value.

[old_tv_nut]

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?

[bhegges]

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.

In Use

• Coil 1: 2.94 ohms (37.6uh to 68.8uh)
• Coil 2 (with center tap): .97 + 3.11 ohms (67.58uh to 120.5uh across full coil)

Spare

• Coil 1: 2.91 ohms (38.27uh to 70.45uh)
• Coil 2 (with center tap): 1.03 + 3.08 ohms (68.57uh to 123.3uh across full coil)

[bhegges]

I have replaced the out of spec capacitors but get the same response. Something interesting; the directions state to set the color level to the mid-point, I found turning the color level to approx 1/4 I could get a much better response curve.

I pulled the lead on the color level pot and did some measuring, the spec is 25k, mine measures 1.78 ohms to 29.624k. The taper appears to be a steep logarithmic curve, my mid-point measures 1.375k, and the 1/4 setting is approx 700 ohms. Someone with math skills could probably calculate what the midpoint value should be for a 25k pot. Directionally I know my midpoint is shifted up since my pot measures high.

[bhegges]

SAMS shows the color level replacement as Mallory part number PP253R which uses their #2 right hand logarithmic taper. The chart shows at 50% it should be at 90% resistance (2.5K where mine measures 1.375K). Now it's possible that Zenith used a special logarithmic taper and the Mallory part is all that was available for the aftermarket.

It's odd that my pot being 20% high measures lower than what the Mallory chart says it should be but also odd I need way less resistance to get a good curve (300 to 700 ohms is the range that appears to be a happy medium when adjusting L17 and L19 yield a proper response curve).

Should I move on or does having to set the color level way high (1/4 turn from max) suggest there are other problems?

*Edit* The other factor is the signal level from the B&K 415 going into C1, I have it maxed out but as I turn it up I am able to back off the color level just a small bit. I suspect the only issue is I don't have enough signal strength going in. In the subsequent Zenith Service Manual (CM-107) for the next roundie series chassis the directions include a note that the signal generator and scope may need to be set near the maximum, but don't overload.

[old_tv_nut]

Are there any instructions for tuning L18? The symbol indicates it is tunable. Since the odd response occurs when adjusting the color level control in parallel with it, L18 may be the problem.

I would not be concerned about the mismatch of the log curve as long as it is in the right direction.

[old_tv_nut]

I'm guessing now that L18 is supposed to be self-resonant at 3.58 MHz, and the purpose of setting L19 to minimum at 3.58 MHz is to flatten that peak. So, it looks to me like L18 is tuned to too high a frequency. When you turn the color level, the peak should go up and down at 3.58 MHz, not at the higher frequency shown.

[bhegges]

Continuing along the next few steps went well.

Color Sync Alignment

• Color switch on, color level set to the halfway point
• Color Bar generator connected to antenna terminals set to Color Bar Pattern
• Connect a .01uf capacitor from test point L to ground
• Ground test point K
• Adjust L34 for zero beat on screen and minimum movement of color bars through picture (they were visible and a very slight adjustment removed these)
• Next set VTVM to +50v DC range
• Connect VTVM to test point V, adjust T20 top slug for maximum indication (very little adjustment was needed)
• Connect VTVM to test point V, adjust T20 bottom slug for minimum indication (very little adjustment was needed here too)
• Note T20 slugs should be on outside end of coil not towards the middle
• Remove ground from Test Point K and .01uf capacitor from test point L
• Color switch on, color level set to the halfway point
• Color Bar generator connected to antenna terminals set to Gated Rainbow
• Connect scope to test point R, adjust L29 Burst Amplifier Plate coil to match Figure 15A (a small amount of adjustment was needed)
• The second and forth bars should be equal in amplitude
• 
• Connect scope to test point S
• If needed touch up T20 bottom slug quadrature coil to match 15B (a small amount of adjustment was needed)
• 5th and 7th bars equal in amplitude
• Note slug should require a very minimal amount of adjustment
• 

Color Level Adjustment

• Color Bar generator connected to antenna terminals set to Color Test Pattern
• Connect scope to test point R and set color level to minimum
• Adjust L18 for minimum color output at test point R (a very small amount of adjustment was needed)
• Essentially the picture should be black and white only

Range Check of Hue Control

• Color Bar generator connected to antenna terminals set to Gated Rainbow
• Connect scope to test point R and set hue control to maximum counter clockwise
• First and third color bars should be equal in amplitude (second color bar maximum)
• If not touch up L29 Burst Amplifier Plate coil for equal amplitude of first and third color bars (a very small amount of adjustment was needed)
• Connect scope to test point R and set hue control to maximum clockwise
• 4th and 5th color bars should be equal in amplitude (they were)
• Note if only the 3rd and 5th color bars can be made equal than this is acceptable
• Connect scope to test point R
• Set hue control for equal amplitude of 2nd and 4th (3rd color bar at maximum)
• Now connect scope to test point T
• Color pattern should appear as shown in figure 15C (No pic but it matched)
• 

Alignment Points

[bhegges]

Originally Posted by old_tv_nut

I'm guessing now that L18 is supposed to be self-resonant at 3.58 MHz, and the purpose of setting L19 to minimum at 3.58 MHz is to flatten that peak. So, it looks to me like L18 is tuned to too high a frequency. When you turn the color level, the peak should go up and down at 3.58 MHz, not at the higher frequency shown.

I just posted the next few alignment steps as I just moved along for now with the color amp alignment as-is. I will go back and see what interactions I can get with the color level, L18, and L19. Note L18 measures ok so I believe it to be good. Also note the voltage going in at C1 from the B&K 415 is 0.3207 volts.

[bhegges]

After going through the other alignment steps here is what the Video Amp test point U with the color level set at the half-way point, it looks better than before.



Also here are some videos (note they should open in Imgur, let me know if they don't work)

• Adjusting the B&K 425 signal down to zero and then back to max, curious if more B&K 415 gain would prove a better response curve: https://imgur.com/jmoT9J0
• Color level starting at the minimum and then turning the potentiometer to the maximum: https://imgur.com/5RBpapN
• Adjusting L19 (2nd color amp) which should be adjusted for minimum 3.58mc response. I starting with the slug at the top and turned it down to the bottom: https://imgur.com/lVtZ9qK

[old_tv_nut]

First video: looks like maximum 415 drive is overloading the set and giving a false flattening of the curve by crushing the peak. Don't try for higher, in fact make sure it's not too much.

Second video: yikes what mess! Stick with the suggested half-way setting for now.

Third video: There is only one point that gives a symmetrical response about 3.58 MHz?

[old_tv_nut]

Seeing the correct two nulls for L19 may be confused because you aren't changing the damping resistor.

Again, this suggests to me to adjust L18 so the peak goes up and down at 3.58 with color level adjustment, and adjusting L19 in conjunction with L18 for most symmetrical response.

[bhegges]

I tried adjusting L18 with the goal of having the 3.58mc peak go up and down with color level adjustment. I also kept the B&K 415 signal turned down a bit from max. I don't know that I got it much better, video: https://imgur.com/A9hrDx1

I have checked all of the parts in the color amp circuits and I believe they are good, I have tried alternate 1st & 2nd color amp tubes, and made coil adjustments but the 4.1mc marker always has a much stronger response and it takes the color level at about 70-75% before the curve looks correct. Note probably unrelated but the last step of the color alignment is the 3.58mc traps which I got confused on and did not adjust the traps yet.

I will reset, look everything over, try again, and report back. old_tv_nut, thank you.