Zenith Color Roundie Alignment with B&K 415

[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.

[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