Zenith Color Roundie Alignment with B&K 415

[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

[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]

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.

[old_tv_nut]

OK, but after adjusting L18, did you try readjusting L19 to get things flatter?

[bhegges]

A few more actions and notes:

• L18 (Color Level Coil) - tried to adjust it against different L19 and L17 coil settings. At the extreme it amplifies the 4.08mc marker which does not help, for now I settled on when the color level is set to the minimum then setting L18 to have what would be the 3.63mc marker at minimum which leaves the 4.08mc and 3.08mc markers at the same level, this is also about what is done in the Zenith color level directions, example below. Note this image is with with the B&K 415 signal turned up, at lower settings this is a flat response.
• 
• L19 (2nd Color Amp Coil) - when adjusting the slug starting with it at the top (nearest to chassis) the 3.58mc marker has the largest response (like a bell curve) with the 3.08mc and 4.08mc markers both lower. As I turn the slug down the 3.58mc start to fall, previously I kept turning the slug until the 3.58mc marker was just to as low as it would go. In doing this the 4.08mc markers would gain strength and throw the curve off, extreme example below.
• 
• Today for L19 I instead only turned the slug down until the 3.58mc marker just fell below the 4.08mc marker, note the 3.58mc marker can be further decreased but at the expense of the over all curve, maybe this is a 1st of the two dips that would be seen with the VTVM.
• L17 (1st Color Amp Coil) - As I adjust this coil there appears to be a center area on the slug which brings the 3.58mc marker to it lowest and then starts to provides some over all curve gain, eventually the 3.08mc marker starts to drop off, I stopped right before this point. Further adjustment will cause the 4.08mc and 3.58mc markers to increase but then the 3.08mc marker will start to decrease.
• Note no combination of adjusting coils would yield individually strengthening the 3.08mc marker response.
• The strength level of the B&K 415 does affect the curve, would it be fair to assume that I should keep the level turned down so that at max color level there is no clipping?
• Several videos below, note I feel like the color amps are turned way down, i.e. the previous setting provided lots (maybe 2x) gain, maybe that is ok...
• Here is a video starting with the color level at minimum and going to maximum, note I have the B&K 415 signal strength set so that a maximum color level there is no apparent clipping (curve flattening): https://imgur.com/YFLdxcx
• Here is a similar video starting with the color level at minimum and going to maximum, but have the B&K 415 signal strength set to maximum: https://imgur.com/l563K5B
• Finally here is where I have landed for now: the response curve with color level set to the midpoint and the B&K 415 signal strength set to avoid clipping.
  

One option would be to pull the 8.2k resistor, add the 100k resistor and via a VTVM adjust the 2nd color amp coil to the 1st dip (nearest the chassis). I am confused by the directions, it notes to disable the sweep but to pass in the 3.63mc marker, the B&K 415 combines these functions so not sure how I would do this. What is the VTVM measuring, I assume overall voltage. Does a single marker act like a signal generator?

[bhegges]

Would there be an affect on the color amp response from the other color circuits such as the oscillator or demodulator? To get the TV up and running I avoided replacing most all ceramic capacitors. Below are the ones that measured way off.

• C92 & C93 - all 4 .001uf measured 18-21% low
• C103 - .001uf measured 19% low, is this just for bypass? or would this affect the 3.58 oscillator
• C104 - 100pf measured 17% high but this was measuring in circuit, I suspect this will measure right on if I pull a lead
• C67 - 100pf measured 28% high but this was measuring in circuit, same thing I suspect this will measure right on if I pull a lead
• C69 - .001uf measured 18% low
• Note capacitors under 100pf may have measured high or low but without pulling them complete out they are hard to measure and I am assuming they are ok.

[old_tv_nut]

https://imgur.com/YFLdxcx -- increase is linear, shape doesn't change - very good measurement conditions.

https://imgur.com/l563K5B -- obviously overdriving and distorting at higher levels

"Would there be an affect on the color amp response from the other color circuits such as the oscillator or demodulator?" - No.

"where I have landed for now" - I'd say leave it this way for now and see how the picture looks, but it still is strange that you cannot get the response higher at 3.08 MHz.

Could you remind me where on the schematic is the sweep signal injected?

[bhegges]

Sweep injection for the color amp alignment is at C1 which is just after the IF picture detector diode. This bypasses all of the IF alignment circuitry accept the 4.5mc filter (which I fiddled with just to see if it had an affect, nope it just shifts response curve left or right of the 4.5mc marker)

I agree it is strange I can not get the response higher at 3.08mc. I would like to think it is not something dumb with my testing setup or tools. Thinking though could an out of spec coupling capacitor pass but attenuate part of the frequency response? Otherwise I am looking at LC circuits which all have been gone through.

Could I create a test point after the first color amp circuit, or at other points to see where I lose the response? Thoughts on where in the circuit?

[bhegges]

For Reference here is the full Zenith CM-106 Service Manual Schematic: