Zenith Color Roundie Alignment with B&K 415

[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:

[bhegges]

Here is what I get off of the cathode of the 2nd color amp tube and a 150 ohm resistor (measured here as this resistor is soldered into a pin that is assessable from the top. Color level set to mid-point, I will assume the peak is at 3.63mc, here too the 3.08mc has a reduced response. As expected at max color level there is no signal as the pot goes to zero. Adjusting L17 only increases or decreases the overall response curve while holding the 3.63mc marker at the peak. I also tried to measure at Pin 7 of the first color amp but the signal was too low to really measure.

With the directions for L19 to minimize the 3.58mc marker I assume that leaves the earlier circuitry to provide an overall response curve wide enough to cover 3.08mc through 4.08mc. Here with the 1st color amp the center point seems like it should be shifted in frequency a bit lower.

[bhegges]

Reading the Color TV Training Manual 3rd Edition. Below is the overview for the chroma bandpass amplifier. Conveniently they picked a Zenith circuit as an example, they note is that the 7pf capacitor blocks the luminance signal, if this capacitor was off spec a bit could it attenuate towards the 3.08mc signal?

[bhegges]

One more post from reading Basic Televisions Principles and Servicing Forth Edition. Here their section on the chroma bandpass explains that the chroma amplifier circuit should peak a little above 4.1mc so when fed from the IF circuit you get the typical textbook response. This would still suggest the response I get out of the 1st chroma amp is not peaked correctly, so who know what the issue is with my chassis.