Zenith Color Roundie Alignment with B&K 415

[old_tv_nut]

I think this looks excellent.

[bhegges]

Next up: Sound and 4.5mc trap, these went well. See below for directions:

Sound

1. Tune in a station, note I used a DVD player that I could pause and then turn up the volume to hear the noise
2. Added an attenuator in-line with the converter box and tuner, set to 20db attenuation
3. Adjust the quadrature coil, intercarrier coil, sound take off coil and buzz potentiometer to minimize hum/buzz/hiss

4.5 Trap

1. Tune in a station
2. Color switch on
3. Ground test point K
4. Connect the standard oscilloscope probe to test point K
5. De-tune slightly with the fine tuning, when doing this I could see the below line on the scope fatten
6. Adjust L7 to minimize the 920kc beat
   

Chassis test point and component locations:

[bhegges]

Moving right along to the Color Amplifier alignment, below are the steps I took. Also posted are the Zenith Service manual and SAMS directions.

• Disconnect the tuner antenna input and set tuner in between channels
• Color switch on, color level set to the halfway point
• SAMS directions say to set the hue to the halfway point so I did this too
• Ground test point K
• Connect -6 volt bias to test point Q
• Connect the oscilloscope to the B&K 415 via the Horz and Vert inputs
• Connect demodulator probe (PR-151) from the B&K 425 Demod connector to test point U which is the cathode of the R-Y demodulator tube
• B&K 415 output set to Chroma, function set to Video
• Connect the B&K 415 signal to test point C1, I used the supplied RF Cable probe (TP-41) set to 75 ohms
• Set the sweep center frequency to 3.6mc, I kept the sweep width at max
• Note the B&K 415 does not have a 3.63mc marker

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.


Zenith Service Manual CM-105 (25MC33 Chassis)


SAMS directions (Folder 757, Set 4)

[bhegges]

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.

[bhegges]

[bhegges]

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.

[jr_tech]

I’m confused... since the scope photo is posted upside down, what is the marker directly above the Tektronix “x”?

jr