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]

[old_tv_nut]

How much does the 3.08 MHz marker move when you adjust L17/A26?

[bhegges]

Here are the curves when adjusting L17 (1st Color Amp Plate Coil) for min and max 3.08mc response:




Also for reference below are the responses when adjusting L19 (2nd Color Amp Plate Coil) which is used to minimize the 3.58,mc response, pics showing min, middle, and max adjustments:




In looking over the full color alignment directions the Zenith manual notes to perform the color amplifier alignment first via adjusting L17 and L19 as the color sync alignment will be affected. Also interesting that SAMS alignment directions are the opposite and have you do the color amplifier alignment last.

Question how much affect will the remaining alignment affect the color amplifier response? I am curious if there is an issue with the color amplifier circuit or if the issue is further misalignment. I might pull the chassis and double check to ensure I didn't make any repair mistakes. The earlier posted schematics highlights replaced parts, mostly way out of spec resistors and then film capacitors.

Note there are a couple of suspect ceramic capacitors that in-circuit measure over the 10% spec:

• C54 - .0047uf 10% 500v measures .003739uf 20% low, maybe an issue?
• C55 - 47pf 10% N75 500v temp measures 54pf 16% high probably good if I pull a lead to measures
• C59- .001uf 10% 1kv measures 840pf 16% low, also maybe a problem?
• C61- .001uf 10% 1kv measures way high probably getting in parallel capacitance somewhere

[old_tv_nut]

C54, C59, c61 are bypass caps not likely to be a problem unless far too small.
C55 looks like it could load T9, maybe check it and the resistors on the other end of it in the burst amp circuit?

I forgot - did you check the 8200 ohm damping resistor connected to T19? How about C58?

[old_tv_nut]

Have you also checked all T19 windings to make sure nothing is open?

[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.