Zenith Color Roundie Alignment with B&K 415

[old_tv_nut]

Question: when you changed output to IF, did you need to adjust sweep width/centering to get the chroma bandpass curve centered on the scope?

[bhegges]

Originally Posted by old_tv_nut

Ok, when you switch the B&K to IF but inject at C1, you apparently get some video detection action, I guess. But there's no telling if it is loading any traps correctly.

The next thing I would try is leaving the B&K output set for IF but moving the injection point to the IF input/mixer test point. With all the IF and trap stages in operation, it may make the curve even closer to ideal.

I would agree and will do this next. Thank you.

[bhegges]

Stay tuned for a troubleshooting interruption. While still connected at C1 I was double checking the response curve and the waveform started to get erratic and now only has a minimal response curve. I pulled the test equipment and I have a very blue screen and basically no color, something failed.

[bhegges]

Not sure what the root issue(s) were with the problems I had. As noted above first there was an issue with the color amp signal which occurred as I adjusted the Heathkit IG-5240 pocket signal generator, I fiddled with the controls which maybe was my problems.

From there after pulling and putting the chassis back in the issues changed to diagonal lines of signal against a white screen and then later a bright but faint image. The IF and color amp signals now looked correct so I poked around in the AGC and video circuit, all parts in these circuits measured ok. The Zenith Color TV Service Manual by R.L. Goodman has a section on actual issues and resolutions. One issue noted: Washed-out picture, no sync. AGC, sync trouble. Set may have to warm up before picture fades out. The cause was a leaky or shorted .01uf capacitor at grid, pin 5 of the sync - AGC tube. The capacitor tested ok but regardless I replaced it, was this the issue, I don't know but the set is back working.



Back to the alignment:

Originally Posted by old_tv_nut

Question: when you changed output to IF, did you need to adjust sweep width/centering to get the chroma bandpass curve centered on the scope?

I did not need to change the sweep width/centering. The only change needed is with the signal output and scope vertical settings to get the response curve to show well on the scope screen. The Zenith Service manual does not show what the curve response should look like when feeding the sweep generator through the tuner and IF sections but I assume it looks good. I still get a lower response for the 3.08mc marker when connecting at C1, maybe this is an artifact of the test equipment? Note when connected at the tuner adjusting L17 does a nice job of balancing the 3.08mc and 4.08mc markers, I assume they should have an even response.

[bhegges]

Question: Which input should I trust for the color amp alignment in regards to feeding the B&K 415 signal into the tuner vs test point C1 (post video detector)?

Connecting the B&K 415 to the tuner with the output set to IF I can adjust L19 to where I see the 3.58mc marker decrease and then have a defined low spot, also L17 allows me to create a flat-ish response with the 3.08mc and 4.08mc markers even.

Below is the optimized response curve with the B&K 415 signal input at the tuner, the 3.58mc marker is at its lowest response, and then the 3.08mc and 4.08mc markers are even. The 2nd pic is then the resultant response curve when moving the input to C1 and setting the output to video, note leaving it on IF works but barely yields a response curve. I am inclined to think I have it set correct this way and the "odd" response curve at C1 is due to the B&K 415 vs a mis-alignment.

[old_tv_nut]

Your final result with the B&K 415 input to the tuner is obviously correct.

It's the overall result of signal going through the whole chain from tuner to picture that needs to be correct. The odd result when going in at C1 may be the result of the video detector circuit being loaded by the signal cable, plus, it is possible that both the SAMS and the Zenith documents are screwed up in not recognizing that the chroma bandpass by itself should have a tilt that compensates for the IF response.

Declare victory and move on!

[bhegges]

I went back through the subsequent color alignment steps to double check everything. I posted directions back a few pages. The only notable item was for the color sync alignment you adjust L34 for zero beat on screen and minimum movement of color bars through picture. This was very dependent on fine tuning and my rf converter and signal bar generator both require a bit of different fine tuning. Adjusting to the signal bar generator probably means the rf converter could be off, maybe there is plenty of "wiggle" room with this adjustment.

The last color alignment step is to adjust the 3.58mc traps. The directions are to connect a color bar generator to the antenna, connect the scope to test points RR, SS, & TT and adjust each trap for minimum for minimum 3.58mc response. My question, what am I looking at on the scope and what component is the 3.58mc response?

[old_tv_nut]

1) It seems odd that the color oscillator beat frequency should be affected by fine tuning, but is very normal that when adjusted using one source it will be different with another source. It depends on whether the sources are close in color subcarrier frequency or not.

2) At the horizontal sweep rate you are using on your scope, adjust the 3.58 traps for minimum "fuzz" (trace thickness), which is caused by residual 3.58 MHz at the output of the demodulators. You may also be able to see the 3.58 MHz in the picture as a fine, moving dot pattern.

[Electronic M]

Sometimes when chroma osc lock is effected by fine tuning it is a result of the horizontal hold being set to the edge of lock and the flyback winding pulse that gates the burst isn't lining up with the burst.

[bhegges]

I am calling it done. The set has a much better picture, probably not perfect but I am very happy. Maybe related to the full alignment or maybe due to the new to me Blonder Tongue BAVM-860SAW modulator but I no longer get horrible noise during commercials transitions which made it un watchable before. Next will be a final color purity and convergence once I get the set brought into the living room.

A special thank you to old_tv_nut. Your help and advice very much appreciated.

[old_tv_nut]

Pictures, please!

[bhegges]

Some pictures below to include the Danish modern cabinet and also my test bench setup. As I look close I see some ghosting or smearing, maybe an artifact of the setup and adjustments, also the CRT in the cabinet should be a bit stronger so the guns and contrast can come down.

[old_tv_nut]

Thanks!