Zenith 25DC56 CC- no color sync

[DavGoodlin]

This chassis uses two color dura-modules, as Zenith dubbed the 9-37 and 9-27. The 9-37 has bandpass amp IC, and a demodulator IC. I swapped this with a PTS rebuilt with no effect on the sync issue.

Looking at the 9-27, a crystal and 221-42 IC were swapped out along with a spare module and sometimes you could lock in but the tint control did not work though flesh tones were close, a tad pink.

If the burst from the 9-37 module (bandpass-chroma amps) is missing, would this also present like a missing flyback pulse, which is not the case here?

Most all voltages are close to schematic values.
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[old_tv_nut]

By definition, if there's no burst to the phase detector, it can't work. Have you checked for the burst with a scope?

Does this chassis have a normal/alignment switch between the two modules as shown here? Just guessing it might be a problem.
https://worldradiohistory.com/Archiv...an-1972-01.pdf

[DavGoodlin]

I will scope a look at waveform 46, which is at the switch.

The schematic for Zenith 19CC19 looks like the same one.

[DavGoodlin]

The burst at the APC switch and horizontal pulse waveform on A6 are per the schematic. I will post a scan of what i am working from.

I assume this would clear the 9-37 module as bandpass amp is feeding the burst in properly.

Question: Is the ACC circuit a possible issue? If not, the 221-42 IC may be bad on both modules, though I swapped them with little change.

[old_tv_nut]

I don't see any waveforms on the page I linked, and I don't have a schematic, so I would just be guessing at this point.

Do you have the chroma oscillator adjustment instructions for the 9-27 module, and do they work?

In any case, it seems like the problem is with the 9-27 module or its connections. It's suspicious that both replacements seem bad.

Not sure what A.C.C stands for here. Automatic Color Control? If so, should not affect oscillator lock, which should be a function of the A.P.C. (Automatic Phase Control of the oscillator).

[DavGoodlin]

The oscillator adjustment stopped working. It is pretty odd, as I was able to adjust the number of bars (frequency) previously. Now the APC control has no effect.

You are correct that Auto Color Control is for the intensity, not affecting 3.58 mc osc.

Here is the Sams schematic with notes, and I will find my Zenith version copy and mark it as well.

The H-pulse and burst waveforms to the sub carrier regen look good on the scope. I am thinking the IC took a crap here, 221-42 or ECG 714. I will see if I can get a new one.


Zenith 25DC56.pdf

[zeno]

If the IC has a socket get rid of it. They didnt age well.
You can also get 9-27's out of 4 tube hybrids.
NOS modules are often not. Often they are duds that didnt get
sent back for credit, pulls or just waiting around for " time to rebuild them".

73 zeno
LFOD !

[DavGoodlin]

The socket looks OK but I have a good working 19DC12 TV, the 4-tube chromacolor. I ordered another 221-42 from Moyers. i will check my attic again but yes Zeno, those old modules are never a sure thing

[DavGoodlin]

All done finally! The color is back, after I decided to replace the 221-42 IC used for subcarrier regeneration on the 9-27 module. Plus a spare module that now also works!

The cross reference listed for a 221-42 was an NTE 714 and Moyer's Electronics subbed and sent me two GE-IC-4, which is the GE replacement. The unopened part bags were dated 1981!
The IC looked like a Motorola, and probably was though simply labeled "GEIC-4" with no date.

Per Zeno's instruction, I cleaned the IC socket with ECG contact cleaner. My DeOxit is running low, and this is a good application for the common stuff.

This set required several non-standard parts including the focus drop resistor that looks like a little white rocket, lucky I had that one!! Now it looks fantastic, even before purity and convergence.

[old_tv_nut]

I am trying to recall the company that supplied a lot of Zenith analog ICs - it wasn't Motorola, who were trying to promote their own designs as universal industry solutions.

[old_tv_nut]

I contacted an old colleague who reminded me that Zenith worked with Motorola to make Zenith-designed ICs at least by the late 70s as they were developing their one-chip monochrome IC. I still think it was some other company that's no longer around for the early 2-chip color system. I looked up the IEEE paper on the color sync IC by John Rennick, but unfortunately he is the only listed author and the manufacturer is not mentioned.

[old_tv_nut]

Another neuron fired - these later chips with automatic color could have been Motorola, I don't know.

[DavGoodlin]

I looked at one of the old IC's it was a Fairchild, the other bagged GE-IC replacement has a 274 code, so RCA on that one. Funny how Zenith used RCA transistors more than they did Motorola as OEM.