Zenith 25MC33 Set

[Electronic M]

Hopefully that 12.1V is the emission measurement, and not the heater setting.

[bhegges]

Originally Posted by Electronic M

Hopefully that 12.1V is the emission measurement, and not the heater setting.

Yes, that would be bad, when I had the tester out and made notes I guess without thinking used the heater voltage scale as the reference. But hey at 12.1 volts I should have excellent emissions, longevity might suffer a bit

[bhegges]

I am trying to minimize new threads but if needed I can start a new thread, also note this post is a duplicate to a thread on Antiqueradios that I started in regards to injecting a composite signal. Finally note this is the same set that I just finished a full alignment on so I believe everything is working correctly.

To summarize the picture will not sync when injecting at test point C1 and am looking for help.

• I built an op-amp circuit to amplify the signal to match the voltage at C1 which is approx. -2.5 volts and 4 volts P-P. Note I added batteries to drive the signal down negative to -2.5 volts.
• Also used a Sencore VA62 to feed signal into the injection point. The VA62 allows you to adjust the drive voltage positive or negative. A positive signal is inverted and the color bars are wrong, a negative signal is correct but again I can not get the picture to sync. I set the VA62 to 4 volts P-P, note measuring at C1 shows no DC voltage, and again no picture sync.
• Finally tried moving back the injection point by lifting the anode of the video diode and then keeping L5, C18 in circuit, this made no difference.

Relevant parts of the schematic with Injection point and switch


Op-Amp


Composite jack and switch

[bhegges]

Did some more testing and while I initially thought the polarity was correct it might not be. From the bits and pieces of out of sync image I see inverted colors.

Assuming the negative drive on the VA62 implies an inverted output this aligns with what I am seeing but I would expect the signal to then be in sync. Also tried to feed the VA62 inverted signal through the op-amp to add the DC offset but that did not help.

Pulling the 3rd IF tube seemed to help and could almost get the picture to sync. Is there something related to AGC or other signals from the IF that are needed for the sync circuit?