Zenith 25MC33 Set

[bhegges]

Originally Posted by old_tv_nut

Can you check with a scope set to DC coupling or maybe just with a voltmeter whether the VA62 output runs negative from ground (like the regular C1 voltage) or floats around zero?

In short there is no DC voltage offset from the VA62. As I look over the schematic it looks like everything that gets signal from test point C1 is capacitively coupled so does the DC offset even matter? Now for more details I Took some measurements from both the VA62 feeding the RF generator into the tuner, and then also the VA62 injecting composite video (currently at L5 & C18 with the diode lifted).

Using the tuner:

• C1: -2.97 Volts DC
• C1: 0.25 Volts AC
• 

Injection:

• C1: 0.043 Volts DC
• C1: 0.25 Volts AC
• Note I did adjust the VA62 signal amplitude up to match the 0.25 Volts AC
• Note the scope pictures are with DC coupling and no changes were made other than adjusting the trigger to get a clear waveform.
• Note with the tuner still tuned to the RF generator which is sending a blank signal I can kind of get the picture to sync, when I turn the tuner to an empty channel the sync from injecting is very scrambled.
• Note due to the camera shutter speed the below image looks way better
• 

[bhegges]

Interesting, see below for the signal diagram from Zenith for this set. Still studying the schematic to see how the sound detector signal feeds the sync circuit, but this would explain the sync issue from just feeding signal at the video detector alone. Maybe lift a lead on L14 and additionally inject video at test point C2.

[bhegges]

Duplicate, see below post.

[bhegges]

Looking at the signals on C1 and C2 to me they look the same, with just the signal at the sound detector attenuated some. How important is the DC offset for each of these? Would it work to splice in the same amplified video signal from the op-amp to C1 but then include a dropping resistor to connect to C2? Is there a need for coupling capacitors between these?



Here is what I measured:

• C1: 0.23 VAC
• C1: -2.98 VDC
• C2: 0.10 VAC
• C2: -1.14 VDC

Also the scope traces look like the waveforms are the same: