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
•