Newbie's First Questions RCA TS 721 Restoration

[Chris K]

With pins 7 and 8 bridged, the voltage goes to -51 volts!

[Kevin Kuehn]

Originally Posted by Chris K

With pins 7 and 8 bridged, the voltage goes to -51 volts!

At least that changed it.

Try removing that jumper and pull V103, the 3rd IF tube. Trying to find out if that voltage changes without the IF noise going through the detector stage.

[Chris K]

Originally Posted by Kevin Kuehn

At least that changed it.

Try removing that jumper and pull V103, the 3rd IF tube. Trying to find out if that voltage changes without the IF noise going through the detector stage.

Jumper removed, 3rd IF tube (6AG5) pulled. Voltage on the CRT Pin 2 is still -36V

[Kevin Kuehn]

Originally Posted by Chris K

Jumper removed, 3rd IF tube (6AG5) pulled. Voltage on the CRT Pin 2 is still -36V

OK it seems to be isolated to the video amp stage. I'm starting to wonder if there's a break between the bottom solder terminal of pin 7 and the top contact in the tube socket. I'm a little dumbfounded how that tube can be passing current with the grid at the same potential as the cathode.

[Chris K]

Originally Posted by Kevin Kuehn

OK it seems to be isolated to the video amp stage. I'm starting to wonder if there's a break between the bottom solder terminal of pin 7 and the top contact in the tube socket. I'm a little dumbfounded how that tube can be passing current with the grid at the same potential as the cathode.

OK...I'll take a closer look

[Kevin Kuehn]

The negative supply makes this seem confusing but with the voltage's shown on the schematic the grid is basically 1 volt more negative than the cathode, which seems reasonable to bias that tube on. Another way to look at it is that the cathode resistor is dropping 1 volt, which makes the cathode 1 volt positive relative to the grid being pulled to -85V . Very similar to a cathode biased class A audio stage.

As Tim mentioned maybe we are chasing a red herring here. Possibly when the video signal is present it biases the grid more negative which would make the tube conduct less, bringing the plate to the positive voltage shown on the schematic.

[Chris K]

Originally Posted by Kevin Kuehn

The negative supply makes this seem confusing but with the voltage's shown on the schematic the grid is basically 1 volt more negative than the cathode, which seems reasonable to bias that tube on. Another way to look at it is that the cathode resistor is dropping 1 volt, which makes the cathode 1 volt positive relative to the grid being pulled to -85V Very similar to a cathode biased class A audio stage.

As Tim mentioned maybe we are chasing a red herring here. Possibly when the video signal is present it biases the grid more negative which would make the tube conduct less, bringing the plate to the positive voltages shown on the schematic.

OK thanks so much Kevin. I appreciate all of your almost real time input tonight. Tomorrow I'll feed a video signal in and check the voltages. It's time for some ice cream and TV...a 60" flat screen! Have a great night and I will update soon! Take care.