Newbie's First Questions RCA TS 721 Restoration

[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.

[Kevin Kuehn]

You're welcome. I enjoy trying to jar my brain cells every once in a while.

[Notimetolooz]

I believe you said earlier that all the voltages on V105, a 12AU7, except pin 6 were correct, that would mean the cathode-grid bias is OK.

This thing began as a question why the intersection of R124 and R125 was -15V.
Up to now I hadn't investigated the math (Ohm's Law).

The math clue.
If one end of R125 is at +150 and the other is at -15V that means there is 165V across it. R125 is supposed to be a 1K resistor. The math says there would be a current flow of 165 ma. The wattage would be current times voltage and this comes to over 27 watts! That would burn it up for sure!
So if the voltages are right the resistance cannot be 1K.
The value of R125 is wrong or is bad or maybe a bad solder joint there.

Note that if one end of R124 is at -38V (V105 pin 6) and the other end is at -15V, the voltage across it would be 23V. R124 is supposed to be 3300 ohms. The current through it would be about 7 ma.

[Chris K]

Originally Posted by Notimetolooz

I believe you said earlier that all the voltages on V105, a 12AU7, except pin 6 were correct, that would mean the cathode-grid bias is OK.

This thing began as a question why the intersection of R124 and R125 was -15V.
Up to now I hadn't investigated the math (Ohm's Law).

The math clue.
If one end of R125 is at +150 and the other is at -15V that means there is 165V across it. R125 is supposed to be a 1K resistor. The math says there would be a current flow of 165 ma. The wattage would be current times voltage and this comes to over 27 watts! That would burn it up for sure!
So if the voltages are right the resistance cannot be 1K.
The value of R125 is wrong or is bad or maybe a bad solder joint there.
Note that if one end of R124 is at -38V (V105 pin 6) and the other end is at -15V, the voltage across it would be 23V. R124 is supposed to be 3300 ohms. The current through it would be about 7 ma.

OK...I might have found it. First for your math, the -15V number I stated earlier was with current limiting through a bulb. The full 125V AC number is -36V DC between R124 and R125. But I just inspected the area closely and R125 doesn't look right. It looks a bit burned on the back side. It ohms out at over 55K! I'll replace and see what happens.