CTC5 special, low 145V source

[CJVx]

My AGC is not working correctly, and the audio cathode generated 145V source measures to be around 90v, much lower than expected. This provides the AGC voltages. In order to get the best picture, AGC has to be turned all the way up, and will not get to the point where the picture overloads or even the best picture. Because of this I do believe my sound is suffering as well.

I injected 145V from an isolated power supply, and the AGC works as expected, and the picture was much better at its optimum point. I did unfortunately lose sound during this test since I suppose the audio tube was way over biased.

Anyway, my 390V and 300V sources being generated from the power supply measured to be close, if not a bit over. The audio output tube is as strong as new. I checked and rechecked all associated resistors and all capacitors in the circuit and throughout the TV have been replaced. I feel like this is a common problem with these TVs but I am struggling to solve it. Any ideas?

[Electronic M]

I may be stating the obvious but in most sets where a B+ rail originates from the audio output cathode, the audio output tube is being used as a B+ dropping resistor... Thus it's quiescent DC current biasing voltages are critical to correct B+. Check all the bias resistors and lift any grid capacitors, and sub the 145V rail filter cap... May also want to unhook the 145V rail from the audio output tube and measure the current it draws from the supply and compare it to the bias current the audio output tube should pass with how it's biased (looking at the schematic voltages and looking them up on the curve in the tubes datasheet should give you design current).

Haven't had this problem with my CTC-5s, but that's how I would approach it if I did.

[Yamamaya42]

Not ever having a CTC-5, I'm not too familiar with the set, but others here do have one, for example, old_tv_nut has one I believe.

That being said, looking at the SAMS, rca_ctc5_sams_358_9.pdf .

It could be one of 2 things, V13 is not able to generate the needed voltage to start with, or something down the line is pulling it down.
If you have not done so already, you may want to check all voltages shown on the SAMS for V13, see if they are OK.
Also take note of the sync sep V9, decoupling capacitor C106, also on that line, it's rare for disc caps to go bad, but it does happen, sometimes.

[old_coot88]

Without a schematic for reference, it's likely the audio tube's control grid (G1) is running too negative with respect to cathode. If there's a resistor in the cathode leg (probably between 200-300 ohm), try shunting it with a resistor of around 1 to 1.5 K or so. That'll make the grid less negative so the tube conducts harder.
If that don't help, look for something downstream on the 145V rail pulling the voltage down.

[Yamamaya42]

According to the SAMS, there are 2 resistors on the grid of the output, 1 820k 5% pull down, and a 1.5m pull up, tied to the tube side of the 470 ohm resister powering the tube, SAMS also says grid 1 voltage should be -14v, reference from the 140/145v source.

[WISCOJIM]

Originally Posted by old_coot88

Without a schematic for reference...

You can find them here if you want to: https://www.earlytelevision.org/tv_s...ams_color.html

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