Newbie's First Questions RCA TS 721 Restoration - Page 13 - Videokarma.org TV - Video

Chris K · #**181** 08-13-2022, 04:49 PM

Quote:

Originally Posted by Kevin Kuehn

One could easily pull V105 and see if pin 2 of the CRT swings positive(indicating shorted V105 cathode to plate). Also if C115 were leaking it could cause the plate and cathode to conduct hard simulating a short. If removal doesn't eliminate the negative voltage on pin 2, then your shorted socket scenario would seem likely.

I pulled V105 and CRT pin 2 went from -38V to +150V. Before I pulled it, the voltage on pin 7 was -85V. BTW, the funky voltage on CRT pin 11 was because of the position of the brightness control. It is 225V and correct when the pot is in the max voltage position.

Kevin Kuehn · #**182** 08-13-2022, 05:01 PM

Quote:

Originally Posted by Chris K

I pulled V105 and CRT pin 2 went from -38V to +150V. Before I pulled it, the voltage on pin 7 was -85V. BTW, the funky voltage on CRT pin 11 was because of the position of the brightness control. It is 225V and correct when the pot is in the max voltage position.

That all sounds correct. Do you have another 12AU7 to try in circuit?

Chris K · #**183** 08-13-2022, 05:05 PM

I forgot to add...V105 Pin 7 -85V AND Pin 8 -84.2V

Chris K · #**184** 08-13-2022, 05:06 PM

Quote:

Originally Posted by Kevin Kuehn

That all sounds correct. Do you have another 12AU7 to try in circuit?

I have a couple of NOS 12AU7s. I'll try them

Chris K · #**185** 08-13-2022, 05:15 PM

Quote:

Originally Posted by Kevin Kuehn

That all sounds correct. Do you have another 12AU7 to try in circuit?

Tried 2 different NOS 12AU7s, the original RCA tube that was in the set, and 2 Mullard, strong testing ECC82s...still a -36V...-37 volts on pin 2 of the CRT. It's not the tube I think we can say

Kevin Kuehn · #**186** 08-13-2022, 05:36 PM

Quote:

Originally Posted by Chris K

Tried 2 different NOS 12AU7s, the original RCA tube that was in the set, and 2 Mullard, strong testing ECC82s...still a -36V...-37 volts on pin 2 of the CRT. It's not the tube I think we can say

And yet removing it made CRT pin 2 return positive.

Makes me wonder if it's suppose to go negative with no video signal from the detector. I need to go look at the conditions for the voltage readings.

Kevin Kuehn · #**187** 08-13-2022, 05:47 PM

The only note I see is that voltage readings were taken with the picture control fully counterclockwise. Without a scope it's hard to say what type of signal you're measuring with your(I assume) digital voltmeter.

Kevin Kuehn · #**188** 08-13-2022, 06:13 PM

You can try tying pin 7(grid) of V105 directly to pin 8(cathode). With the cathode and grid both at the same -85v it should cut-off the tube from conducting current and allow pin 6 (plate) to go positive. With the grid at -85 and -84 on the cathode it should be cut-off with no IF signal present. Check R120, R121 for correct resistance. Something needs to give.

Chris K · #**189** 08-13-2022, 08:01 PM

Quote:

Originally Posted by Kevin Kuehn

You can try tying pin 7(grid) of V105 directly to pin 8(cathode). With the cathode and grid both at the same -85v it should cut-off the tube from conducting current and allow pin 6 (plate) to go positive. With the grid at -85 and -84 on the cathode it should be cut-off with no IF signal present. Check R120, R121 for correct resistance. Something needs to give.

R120 is 58 ohms, R121 is 1.03 Meg. Can I run the CRT with the grid and cathode tied on V105 to get my voltage?

Kevin Kuehn · #**190** 08-13-2022, 08:06 PM

Quote:

Originally Posted by Chris K

R120 is 58 ohms, R121 is 1.03 Meg. Can I run the CRT with the grid and cathode tied on V105 to get my voltage?

I think that will be fine. Just long enough to take measurements.

Chris K · #**191** 08-13-2022, 08:06 PM

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

Kevin Kuehn · #**192** 08-13-2022, 08:14 PM

I was just looking at your new Leader signal generator and notice it has an adjustable video output. I don't see any switch to change from positive to negative going video signal, but with some luck you may be able to connect that video to the input of the 1st Video amp through a .05uf cap and therefore bypass your tuner and IF stages. Worst case if it's the wrong polarity signal you'll have a negative image.

Chris K · #**193** 08-13-2022, 08:22 PM

Quote:

Originally Posted by Kevin Kuehn

I was just looking at your new Leader signal generator and notice it has an adjustable video output. I don't see any switch to change from positive to negative going video signal, but with some luck you may be able to connect that video to the input of the 1st Video amp through a .05uf cap and therefore bypass your tuner and IF stages. Worst case if it's the wrong polarity signal you'll have a negative image.

Hmmmm...I don't see that option either. All this is very confusing.

I also bought a Tektronix 2215A Scope that should be here by next weekend so That'll give me a bit more insight. Frustrating as hell.

Kevin Kuehn · #**194** 08-13-2022, 08:23 PM

Quote:

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 · #**195** 08-13-2022, 08:28 PM

Quote:

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