Newbie's First Questions RCA TS 721 Restoration - Page 14 - Videokarma.org TV - Video

Kevin Kuehn · #**196** 08-13-2022, 08:36 PM

Quote:

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 · #**197** 08-13-2022, 08:42 PM

Quote:

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

Chris K · #**198** 08-13-2022, 08:48 PM

I checked continuity between the top of the pin socket and the solder terminal and it's fine...crap

Kevin Kuehn · #**199** 08-13-2022, 09:01 PM

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 · #**200** 08-13-2022, 09:07 PM

Quote:

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 · #**201** 08-13-2022, 09:16 PM

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

Notimetolooz · #**202** 08-14-2022, 09:30 AM

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.

Boobtubeman · #**203** 08-14-2022, 01:43 PM

14 pages later, and Still not running.... Thats ONE-- STUBBORN-- TELLY!!

Keep up the fight... Success will prevail...

SR

Chris K · #**204** 08-14-2022, 04:05 PM

Quote:

Originally Posted by Boobtubeman

14 pages later, and Still not running.... Thats ONE-- STUBBORN-- TELLY!!

Keep up the fight... Success will prevail...

SR

Well...the Captain at the helm ain't exactly performing the best!

Chris K · #**205** 08-14-2022, 04:15 PM

Quote:

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.

Chris K · #**206** 08-14-2022, 04:37 PM

Jesus...what an ordeal chasing my tail. I'm sure I checked that resistor and got 1K. Maybe something happened to it while I was working in there. Anyway...I got it...sorta. It's supposed to be +75V and now it's +70. Close. So for the CRT, Pin 1 and Pin 12 are perfect at 140V. Pin 2 at 70V Pin 11 is 225V. Pin 10 is still an issue. I've got 275V. The voltage table I have in the Sams says it should be 290V but the earlier schematic we've been looking at says 330V. I believe 290V is correct for this chassis. It's a later model that the schematic in the Riders. It's #1 off the flyback.

Chris K · #**207** 08-14-2022, 04:42 PM

Quote:

Originally Posted by Chris K

Jesus...what an ordeal chasing my tail. I'm sure I checked that resistor and got 1K. Maybe something happened to it while I was working in there. Anyway...I got it...sorta. It's supposed to be +75V and now it's +70. Close. So for the CRT, Pin 1 and Pin 12 are perfect at 140V. Pin 2 at 70V Pin 11 is 225V. Pin 10 is still an issue. I've got 275V. The voltage table I have in the Sams says it should be 290V but the earlier schematic we've been looking at says 330V. I believe 290V is correct for this chassis. It's a later model that the schematic in the Riders. It's #1 off the flyback.

R142 is giving me 69K. It's supposed to be 56K. I'll replace and see what that does to the voltage to pin 10.

EDIT: Replaced and up to 282V. Still low.

Notimetolooz · #**208** 08-14-2022, 05:11 PM

I think that was the major cause of the problem.
Getting pretty close though. The voltages can be off 10% or more and it should work OK.
CRT pin 11 is variable with the control. Pin 10 voltage could be effected by the horizontal sweep performance.
I haven't seen the SAMS, just Rider's.
So what does the screen look like now?

Chris K · #**209** 08-14-2022, 05:20 PM

Quote:

Originally Posted by Notimetolooz

I think that was the major cause of the problem.
Getting pretty close though. The voltages can be off 10% or more and it should work OK.
CRT pin 11 is variable with the control. Pin 10 voltage could be effected by the horizontal sweep performance.
I haven't seen the SAMS, just Rider's.
So what does the screen look like now?

I don't have the tube in right now. Tomorrow evening I'll get the chance...or maybe I'll sneak down to the shop later tonight and try it. Dinner and a movie with wifie tonight. Did you see I bought a working Textronix oscilloscope on eBay? Should be here Thursday. Maybe we can get this thing working reasonably well soon. I so, as always, appreciate the willingness of you guys to share your knowledge with me! I'll keep all of you posted!

Boobtubeman · #**210** 08-14-2022, 06:16 PM

Quote:

Originally Posted by Chris K

Well...the Captain at the helm ain't exactly performing the best!

Stay with it... When the fault prevails itself, youll kick yourself over it for days sayin"how did i missed that?!"

Had that with an RCA with an audio problem. About a month later i discovered a ground wire tucked to the side i forgot to resolder when restuffed an e-cap.. Found it while going through some before pics.

SR