Ok. Let’s try this again…RCA 9TC240 Alignment

[Chris K]

I wish I knew what to do with this stupid iPhone camera to take these images without blanking but I'm happy with the image on the CRT

[Penthode]

That is great.
As far as the camera is concerned, my cell phone camera I have to photograph closer to the CRT to fo the shutter to slow sufficiently to obtain a picture field without the black bar.

Have you finished the sound alignment?

[Chris K]

If step 4 aligning T-112 is the last step in the sound alignment, then yes it's complete. The next step, Picture IF and Trap alignment first step is, for me, a "Say What" moment! A potentiometer between pins 5 and 6 of the V107 socket...after removing the tube? Yeah, I need help!

[Penthode]

Yeah removing the tube and adding the pot is standard process for this chassis. You just attach the pot where indicated and use it to set the proper bias to the IF amplifiers while doing alignment.
The trap alignment is easy with the HP combo set up. You select CW on the 8601a generator and the 8600a marker. Then as instructed set the frequency on 8601a and the adjust the traps ( and there a lot of them in this set) for a null or minimum on the VTVM.

[Chris K]

What kind of potentiometer? A variable resistance pot? In what range? How do you use it in a way that makes connecting it in the sockets secure while you adjust it? Do I need to build some sort of rig/tube base with a pinout?

[Chris K]

Originally Posted by bandersen

Also be aware the PR57 output voltage meter is not very accurate. I put mine about 5 volts about 117 to get 117 out with a set drawing a couple amps.

You should be able to go all the way up on the brightness. I wonder if your HV rectifier tube is weak?

So when I set the PR57 to 117v and turn on a tv, yes the voltage drops. It never occurred to me that wouldn’t happen with straight line voltage. So if I’m monitoring voltage, after the tv is on and up to full power I should readjust the voltage back up to 117V?

[bandersen]

Yes but also I wouldn't trust the built in meter. Mine is off by about five volts

[Penthode]

Originally Posted by Chris K

What kind of potentiometer? A variable resistance pot? In what range? How do you use it in a way that makes connecting it in the sockets secure while you adjust it? Do I need to build some sort of rig/tube base with a pinout?

It is effectively a rheostat or variable resistance. Just wire two leads to the pot wiper and element end and attach it to the pins five and six. I typically take a longish cheap croc clip lead, cut it in two, solder it to a pot and then clip the leads to pins five and six.

[Penthode]

It says what it is in the manual doesn't it? 250k or something like that? (I don't have the manual in front of me at the moment).

[Chris K]

Originally Posted by Penthode

It is effectively a rheostat or variable resistance. Just wire two leads to the pot wiper and element end and attach it to the pins five and six. I typically take a longish cheap croc clip lead, cut it in two, solder it to a pot and then clip the leads to pins five and six.

I’ll have to look closer for the resistance value. I don’t have any pots that go that high so I’ll need to check the Riders before I order one. The clip lead idea sounds doable. I was thinking of getting fancy and getting a test adapter for the socket wiring the pot to that.

[Penthode]

A real cool way if you were doing lots of these sets would be to use and old octal tube sockets with pins five and six wires to the rheostat. Then all you would have to do is just remove the tube and plug the rheostat in

[Chris K]

Originally Posted by Penthode

A real cool way if you were doing lots of these sets would be to use and old octal tube sockets with pins five and six wires to the rheostat. Then all you would have to do is just remove the tube and plug the rheostat in

Yeah that’s what I’m thinking about because I have about a dozen RCA sets in storage none newer than 1950. Today I picked up a 1949 Emerson and a Dumont RA103-A with the FM radio and a CRT I’ve never seen before, a 15DP4. It tests great…almost new. One owner set.

[Penthode]

Note for the trap adjustment with the VTVM across the video detector load resistor, the case of the VTVM must not be grounded to the mains supply ground. That would have to be lifted. You can be cautious and do it this way. Or alternatively if you have a digital meter that measures trends rather than just an absolute, that may be safer.

[Chris K]

Not sure I understand what you’re saying about the VTVM. So I shouldn’t connect it directly to the 120V AC supply? Would running it through the isolation transformer help with this or am I way off base with what you’re talking about?

[Penthode]

The isolation transformer won't help.

Look at it this way: you want to differentially measure a small dc voltage that is not referenced to the chassis ground. A three pronged plug on your VTVM, one is hot, one is neutral and the round pin is case ground. The metal case is grounded via the third pin to protect against you getting a shock if the case is not at ground potential.

If you try to connect your IM-18 VTVM to the load resistor in the next step without removing the ground pin connection on the power plug, you will short circuit the minus supply in the TV set and cause damage to the TV and likely your test equipment.

Background: in your chassis, to avoid adding a DC restorer circuit to clamp video black level, the designers used DC coupling to pass the video DC component from the video from the detector to the CRT grid through two stages of video amplification with the 12AU7 tube. To DC couple the video stages, the video detector is at about -120v DC. The -19.2v DC level on the plate of the first amplifier is coupled to the grid of the second amplifier. And the second amplifier plate at 134v DC is coupled to the CRT grid. The designers did not provide 100% DC coupling to avoid changing brightness as the 12AU7 warmed and current drifts. But the DC coupling is sufficient to give consistently good black level clamping.

You therefore must be aware that because of this design, you have to take precautions. RCA warns you a little bit. Their audience is trained electronic engineers and technicians and the warning would make them consider how to make the measurement under these circumstances. I hope this is clear and you understand. If not please ask more questions. If you do not understand the design concept of the video amplifier in this set, now is a good time to learn.