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

[Chris K]

I understand the description of the danger inherent in the VTVM measurement. My VTVM does not have a ground pin on the plug, just a non-polarized plug that was standard back in the day. I’ll be very careful regardless. As for the circuitry description I am a little lost on some of it but I’ll be back with some questions after a bit of research for sure. Thanks so much for all of this knowledge and your generous time and guidance!

[Penthode]

I should add the short circuit would occur without removing the third prong ground of the VTVM, -120v of the VTVM ground to mains ground thru the case ground of the HP8601a where the signal ground lead in turn is connected to the chassis. So you will be directly shorting out the -120v DC supply through both test equipment.

[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 os 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 you IM-18 VTVM to the load resistor in the next step without removing the ground pin connection, you will short circuit the minus supply in the TV set and cause damage.

Background: in your chassis, to avoid adding a DC restorer circuit to clamp video black level, the designers coupled the DC component of the video from the detector to the CRT grid through two stages of video application through the 12AU7 amplifier. To DC couple the video stages, the video detector is at about -135v. The DC level on the plate of the first amplifier is coupled to the grid of the second amplifier. And the second amplifier plate 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 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 design concept in this set, it is a good time to learn.

[Chris K]

Scaring the crap out of me! I don’t want to destroy the equipment I just spent a month acquiring. My VTVM does not have a ground pin on its plug. Is my equipment safe if I use it?

[Penthode]

If there is no ground pin it is safe to use. However you must keep safe. The VTVM case must be isolated from chassis ground as it will be at -120vdc. I keep mine away from the chassis. And do not touch the VTVM when the power is on.

You will get a very bad jolt of you are touching the chassis with one hand and the VTVM with the other. When I was young long ago, I was taught the one hand rule: place one hand in the pocket and only use the other hand to probe and test. I cannot emphasize the danger if you do not take care.

Just be methodical and understand what you are doing. You should have no problem if you just setup the VTVM, isolate the case and just observe it as you make the adjustments. Just refrain from touching it.

[Penthode]

And the generator will be fine as long as you have a good ground connection.

[Chris K]

Originally Posted by Penthode

If there is no ground pin it is safe to use. However you must keep safe. The VTVM case must be isolated from chassis ground as it will be at -120vdc. I keep mine away from the chassis. And do not touch the VTVM when the power is on.

You will get a very bad jolt of you are touching the chassis with one hand and the VTVM with the other. When I was young long ago, I was taught the one hand rule: place one hand in the pocket and only use the other hand to probe and test. I cannot emphasize the danger if you do not take care.

Just be methodical and understand what you are doing. You should have no problem if you just setup the VTVM, isolate the case and just observe it as you make the adjustments. Just refrain from touching it.

Thanks again! I have a very healthy, I think, respect for electricity. I do not like shocks. Doesn't matter if it's a tingle from an insufficiently discharged capacitor or a 350V DC B+ shock...I don't like them. I am especially cautious inside tube equipment. I have a 16" RCA with a metal cone CRT carrying 15kV that I'm terrified to work on. It's a shame because the cabinet for this TV is perfect. Literally owned by a little old lady who only drove it to church on Sunday's.

[Chris K]

Took a trip up to a little town about 35 miles north of here and picked these up Sunday. Sorry, I know it's off topic but I just couldn't help it!!!

[kvflyer]

Originally Posted by Chris K

Took a trip up to a little town about 35 miles north of here and picked these up Sunday. Sorry, I know it's off topic but I just couldn't help it!!!

You have been infected! The bug has bitten you. Oh, welcome to the club as they say. But seriously, nice...

[Penthode]

I started on the vintage TV thing 55 years ago this year with the late 1940's RCA. I suppose what amazes me most is in 2024 there appear to be more olds becoming available than I recal seeing 40 to 50 years ago.

I would take time travelling through the US North East and I saw very little. Now they seem to be jumping at me.

I don't know what others think.

Nice catches! I especially like the Dumont. But the Emerson is nice too!

[Chris K]

There's no question the vintage electronics "business" has exploded in the last 5 years. Vintage audio prices have gone through the roof. It's amazing the entry level units from elite manufacturers like Marantz, Pioneer, Sansui are going for thousands of dollars on eBay. Vacuum tubes are being sucked up by tube amp devotees by the tens of thousands. Maybe that, and the ease of selling through virtual auction and market sites are prompting people to dig out Grandpop's old, heavy TV out of attics and basements. Whatever the reason, I love it. These RA103 based Dumont TVs are a favorite of mine too. I restored a Doghouse Chatham to like new condition right down to completely stripping the cabinet and replacing decals etc. There's more evidence of the craze...the fact that decal kits for these old TVs and radios are readily available. Amazing.

[Chris K]

Originally Posted by Penthode

I started on the vintage TV thing 55 years ago this year with the late 1940's RCA. I suppose what amazes me most is in 2024 there appear to be more olds becoming available than I recal seeing 40 to 50 years ago.

I would take time travelling through the US North East and I saw very little. Now they seem to be jumping at me.

I don't know what others think.

Nice catches! I especially like the Dumont. But the Emerson is nice too!

My pride and joy!

But seriously, I'll be back on the alignment tonight. I still don't know what resistance pot I'm supposed to use. I'll double check the service data and see what I can find

[Chris K]

It is 250K

[Chris K]

I do have a 500K rheostat I pulled out of a parts chassis. Would that be useable?

[Penthode]

yeah a 500k should do. If it is has an audio taper, set the control mid way and connect the wires to the side with the lowest resistance to center.