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

[bandersen]

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

[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.

[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.

[Electronic M]

Originally Posted by 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.

.

I know this is a settled matter, but there's one thing I find odd. Your assumption that an isolation transformer will pass it's plugs ground through to its outlet and that the VTVM would have a 3 prong cord... While I wouldn't doubt there are ones that do out there somewhere, I've never seen an isolation transformer built that way out of better than half a dozen I've had. Most isolation transformers I've seen don't have 3 prong cords and many that do don't ground the outlet or use a 2 prong outlet.
Likewise the vast majority of VTVMs made are only 2 prong...It wouldn't surprise me if some lab or mil spec units or some last gasp 80s units might have been built with 3 prongs, but that seems like assuming the exception rather than the rule.... Better to assume the rule and warn to check for the exception in most cases.

[Penthode]

Originally Posted by Electronic M

I know this is a settled matter, but there's one thing I find odd. Your assumption that an isolation transformer will pass it's plugs ground through to its outlet and that the VTVM would have a 3 prong cord... While I wouldn't doubt there are ones that do out there somewhere, I've never seen an isolation transformer built that way out of better than half a dozen I've had. Most isolation transformers I've seen don't have 3 prong cords and many that do don't ground the outlet or use a 2 prong outlet.
Likewise the vast majority of VTVMs made are only 2 prong...It wouldn't surprise me if some lab or mil spec units or some last gasp 80s units might have been built with 3 prongs, but that seems like assuming the exception rather than the rule.... Better to assume the rule and warn to check for the exception in most cases.

You unfortunately misinterpreted what I said. Look at the instructions in manual for adjusting traps. I said the isolation transformer will not prevent the measurement from being a shock hazard.

And my Heathkit VTVM I bought new in 1971 was provided with a power plug third prong or ground. My three earlier VTVMs (two RCA's and a 1950 Heath) did not have the ground connection but my later Heathkit one did.

I only had to emphasize that the dangerous measurement using a VTVM with a metal case at 120 volts B minus potential can cause quite a jolt and one must be aware.

[Chris K]

Originally Posted by Penthode

You unfortunately misinterpreted what I said. Look at the instructions in manual for adjusting traps. I said the isolation transformer will not prevent the measurement from being a shock hazard.

And my Heathkit VTVM I bought new in 1971 was provided with a power plug third prong or ground. My three earlier VTVMs (two RCA's and a 1950 Heath) did not have the ground connection but my later Heathkit one did.

I only had to emphasize that the dangerous measurement using a VTVM with a metal case at 120 volts B minus potential can cause quite a jolt and one must be aware.

...and I appreciate that very much!

[Electronic M]

Originally Posted by Penthode

Look at the instructions in manual for adjusting traps.

I don't have time to read any manual.... I'm very close to ghosting videokarma for the next few months. Dieseljeep died back in January. I bought up most of his monochrome sets and a few color ones that his family didn't send to the dump. I've got borderline too many projects to complete before winter and reorganize.

[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!

[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!!!