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

[Chris K]

No I’m using a new digital unit from Klein Tools

[Penthode]

Something does not seem right. Where and how did you measure the filament voltage? What is the Mains supply voltage?

[Chris K]

I pulled the audio output tube and put the meter probes in holes 2 and 7. I’m running it on 117 volts AC through a Sencore isolation transformer

[Chris K]

I was working on it from the top of the chassis and not under so I did a quick voltage check

[Chris K]

The CRT is dim but has strong emissions on testing. The set only makes a passable picture when the brightness is turned up about half way. Any further up it goes dark and it seems to get bigger before it fades to no light

[Chris K]

Ion trap is adjusted to best brightness

[Chris K]

All this is with the original 10BP4 but it was behaving the same way with the 5” test CRT

[Penthode]

Why are you using an isolation transformer? And did you measure the supply AC with it switched on? As it is on the bench, I'd like to see the supply voltage when on, the rectifier DC to ground, the negative rail (from the transformer center tap) and the filament voltage.

[Chris K]

Yes I’ve measured the output before but not during this project. I’ve just always been told that while you work on these old TVs you should use an isolation transformer setup. You’ve probably seen it in some of the photos. It’s the Sencore unit most of these guys use. Are you saying it’s ok to run it on straight 120V line voltage?

[Penthode]

It is a transformer set. When working on hot chassis sets, I always use an isolation transformer. On these set? No need.

What you do is replace the AC filter capacitors. The ones from the AC supply in to chassis. Use only 1000v dc capacitors or ones rated for 600v ac. Once replaced use an ohmmeter from each prong to chassis to confirm no problems. The is no resistive leakage here so the reading will be more than 50 Megohms so off the scale of most of your meters. And thats it.

So no on this and other full transformer sets like this, no isolation transformer is needed.

I suspect your isolation transformer is dropping the supply to 95 volts ac.

But put it this way: if th supply is 120vac, I would expect the filaments to be about 6.5v ac. Because it is only copper wire and a fixed transformer ratio, it is impossible to have the low filament voltage you cited unless there is a resistive drop someplace (which will dissipate a lot of heat) or the losses in the isolation transformer are causing the supply to the set droop when on.

[Chris K]

Ok that’s great information to have! I have the XY safety caps rated for 600V AC. The caps in the tv are 0.01uF. I have safety caps with a variety of capacitance values. Should I replace the old ones with the same value capacitors?

[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?

[Chris K]

Absolutely right on the isolation transformer drop. With no load and the AC output set to 117 volts I turned on the tv. I’ve had it set to the amp scale and not the voltage scale. When the TV came on full power, the transformer output voltage dropped just short of 10 volts

[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?

I had put a NOS 1B3 in it but I can try another. NOS doesn’t mean much when it comes to 70 year old vacuum tubes

[Chris K]

It was the transformer. I put it on straight line voltage at 121V AC and the CRT is bright and I've got decent resolution on the multiburst bar sweep