1950 Dumont RA-109 Tuning Issue

[Yamamaya42]

All I can find is the schematic for the Dumont RA-108 , which I'm hoping is close enough.
Could it be perhaps that V1 RF amp is not working at all, or very weakly, and that the pattern generator in question is just putting out such a strong RF signal that it's able to blow past the defective stage, where the modulator lacks the power to do the same?
Lack of snow with nothing connected would be a clue of this, which is what I believe old_tv_nut alluding to.

[HiFiasco]

Originally Posted by old_tv_nut

Do you get snow with nothing attached?

There's no snow or sound, only raster.

Originally Posted by old_tv_nut

Is it possible your custom cable wiring is shorting to the cathodes of V1, so whether it works depends on the source being floating?

I wondered about that, however running the pattern generator through the custom cable verifies it's working

Originally Posted by Yamamaya42

All I can find is the schematic for the Dumont RA-108 , which I'm hoping is close enough.
Could it be perhaps that V1 RF amp is not working at all, or very weakly, and that the pattern generator in question is just putting out such a strong RF signal that it's able to blow past the defective stage, where the modulator lacks the power to do the same?
Lack of snow with nothing connected would be a clue of this, which is what I believe old_tv_nut alluding to.

Good call, it's been a minute since I last had a look at it, but seems like I tried the old pull test to see what happened to the picture. With a pattern generator attached, the test pattern disappeared, but to your point yes it may just be passing the signal without amplification.

The print for this set is on Earlytelevision Foundation's web site, but it's a Sams which as usual and especially with this set isn't exactly a bastion of accuracy!

[old_tv_nut]

Originally Posted by HiFiasco

I wondered about that, however running the pattern generator through the custom cable verifies it's working

What I meant was, is the pattern generator the only thing that is grounded only to the cable input (or not even that) and completely floating from external grounds?
Edit: have you tried checking with an ohmmeter between the RF stage and the cable?

[Kevin Kuehn]

I just had a similar thing going on with a Motorola set that turned out to be the sets antenna input Balun was blown out by lightning. Have you checked for continuity on both the primary and secondary of L2(sams), the antenna coupling transformer? Odds are the portable signal generator has a stronger signal than the rf modulator and a much higher contrast level with only lines being displayed.