1950 Dumont RA-109 Tuning Issue
 

Hello! I've been in the process of documenting the restoration of a Dumont RA-109 I found recently. I completed the chassis rebuild without much incident, but was hoping to get some input on an apparent tuning sensitivity issue it seems to be having. I've made a quick video to help show what's happening:

https://youtu.be/r2SW5u6Ezcw


This is what I have:

-The TV tunes in a crisp bright picture with a battery powered pocket signal generator attached, even without a proper matching transformer.

-It won't display as much as snow with an RF modulator attached, even loosely coupled with wires laid on top of one another to try and eliminate the possibility that the signal from the modulator is too strong and throttling back the RF/IF gain. Blank raster only with no sound.

-Attaching the signal generator through the custom RF input cable I made instead of directly to the antenna jack works fine, so there doesn't seem to be an issue with the cable.

-With the signal generator routed through the modulator (modulator off) the signal is greatly attenuated, some picture with light interference.

The set uses the later style Mallory Inductuner without the ceramic shaft.

I feel like I've exhausted all my ideas on how to diagnose this thing. Unfortunately I don't have any alignment equipment handy. Does any one have any ideas or had similar experience with this type of problem?

Do you get snow with nothing attached?

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

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.

There's no snow or sound, only raster.

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

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!

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?

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.

Also possible but less likely, it could be a alignment issue.
For example on my FADA 895 (RCA 630ts clone), I get no snow when there is nothing hooked to the RF input, just a raster, but I can tune in things on all channels if I try, not very well mind you, but it works, when the video is at peak clarity, the audio is a bit fuzzy, and vice versa, I can find a decent happy medium when video source is DVD or laser disc, ( channel 3), but it's BUZZ-O-MATIC with most most input from the cable box, which is typical for cable input, :(

So, I know that mine can do with an alignment, which I have been avoiding because I don't want to pull the chassis again unless I really have to, and I have the direct video input option anyway in the meantime, but I will try it someday, though I hear it's an arduous task to get it right.

Anyway, this also may or may not be similar to the problem you are having with your set, just a guess.

Weird thing is, with mine, with nothing but an alligator clip lead hooked to the antenna input set to CH 3, and adjusting FT through it's full ranges, at one spot, it locked on to a FM station, not sure which one.
:scratch2:

Could be. But keep in mind the early RCA clones like your Fada have no automatic gain control(AGC). They had the so called contrast control that actually manually controls the gain of the front end and first few video IF's. That's why you don't automatically get high contrast snow when there's no signal being feed to the tuner, because the gain doesn't increase. If I'm not mistaken your TV has split sound, so you may be able to tweak the sound alignment over to match best picture, provided the picture looks acceptable at some point within the fine tuning range. The fact that the picture and sound are split before the video IF is also what makes the sound alignment so critical. You probably wouldn't need to adjust the sound much to get it to match best picture. The OP's DuMont on the other hand has AGC and a modern contrast control which controls the gain of the first video amplifier.