Injecting video in DuMont RA-102?

[Notimetolooz]

I think I know why your voltage readings didn't make sense to me. First I made a math mistake, but also I thought you built old_coot88's version and I guess you built vts1134's. In that version the collector voltage varies as you adjust the pot to change the AC gain. I think if you check, the pot was adjusted to about 1.6K. That would mean the AC gain is about 2 instead of about 4 with the pot at 3.3K. Anyway you can't argue with success!
More on that regulator; you can see the relevant data by clicking on the link on that page. The pages for the 9V version is on pages 4 and 9. You need at least a 0.33 uf on the input and at least 0.1 uf on the output. The regulator should be fed by at least 12 V.

[Phil Nelson]

Originally Posted by Notimetolooz

I thought you built old_coot88's version and I guess you built vts1134's.

Yes, I built what vts1134 posted, with a couple of changes. I used a 5K pot rather than 3.3K, because that's what I had on hand. I also changed the output capacitor from .002 mfd to .2 mfd, and changed R23 on the TV chassis from 5.6K to 56K. Here's a sketch of what I built, with some labels



The preamp/inverter seems to work well using either a 9V or 12V battery.

As for powering it from the TV chassis rather than a battery, old_coot88 suggested:

Originally Posted by old_coot88

You could rectify it right offa the 6.3V heater supply using a single diode and a filter cap. That's assuming one side of the supply is grounded. It would give you real close to 9V DC.

I tacked this together using parts on hand:



Using only one filter cap produced a faint moving hum bar, so I added a second. The preamp/inverter seems to work at this lower supply voltage (around 7.5V DC), although so far I haven't looked at anything except test patterns.

This is an interesting experiment, but I don't know whether I'll install this preamp/inverter. Yes, injecting the video produces a somewhat better picture, but the improvement is incremental, not night-and-day. In its current condition, the TV makes a watchable (although not perfect) picture without injection, as seen in this photo and video clip that I took a while back.



http://antiqueradio.org/art/DuMontRA...inaryInput.mp4

(The horizontal bands in the video are camera artifacts.)

I've been slogging along with this project for a VERY long time. It would be nice to get this whale off my workbench and putter with something else for a change. Maybe I'll put the TV in its cabinet and try watching for a while. If I can't live with it, I can always install the A/V patch later.

Thanks,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html