Injecting video in DuMont RA-102?

[Phil Nelson]

Originally Posted by old_tv_nut

The 4.7 is 4.7 microfarads, right? The + side should be connected to the transistor base.
You are going into pin 4 (grid) of V4 (6AC7), right? Did you disconnect the detector diode?

Yes to all questions. 4.7 mfd electrolytic with + going to base.

Originally Posted by old_tv_nut

Since 5.6 k is close to your collector load, it will reduce the AC gain, so disconnect it and replace it with something like 56k.

There is no 5.6K resistor in the schematic supplied by vts1134. Do you mean R23 in the RA-102 schematic, the 5.6K connected to pin 4 of the 6AC7?

Thanks much for the various suggestions. I will tinker some more and report back (possibly tomorrow).

Phil Nelson

[old_tv_nut]

Originally Posted by Phil Nelson

...

There is no 5.6K resistor in the schematic supplied by vts1134. Do you mean R23 in the RA-102 schematic, the 5.6K connected to pin 4 of the 6AC7?

...

Phil Nelson

Yes, exactly. For AC signals coupled through the amplifier output capacitor, the 5.6k appears to be in parallel with the collector resistor and lowers the gain. Also, the detector diode may present a non-linear load and distort the signal.

[Phil Nelson]

Progress! I replaced R23 with a 56K resistor and replaced the .002 coupling cap with an .01 cap. The picture was nice but the vertical was still somewhat unstable. I doubled the value of the coupling cap to .02 and now the vertical is very stable.

Here's the video signal direct from the generator:



Here's the signal coming out of the preamp/inverter:



And here's a DVD using the preamp/inverter:



The screen image is so strong and bright that I had to turn it way down to avoid washing out in the photo.

I tried using a 9V battery in place of 12V, and it seems to work fine with 9V.

It would be nice to use this in a switchable A/V adapter like the one described in http://antiqueradio.org/A-V_AdapterForVintageTVs.htm . Then you can just flip a switch to change from A/V input to ordinary input from the antenna. Changing the value of R23 complicates that scheme, but I suppose there's a way.

If I installed this permanently in the chassis (with or without switching), it would be nice to find (or generate) a +9V source somewhere in the chassis rather than rely on a battery.

Phil Nelson

[old_coot88]

Phil said:

If I installed this permanently in the chassis (with or without switching), it would be nice to find (or generate) a +9V source somewhere in the chassis rather than rely on a battery.

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.

[mrjukebox160]

You could rectify and filter the filament voltage should come out to 9 volts.

[mrjukebox160]

It happened again! LOL

[Electronic M]

You could probably apply 6.3V heater current to a voltage doubler, then apply the ~12VDC form the doubler to a simple LM317 based regulator circuit to drop it to 9V and eliminate ripple (with suitable caps in the input and output of the reg)... Just watch for DC potentials on the heater line from preexisting circuits, and Heater cathode breakdown voltages.

[EDIT] Seems others beat me to my first point...I still recommend having that active regulator there to reduce hum.

[Notimetolooz]

Phil; I've gone back and read through this conversation and I thought I would add some things. Before you go too far with this it would be a good idea to measure the DC voltages on the transistor. My back of a envelope calculations indicates the collector might be sitting at more than 8 V if the supply is 9 V, which would leave little "head room". My calculations predict a current draw for this circuit is about 3 ma. Also you can't expect very good high frequency performance with the long wires in your test setup.

[Phil Nelson]

Thanks, I measured those DC voltages and got 4.9V at the collector, 2.8V at the base, and 2.0V at the emitter.

If I decide to install this gizmo, I'll certainly use much shorter leads.

Regarding a voltage regulator for a 9V p-s, are we talking about something like the following?



Regards,

Phil Nelson

[old_coot88]

Considering the low current draw of that one transistor, all you need is a simple half wave supply using a diode and filter cap. A fancy active regulator is good when you're dealing with high current draw.

Others' opinions may vary.

[Notimetolooz]

Here is a link to a three pin fixed 9 V regulator. You only need to add two capacitors to it. Part of the 78L series. The TO-92 package (transistor size) can provide 100 ma.
http://www.digikey.com/product-detai...1-1-ND/5253742

Phil, thanks for the voltage readings. Its good documentation for the future.
However I'm not sure everything adds up. You are using 10K, 4.7K, 820 and 3.3K resistors and a 9 V supply?

[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

[M3-SRT8]

https://www.youtube.com/watch?v=1c96vgKZ1CM

[Notimetolooz]

Originally Posted by M3-SRT8

https://www.youtube.com/watch?v=1c96vgKZ1CM

Ha-ha! I've been discovered! Yes, that's where I got it from.