Injecting video in DuMont RA-102?

[old_tv_nut]

You are going into pin 4 (grid) of V4 (6AC7), right?

Did you disconnect the detector diode?

Also, .002 microfarad is too small to drive 5.6k. The low frequency cutoff will be about 14 kHz, causing vertical sync problems.

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. This will lower the cutoff to 1.4 kHz, still too high. Then, to get the low frequency response down to 30 Hz, the coupling cap has to be increased to 0.002x1400/30 = 0.09 (use 0.1 microfarad).

[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.