Injecting video in DuMont RA-102?
 

After much travail, my DuMont RA-102 works reasonably well, but I'm curious to see how it performs if I inject video & audio signals directly. The audio injection is simple and it sounds peachy. The video is another matter.

My first instinct was to lift the 1N34 video detector diode from pin 4 (1st grid) of the video amp tube V4 and inject video there, but the results were poor: wobbly sync and strange contrast, sometimes with almost-negative images. To check whether it's a polarity issue, I unhooked my DVD player source and substituted my BK 1077B Analyst. The results seemed equally rotten whether I switched the video polarity negative or positive.

This set has only one stage of video amplification. The pertinent schematic snippet is here:

http://antiqueradio.org/art/DuMontRA...oSchematic.jpg

The RA-102 Riders manual calls for a nominal -1.5 volts at the video amp grid (pin 4).

Will video injection require adding a little video preamp? I built one such years ago for my RCA CT-100 (http://antiqueradio.org/VideoAdapter...Television.htm) with good results. I'm not hell-bent on doing this now, but it would be nice to have a clearer idea of what the RA-102 would require.

You can find the full schematic at these links:

http://antiqueradio.org/art/DuMontRA...chematic01.jpg
http://antiqueradio.org/art/DuMontRA...chematic02.jpg

Regards,

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

Phil-

Check out this thread about direct injection and my Crosley "Swing-A-View" http://videokarma.org/showthread.php?t=268178. My set has only one stage of amplification also.

Thanks, that's interesting. I guess the executive summary is that you're close to making it work, but still have an interference issue. Is that a fair statement?

Regards,

Phil Nelson

Phil,



The 1.5v you mention is likely a DC bias voltage, not a P-P video signal. Experiment with it, try negative and positive going video to see which it likes more. You may need a higher amplitude signal at that point for it to properly pick up, I've had varying degrees of success with different sets in the past when doing this. For example, the 52T-1634 Philco I had as my first set made a dazzling monochrome picture with nothing more than normal composite output from just about any source. Color sets less so, they tend to need a stronger signal hence the amplifier to boost it up a bit.

You say it works 'reasonably well' right now, what exactly is keeping you from giving it the ole siskel and ebert? I ask because there's a good chance that what's keeping it from being outstanding now will likely still keep it from performing it's best later, so why not get it where it should be first then try injecting video? My experience has always been that small problems become bigger ones post modification, and become harder to trace down. Only exception of course being tuner/IF troubles, I eliminate a lot of problems by bypassing that early in a resto.

Well, I suspect it would benefit from a video IF alignment, but I'm not equipped for that, so the question is how well it can perform without realignment. (Or, if you know anyone within a 3-hour drive of Seattle who would like to align a Dumont RA-102, please send me that info.)

In light of that, here are the remaining issues:

1. Weak audio. Sounds OK if you crank it, but you shouldn't need to turn volume halfway up before hearing anything. When I injected audio on the RA-102, it was strong like my RA-103. (Yes, I did tweak the audio adjusters to get best (under the circs) volume & fidelity at the "best video" tuning point.)

2. The tuning is twitchier (narrower?) than my RA-103, and the tuning meter is pretty unresponsive. The needle does move a bit while you tune, but way less than I'd expect. Again, if bad alignment equals weak signal, I guess these symptoms aren't surprising. (I have cleaned the devil out of the tuner, its tube sockets, etc., and both tested and tried subbing all of those tubes.)

3. Vertical jitter. There is still a slight jitter, even after I replaced a vertical xfmr and most other vertical components. Not noticeable when watching ordinary program content, but visible if you display something like a cable TV program guide with lots of horizontal lines. Dumont published a service note with a modification to reduce jitter; I have read that, but not tried the mod.

4. The picture blooms and loses focus if you have set the contrast/brightness to correctly display ordinary content, and then the station displays a very bright scene, like a white ad screen with small dark lettering. Poor HV regulation? In the course of getting the HV to work at all, I replaced nearly every related component on the p-s chassis.

True, problems usually don't cure themselves. I'd be happy with this set if I only improved 1, 3, and 4 in the above list. If I could inject video, that might help to evaluate these issues. For example, is the jitter present when injecting video vs. receiving signal through the antenna? And so on.

Regards,

Phil Nelson

Don't you have a B&K 1077 analyst? Those are great for test video injection...Give it a try and see what it does. It will be a lot quicker and easier than building a video amp.

Thanks, I tried using my 1077B analyst (see first post) without noticeably better results.

Regards,

Phil Nelson

No that interference was not in the video amplifier circuit. I moved my discussion on that video amplifier to a thread about the restoration of the whole set. I should update the video amplifier thread to point to the new one. The circuit works really well, although I am using a 12v battery for the power source. I may circle back to the power supply after fixing other issues with the set. The trick is finding a DC power source "clean" enough to not inject problems of its own.

It's really odd that the 1077 isn't giving you results when you switch the polarity of the video output to positive. Did you try going "up the river" with the 1077 and injecting video on the output of the video amplifier with the gain cranked and the video negative on the 1077?

The thing expects positive going sync pulses at that grid, and an average
video signal that has a positive DC average value. And there is no gain
control. You probably need a rather large video signal with an adjustable
DC level to get the signal in the center of the range at the amplifier plate.

Use your scope at the plate. (Yes, I'm a scope fetishist.)

If you are trying to borrow power from the set I highly recommend using a LM317 family linear regulator to drop some voltage...It will make the supply much cleaner with reasonable filtering on both sides of the reg.

Try hooking your scope to your 1077's video output and verify that it's capable of about a 5 volt video signal. You can also place a jumper between the 1077's positive bias supply over to the video jack which will allow you to vary the positive bias that the video signal rides on. You'll want your scope set to DC coupling to see the bias level change. That bias supply is floating from ground, so you'll need to ground it's negative side for positive bias. You can also reverse the plus and minus and end up with an adjustable negative bias.

[Edit] I just got to thinking - The 1077's 25 volt bias supply is going to be pretty sensitive to adjust down around 1.5v, so you might put something like a 100k resistor in series between the bias supplies positive output and the video out jack. That should knock the DC bias down to a more manageable level.

OK, more data. Injection with the BK Analyst does work, after all (don't ask me what happened before). I lifted the 1N34 video detector diode from the junction of R53 and pin 4 of the video amp, injecting at that point with the Analyst's video output control set at midrange and the polarity positive:

http://antiqueradio.org/art/DuMontRA...hBKAnalyst.jpg

(Excuse the rotten handheld photo; the set is propped on its side. The RA-102 actually has better linearity than appears here -- I think my Analyst is due for a service visit of its own.)

That image is stable and has decent resolution. In the unwelcome side effects category, the Contrast control is now completely inoperative. And increasing the Brightness control only makes the picture bloom, as seen here:

http://antiqueradio.org/art/DuMontRA...KAnalyst01.jpg

For kicks, I tried injecting a signal from my Philips PM 5518 TV pattern generator. I guess the polarity is wrong. The image is negative, with awful sync (the correct image would be a white circle on a black background):

http://antiqueradio.org/art/DuMontRA...nGenerator.jpg

Same deal when I inject video from a DVD player:

http://antiqueradio.org/art/DuMontRA...hDVDPlayer.jpg

For comparison, here is that same DVD menu page when I was experimenting with audio injection and the video was coming through the antenna:

http://antiqueradio.org/art/DuMontRA...hDVDPlayer.jpg

To summarize, I guess a signal roughly the same strength as the BK Analyst output will work at that injection point. The disabling of the Contrast control is a bummer; there is only a narrow range of adjustment on the Brightness control where the picture is watchable, with no contrast adjustment at all.

Regards,

Phil Nelson

Phil, you're in the exact same boat that I am in. You need to build the 1 transistor video amplifier that everyone guided me to build. This will give you video with the correct polarity and you can add a potentiometer in the circuit that will give you your "picture" control back. I plan on rewiring the control in my set to control the video amp instead of the IF gain as I plan on permanently using direct injection.

Thanks. I don't think I want to permanently modify this set on that scale.

The image resolution that I got when injecting video with the 1077B Analyst was a bit better than when receiving a signal through the tuner and IF, but not dramatically better. Which basically answers my "how good could it look" question.

The video preamp is a cool idea, though. When you're finished with your installation, maybe you can post a schematic of the whole thing.

Regards,

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

I will post a schematic of the final solution when I'm finished with the set. It's the least I can do in return for the immense help I receive from this forum.

This is what I ended up with for the video amp/inverter...

http://i1075.photobucket.com/albums/...ps0tqooz3x.jpg

Thanks, that looks pretty simple. What type of transistor did you use?

Regards,

Phil Nelson

Out of curiosity, I built that preamp/inverter on a piece of perfboard, using a 12V battery supply and a 2N3903 transistor:

http://antiqueradio.org/art/DuMontRA...eoPreamp01.jpg

It works, after a fashion, but it isn't great. The first pic shows the video signal (color bars) direct from the pattern generator:

http://antiqueradio.org/art/DuMontRA...eoPreamp02.jpg

And below is the signal coming out of the preamp. The pattern is inverted, but kinda "slenderized," and there seems to be some noise or instability:

http://antiqueradio.org/art/DuMontRA...eoPreamp03.jpg

The screen image is disappointing. It has decent contrast and detail, and the pot on the collector acts as a contrast control, but the vertical is quite unstable and retrace lines are visible nearly all of the time (not much in this pic, though):

http://antiqueradio.org/art/DuMontRA...eoPreamp04.jpg

I wonder if the 2N3903 transistor wasn't the best choice? The local supplier didn't have much to pick from. Here's the 2N3903 data sheet:

http://www.brown.edu/Departments/Eng...s/2N3903-D.pdf

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

Looks like lack of bandwidth if you ask me, did you sweep it to make sure it's passing all frequencies up to about 4.2mhz? Probably could use some peaking.

I wonder if the transistor is too "slow". You might need a true video amp transistor.

You beat it to the post miniman82...LOL

How would I do that?

Any suggestions? I know diddly about solid state.

Thanks,

Phil Nelson

I would modify it slightly so you're gain-controlling it from the input side.

Do you have Multiburst signal on your generator? If so, feed that through it.

http://hometheaterhifi.com/volume_7_...-wfm-large.jpg

+1 on any suggestions.

The 4.7 is 4.7 microfarads, right?
The + side should be connected to the transistor base, but the way you have it drawn, the negative side is connected to the base. Which way do you really have it?

That transistor should be OK for video frequencies (Ft is 250 MHz, which means the internal capacitances reduce the gain to unity at 250 MHz).

Are you using a 10x probe on the scope? A 1x probe may add too much stray capacitance and reduce the high frequency response. You can detect this by watching the picture while you apply and remove the probe.

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).

Once you have made the changes, your waveform should look much better. Then, if the chroma (or high frequency multiburst) amplitude is a bit low, we can talk about adding some peaking.

By the way, if you don't like replacing the 5.6k ohm resistor, you could add an emitter follower to your amplifier to drive the lower impedance - but you would also have to increase the coupling cap from 0.1 uf to 1 uf, a nuisance because you are getting back into the range of polarized caps.

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

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

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.

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:

http://antiqueradio.org/art/DuMontRA...eoPreamp05.jpg

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

http://antiqueradio.org/art/DuMontRA...eoPreamp06.jpg

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

http://antiqueradio.org/art/DuMontRA...eoPreamp07.jpg

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

Phil said:
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.

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

It happened again! LOL

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.

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.

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?

http://antiqueradio.org/art/lm317-ad...-schematic.jpg

Regards,

Phil Nelson

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. :smoke: