Videokarma.org

Go Back   Videokarma.org TV - Video - Vintage Television & Radio Forums > Early B&W and Projection TV

We appreciate your help

in keeping this site going.
Reply
 
Thread Tools Display Modes
  #1  
Old 01-30-2017, 05:49 PM
Phil Nelson's Avatar
Phil Nelson Phil Nelson is offline
VideoKarma Member
 
Join Date: Jan 2006
Posts: 4,030
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

Last edited by Phil Nelson; 01-30-2017 at 06:07 PM.
Reply With Quote
  #2  
Old 01-30-2017, 06:20 PM
vts1134's Avatar
vts1134 vts1134 is offline
Looking For Time
 
Join Date: Apr 2011
Location: Pittsburgh, PA
Posts: 1,532
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.
__________________
John
Reply With Quote
  #3  
Old 01-30-2017, 08:01 PM
Phil Nelson's Avatar
Phil Nelson Phil Nelson is offline
VideoKarma Member
 
Join Date: Jan 2006
Posts: 4,030
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
Reply With Quote
  #4  
Old 01-30-2017, 08:37 PM
miniman82's Avatar
miniman82 miniman82 is offline
First Light: 1952-2011
 
Join Date: Jun 2009
Location: Great Mills, MD
Posts: 4,159
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.
__________________
Evolution...
Reply With Quote
  #5  
Old 01-30-2017, 10:50 PM
Phil Nelson's Avatar
Phil Nelson Phil Nelson is offline
VideoKarma Member
 
Join Date: Jan 2006
Posts: 4,030
Quote:
Originally Posted by miniman82 View Post
You say it works 'reasonably well' right now, what exactly is keeping you from giving it the ole siskel and ebert?
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.

Quote:
Originally Posted by miniman82 View Post
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
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
Reply With Quote
Audiokarma
  #6  
Old 01-30-2017, 11:05 PM
Electronic M's Avatar
Electronic M Electronic M is offline
M is for Memory
 
Join Date: Jan 2011
Location: Pewaukee/Delafield Wi
Posts: 14,758
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.
__________________
Tom C.

Zenith: The quality stays in EVEN after the name falls off!
What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
Reply With Quote
  #7  
Old 01-30-2017, 11:20 PM
Phil Nelson's Avatar
Phil Nelson Phil Nelson is offline
VideoKarma Member
 
Join Date: Jan 2006
Posts: 4,030
Thanks, I tried using my 1077B analyst (see first post) without noticeably better results.

Regards,

Phil Nelson
Reply With Quote
  #8  
Old 01-31-2017, 08:52 AM
vts1134's Avatar
vts1134 vts1134 is offline
Looking For Time
 
Join Date: Apr 2011
Location: Pittsburgh, PA
Posts: 1,532
Quote:
Originally Posted by Phil Nelson View Post
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
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.
__________________
John
Reply With Quote
  #9  
Old 01-31-2017, 08:54 AM
vts1134's Avatar
vts1134 vts1134 is offline
Looking For Time
 
Join Date: Apr 2011
Location: Pittsburgh, PA
Posts: 1,532
Quote:
Originally Posted by Phil Nelson View Post
Thanks, I tried using my 1077B analyst (see first post) without noticeably better results.

Regards,

Phil Nelson
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?
__________________
John
Reply With Quote
  #10  
Old 01-31-2017, 10:02 AM
dtvmcdonald's Avatar
dtvmcdonald dtvmcdonald is offline
VideoKarma Member
 
Join Date: Nov 2012
Posts: 1,191
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.)
Reply With Quote
Audiokarma
  #11  
Old 01-31-2017, 10:10 AM
Electronic M's Avatar
Electronic M Electronic M is offline
M is for Memory
 
Join Date: Jan 2011
Location: Pewaukee/Delafield Wi
Posts: 14,758
Quote:
Originally Posted by vts1134 View Post
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.
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.
__________________
Tom C.

Zenith: The quality stays in EVEN after the name falls off!
What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
Reply With Quote
  #12  
Old 01-31-2017, 12:08 PM
Kevin Kuehn's Avatar
Kevin Kuehn Kevin Kuehn is offline
Workin' Late Again
 
Join Date: Jan 2011
Location: WI
Posts: 3,824
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.

Last edited by Kevin Kuehn; 01-31-2017 at 12:43 PM.
Reply With Quote
  #13  
Old 01-31-2017, 05:13 PM
Phil Nelson's Avatar
Phil Nelson Phil Nelson is offline
VideoKarma Member
 
Join Date: Jan 2006
Posts: 4,030
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:



(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:



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):



Same deal when I inject video from a DVD player:



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



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
Reply With Quote
  #14  
Old 01-31-2017, 06:52 PM
vts1134's Avatar
vts1134 vts1134 is offline
Looking For Time
 
Join Date: Apr 2011
Location: Pittsburgh, PA
Posts: 1,532
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.
__________________
John
Reply With Quote
  #15  
Old 02-01-2017, 05:13 PM
Phil Nelson's Avatar
Phil Nelson Phil Nelson is offline
VideoKarma Member
 
Join Date: Jan 2006
Posts: 4,030
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
Reply With Quote
Audiokarma
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 08:56 AM.



Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
©Copyright 2012 VideoKarma.org, All rights reserved.