Adding Composite Input (Capacitor Question)

[ARC Tech-109]

Image looks like my Magnavox T940 with composite input... thus the reason I'm going back to RF/IF as the Mag did not play well with the external input despite using an op-amp interface, big impedance mismatch. I used a LT-1252 op-amp in this instance.

[TinCanAlley]

I really would like to use the composite input on this set, but it'll have to wait for another day. Main reason is to see if the tuner/IF is the source of my jailbars. Pretty much check all other areas of the set. Oh well.

[ARC Tech-109]

Jailbars? Are you dealing with poor quality RF modulators or front end overload as I am? My reason for the composite input on my Magnavox was due to the IF board getting damaged while I was trying to deal with some similar issues caused by the poor quality signals coming from cheap RF modulators but the composite input introduced a number of new issues related to impedance matching and the raw input levels needed to get thru the video amps, I've since gone back to RF modulation which has brought me back to square-one and I believe the solution may be the use of CATV head end RF modulators that use SAW filters. As I understand they're far more robust and have a much cleaner output signal than the cheap-o's used in home equipment, I do have experience with these as our local senior care center has gone to great lengths to keep legacy analog MATV going in the building so those with older TV's can still use them. It's a memory care type facility and many of the residents can't operate the "new fangled" flatscreens and need their old rotary tuners from long ago, just doing our best to help those in need.

As it stands right now my issues are with a limited AGC range and what appears to be some type of "saturation" that is washing out the colors on all but the strongest saturations, set chassis in question is a T940 and thanks to member Damen I was able to get another good IF board. Now it's a matter of undoing all of my composite input mods and bring the set back to original.

[TinCanAlley]

Originally Posted by ARC Tech-109

Jailbars? Are you dealing with poor quality RF modulators or front end overload as I am? My reason for the composite input on my Magnavox was due to the IF board getting damaged while I was trying to deal with some similar issues caused by the poor quality signals coming from cheap RF modulators but the composite input introduced a number of new issues related to impedance matching and the raw input levels needed to get thru the video amps, I've since gone back to RF modulation which has brought me back to square-one and I believe the solution may be the use of CATV head end RF modulators that use SAW filters. As I understand they're far more robust and have a much cleaner output signal than the cheap-o's used in home equipment, I do have experience with these as our local senior care center has gone to great lengths to keep legacy analog MATV going in the building so those with older TV's can still use them. It's a memory care type facility and many of the residents can't operate the "new fangled" flatscreens and need their old rotary tuners from long ago, just doing our best to help those in need.

As it stands right now my issues are with a limited AGC range and what appears to be some type of "saturation" that is washing out the colors on all but the strongest saturations, set chassis in question is a T940 and thanks to member Damen I was able to get another good IF board. Now it's a matter of undoing all of my composite input mods and bring the set back to original.

I've ruled out external RF as it happens with a stand alone RF modulator, Laserdisc player's RF output and a number of other units I've tried. That's what has lead me to a possible tuner/IF issue. The replacement IF I got didn't work as a direct swap. It over saturated colors and contrast and no adjustment of color or tint made any change in color or hue. I think it's the detector voltage level. I need to set up my power supply to inject a voltage, read it on another test point and then adjust the pot in the IF for that. Since the image was there and stable, I'm sure the unit is aligned properly, just not the one voltage adjustment. That sets the voltage on the detector output going to the chroma and video board.

If the new IF doesn't fix the jailbars, I'm at a loss as I've replaced and tested every component that could cause the bars.

[ARC Tech-109]

Okay so are you saying the color & tint controls have no effect at all? I know you're working with a Zenith set but what series or model? This almost sounds like broken connections between the tint/color pots somewhere along the lines.

[TinCanAlley]

Originally Posted by ARC Tech-109

Okay so are you saying the color & tint controls have no effect at all? I know you're working with a Zenith set but what series or model? This almost sounds like broken connections between the tint/color pots somewhere along the lines.

No, they stop working only with the new IF module installed. They work with the original IF module. So I guess they are working with the new one, just something is so far out of wack that adjustments have no affect.

It's a '74 Avante with the 25EC58 chassis.

[ARC Tech-109]

Guess we're sorta in the same boat here. I'm working with a Magnavox T940 chassis that seems to have its own ideas on how to deal with external video signals. Problem started out when I broke my IF strip board and destroyed a coil so I decided it would be easier to just inject a video signal in behind the detector diode, this introduced some impedance matching issues which were sorta solved with a simple matching network but like in your case the internals of the set don't use the "standard" video levels and the picture was weak & blotchy. Some other members pointed me to a composite input circuit used by an earlier RCA (don't recall the chassis) and I built it up on a perf board tube socket and all but it introduced a new set of challenges and was superseded by an op-amp design using an LT1252 and configured to produce up to 8 volts P-P and it worked... sorta. Other issue I was having involved sync and DC restoration which I tried to inject using a diode based circuit but the Magnavox would not play nice no matter what I did.
I bypassed the 6GH8 sync separator completely and went with an LM1881 sync sep/countdown chip from TI and configured it to feed the horizontal & vertical directly which resulted in a very stable lock no matter how much things varied but the picture quality itself was never 100%. The set has been in the family for 56 years now so I know what it's capable of... well back in the day and now no matter what I do or how I do it I can't get to that level again. Pix tube tests very good on my B&K 467 showing 1.6ma on each cathode and over the past several months I've been slowly replacing resistors and other parts that don't measure up in their tolerances so everything is what's specified on the schematic (Sams 1144 folder 1) at least academically.
So recently I put out a want for a replacement IF board as I wanted to bring everything back to factory stock but with all the new parts and a fellow member pointed me in the right direction, two weeks later a carefully removed IF board and the service manual for that donor set arrived at my door. Just want to say Thanks Again to damen for helping me with this.
That afternoon the IF board was carefully transplanted with the hopes everything would fall into place... then reality hit. I had to go back on all of my mods and restore the sweep board back to original then double-double check all the wiring points, a picture appeared on the screen but it was weak and washed out with the same color issues I was having with the composite input.
Problem I'm having with the color is in the areas of strong or saturated colors they're very bright and saturated on the screen but in areas not as saturated or more like a shading on say a scene with a wall or background of trees the color is missing or only shows the areas that "rise above" some arbitrary threshold like some amplifier is not biased correctly. Think of it like a contrast control is set too high and the gray details are all but lost. I've gone thru the chroma sections replaced resistors and screen bypass caps, all the voltages are correct compared to the schematic but I can't figure this one out.

So what I'm fighting on the IF board is a low video level coming from the detector, it will not go above about 2.5V P-P and the AGC is very aggressive, I have almost no gain no matter how things are adjusted and any thing short of 100% on the AGC pot cuts everything off. Voltage on the IF tubes are within a few points of the schematic and the tubes themselves check good, tried known good subs and I get the same thing. Video/RF modulator is a Blonder Tongue AMCM 860D CATV head end that's padded down to -70 dbm. I'm going to do a raw video injection using my op-amp circuit again this evening right behind the detector diode and look at what's showing up on the grid of the 12BH7 video out tube just to cover this path and I will post my findings once I know more.

[kf4rca]

Can you post a scan of the schematic so we can see what you're doing?

[ARC Tech-109]

The best I can do is a hi-res photo of the chassis schematic, very little online on the Magnavox sets of this era, I don't know where my notes & schematic of the op-amp video interface are so I'm working from memory on this one, they're around here... somewhere.


Okay so this afternoon I went for broke and literally jammed the output lead of my video op-amp into pin-2 of the 6MU8 video out tube without making any other changes to the IF or the video detector side of the board, hit it with some color bars and the most "watchable" picture appeared on the screen. Going this way to me seemed like the most logical in terms of maintaining the DC loop thru the sound trap despite the impedance mismatch so maybe this is something you might want to consider with your Zenith.
I couldn't find a free online schematic of your EC chassis so I'm no able to follow along, if you know of one point me in the direction and I will take a good look at it.

So the op-amp circuit I'm working with is a simple non-inverting layout using an Analog Devices LT1252 in the 8-pin DIP package. Input is direct coupled and the gain is set to 8V P-P output which is then fed to a 100 ohm pot to adjust the final output level, I'm using the suggested layout of the datasheet and it's powered by a regulated and well filtered +/-9V onboard linear supply. The input signal is also directly coupled to an LM1881 8-pin DIP sync separator which can supply raw vertical, horizontal and composite sync plus a burst/back porch and odd/even output, early on I was using the v-sync to drive the vertical section directly and H-sync to the AFC diodes thru an NPN as an inverter. In this arrangement raster lock was absolute and the horizontal would remain locked thru the entire range of the control pot. The big downside to all of this was an uphill battle with DC restoration and this appears to be far more stable now using the brute force video injection to pin-2 (grid-1) of the 6MU8, schematic of the video amp below.

Now I'm waiting on a fresh 6MU8 to arrive tomorrow, want to make all the adjustments using a known good tube.

schematic of the video amp used in the Mag T-940. Flag 56 feeds the input of the 6GH8 sync sep and between R726 & R727 is the feed to the AGC grid-1. Plate section of the triode goes thru a parasitic suppressor then the delay line while the grid gets a blanking pulse from the vertical. Flag 22 goes thru a 22k resistor then to the low side of the brightness control which has the high side on a divider and +140V. For some reason imgur is not allowing me to post more images so I can't include the wider view.


Okay an addendum to all of this.

I have finally integrated my video/sync buffer amp into my Magnavox with excellent results, most of the work was housekeeping and finding the most suitable places to inject the signals and at 03:30CDT this morning everything came to life. I ended up connecting a shielded wire directly to Pin-2 of the 6MU8 1st & 2nd video amp tube leaving the entire video detector circuit intact and removing the 3rd IF coil connection from the plate of the 3rd video IF tube, from there it was run a pair of shielded wires from the buffer amp vertical pulse output to feed the vertical osc which was isolated from the sync separator by lifting one side of R-515. Horizontal is still keyed off the sync separator which has now become essentially an amplifier to feed the needed 35V P-P to the AFC diodes, one side of R-725 was lifted to inject the composite sync out of the buffer amp. The reason for this is twofold; one I can now use sources with non-standard sync levels to maintain a raster lock and two maintaining the sync separator maintains the original timing of the horizontal to the burst gate. I'm using the Texas Instruments LM-1881 video sync separator chip for the timing as they're very simple to use and have plenty of drive, the composite sync feeds a 2SC945 which is emitter coupled to the 6GH8 sync separator via C-724.

The biggest problem I'm having is uploading images to the videokarma server and as a result they are not available for this post. If you want to see the images you will have to send me a private message with your email or other means of file transfer as downsizing them to fit the antiquated requirements cuts too much of the details out and is completely unacceptable in this day when the minimum image from a typical phone camera is several megabytes in size with little one can do to reduce this.
The alternative is to follow along on another site where I have also posted my progress on this and many other projects over the years on my dedicated thread called "On The Bench" that is found in the Ham Amateur Radio section of smokstak.com and my username there is Brhatweed.

[kf4rca]

A tube set might require as much as 4V P-P video.
I have to shrink my images down to 640x480 to get them to post. You can do this in "Paint". What OS are you using?
I use a Canon 12Mp camera and transfer them to a Dell D630 laptop running W7. W7 has the software built in so you can transfer pictures from your camera to the laptop. I then crop or reduce them as required in Paint, then upload them via Firefox 32 bit to Videokarma.

[ARC Tech-109]

Running Linux on my end but regardless of the OS it comes down to principals. If videokarma wants support then they need to offer something to support. I resized the images down using Gwenview to fit the videokarma constraints shown on the upload panel and they were still refused, I'm not the one running the same software from before the karma split and have to say this is doing none of the members any service.

As for the set itself the new 6MU8 arrived this morning and it turned out to be a NOS dud that resulted in a washed out picture with retrace lines, going back to the original tube clears up the problem so it's definitely the replacement tube. Going to give it one more try and buy another 6MU8 as they're $5.95 while this one is left to burn in for the afternoon.

The full write-up with large detailed images can be found here https://www.smokstak.com/forum/threa...5#post-2110545 Small images are in the post below

Computer here is a HP EliteDesk G4 i5-8500 running ubuntu 24.04

[ARC Tech-109]

Images for the above post. Tried uploading to Imgur but that's an even bigger pain so I've cropped them way down to a Windows 95 dial-up friendly size. If you want the real-deal email me and I will reply with the full size images.
My whole point to all this is to share the details and maybe spark some ideas on keeping the old sets going in the modern world of unfriendly tech. I'm still dealing with some color shading issues and will post them in the near future as they have me completely baffled.

[kf4rca]

There is something wrong with your buffer detail. The emitter should be negative with respect to the collector if it is an NPN transistor. That circuit would call for a PNP transistor. You really need TWO stages of amplification. NOW, you can get amplification out of one stage but it will be INVERTED which is NO GOOD. SO, you really need two stages. The first inverts (with gain) and the second re-inverts. SO, now you are "right side up." Remember, in video, polarity matters.

[ARC Tech-109]

That only handles the composite sync for the horizontal not video. In the bigger picture the buffer has been sidelined for the time in favor of the original IF strip as the op-amp itself doesn't have the swing the set needs on the 1st video amp. The issue with the IF strip turned out to be an open parasitic suppressor coil which was only letting a little signal thru on the internal 12K resistor and this was discovered using an IF sniffer connected to a later Panasonic TV.
As it turns out the 1st video is looking for a signal with 10 volts P-P to make the magic happen so an additional stage may be added at a later point, right now it's get a stable image on the screen and the chroma section working properly before I continue with the composite input.

Problem I'm having with the chroma has to do with this green shading that seems to be some sort of 3.50 osc leakage that's getting amplified by the XZ demod and equally imposed on the R-Y and B-Y. It goes away if I advance the color killer or pull the 6HS8 demod otherwise it's always present and baffling me. Just another bug to iron out.

[kf4rca]

The circuit I posted might be able to give you 10V P-P if you use 24V as your supply. Transistors like 2N2222 and 2N2907 are rated at 40V and should do the job.
What is wrong with the existing sync separator that you need an outboard one?
The green shading, if it is not a purity setup problem, could be inadequate decoupling in a video circuit getting crosstalk from a sweep circuit due to a weak cap.