I'm about to test inserting a composite signal into my Zenith and have a question about the capacitor orientation.

I am creating the cable to do this insertion and so far I've taken a female RCA jack and put a 75 ohm resistor across it. From videos I've seen on this, they also use a 47uf capacitor. However, the one I saw had the negative side of the coupling capacitor on the signal side. My brain tells me that since it's the signal side, that should be the positive lead of the cap. Upon reading more about coupling caps, it seems the negative side goes to the "most" negative side of the circuit. That if the signal side is lower voltage than what it's connecting to, than the negative lead is connected there. If this is true, then once I remove the IF module, I need to measure voltage on the the insertion point and if it's less than composite signal voltage, the negative goes to that side. But, if it's higher than the composite signal, the positive side goes to it.

Do I have this correct?

There is no fixed DC polarity to the composite video signal. You either determine polarity based on the injection points DC value with respect to ground. Or you use a non-polarized capacitor for the connection...Which you can make out of 2 polarized lytics of double the desired capacitance connected in anti-series...(Not parallel but rather) Series with one cap reverse polarity from how you'd connect 2 batteries in series.

There is no fixed DC polarity to the composite video signal. You either determine polarity based on the injection points DC value with respect to ground. Or you use a non-polarized capacitor for the connection...Which you can make out of 2 polarized lytics of double the desired capacitance connected in anti-series...(Not parallel but rather) Series with one cap reverse polarity from how you'd connect 2 batteries in series.

So if I put two 100uf caps negative to negative I would get a 50uf non-polarized cap, right?

So if I put two 100uf caps negative to negative I would get a 50uf non-polarized cap, right?

No. One cap going "reverse" is essentially a short to the the waveform so it's going to be about 100uf as the other is running in the "correct" polarity. I'd start off with a pair of 22uf 16V caps back-to-back and work up from there. Remember sync is negative going with respect to ground and visual is positive going so the capacitors should be equal value however playing with the values be it negative or positive going may help work out some balance issues.

Now my question is where are you feeding the composite signal into and is this a televison receiver or real monitor? Reason I ask is twofold. One is the possibility of a hot (AC) chassis with respect to ground and two some TV receivers don't like anything other than what comes from the video detector as I'm fighting this issue now with a Magnavox T940

I have done this but I used DC coupling into the first video amp. You're probably going to need some amplification as the set probably wants 2V P-P for full brightness. Check the schematic. I did this on a Heathkit using a MAX405 op amp and an RCA using a 2 transistor amplifier. I would go the transistor route since the MAX405 op amp is probably no longer available.

Yes, check for the p-p video required. In some sets it could be as much as 4 V p-p. It not only afffects the picture brightness/contrast, but also the sync circuits.

No. One cap going "reverse" is essentially a short to the the waveform so it's going to be about 100uf as the other is running in the "correct" polarity. I'd start off with a pair of 22uf 16V caps back-to-back and work up from there. Remember sync is negative going with respect to ground and visual is positive going so the capacitors should be equal value however playing with the values be it negative or positive going may help work out some balance issues.

Now my question is where are you feeding the composite signal into and is this a televison receiver or real monitor? Reason I ask is twofold. One is the possibility of a hot (AC) chassis with respect to ground and two some TV receivers don't like anything other than what comes from the video detector as I'm fighting this issue now with a Magnavox T940

Isn't two caps connected negative to negative what is meant by "back to back"?

This is a television, but not a hot chassis. It has a full transformer. I'm feeding it into C1 test point which is after the detector stage and where it splits off to the video amp, chroma module, horizontal module and vertical module. This test point is where you attach a scope to see the video signal from the IF.

I have done this but I used DC coupling into the first video amp. You're probably going to need some amplification as the set probably wants 2V P-P for full brightness. Check the schematic. I did this on a Heathkit using a MAX405 op amp and an RCA using a 2 transistor amplifier. I would go the transistor route since the MAX405 op amp is probably no longer available.

I think this may be the case as the image I just got was way to dim and looks the way it does when the brightness is down too low. According to the schematic, the signal from the IF out is 1.4V for the waveform.

Yes, check for the p-p video required. In some sets it could be as much as 4 V p-p. It not only afffects the picture brightness/contrast, but also the sync circuits.

Attached is the waveform from the schematic. It shows 1.4V. Thing is, with the IF module in place, there is just over 6V on the test point and 4V with module removed. Not sure where to go from here as setting it up with the 75ohm resistor and a 75uf cap (negative towards signal side since its voltage is less than the connecting point on the chassis) gave me a very dark image with bad sync.

Also, tried the "new" IF module and it is either not new, or not aligned. When I installed it, the color went full and the color control wasn't functional, or it was but had no affect.

I think I may see one issue. The output of the IF is 6.99V and the signal off that lead goes to the chroma module and that input is 6.99V. So if I remove the IF module, it drops to 4V. This where my understanding of electronics lacks. If the IF is outputting 6.99V, but the signal in the wave form is 1.4V, how is this possible? Is it possible to have the composite signal of 1.4V inside the 6.99V? If so, how does the signal not go from 1.4V to something higher?

I think it time to break out the Tek scope and look for the video signal on that test point. I tried the small digital scope I got, but it was all over the place trying to lock onto it.

Okay, here is the waveform from C1 test point. I used a gated rainbow test pattern. First one is 30us and second is 20us. I used a 1x probe with 1V per division. The SM waveform says 1.4V, but this is definitely 2V p-p.

1) If the DC voltage is 6.99, that is the average of the video waveform, that is, the high parts of the waveform are more than 6.99 V, and the low parts (sync pulses) are less than 6.99. The difference between the max and min is 1.4.

2) The gated rainbow pattern does not go to full white level, so it's peak-to-peak range would be less than a test signal containing full whites. Does the service info say what test signal is used to get the 1.4 volts?

3) Having 2 volts p-p could be from an AGC misadjustment, but before jumping to conclusions, how does the picture look? Overly bright and unstable, or OK?

1) If the DC voltage is 6.99, that is the average of the video waveform, that is, the high parts of the waveform are more than 6.99 V, and the low parts (sync pulses) are less than 6.99. The difference between the max and min is 1.4.

2) The gated rainbow pattern does not go to full white level, so it's peak-to-peak range would be less than a test signal containing full whites. Does the service info say what test signal is used to get the 1.4 volts?

3) Having 2 volts p-p could be from an AGC misadjustment, but before jumping to conclusions, how does the picture look? Overly bright and unstable, or OK?

I used the gated rainbow to have a stable waveform using RF, but when I tested the composite insertion on C1, I used a DVD playing video. What I got was way too low brightness and no sync. I did that with the IF removed, so today I plan on trying it again with the IF installed, but the tuner lead disconnected.

I believe the lack of brightness and sync was due to the IF module being removed. The chroma module input being fed from the IF module is also 6.99V and it probably couldn't compensate for the 4V with the module removed. The sync going to the video module is 6.24V and that one also didn't like the lower voltage with the IF removed.

I haven't adjusted the AGC since it said to do it with a weak channel, without snow. If I do it, the signal I feed it will not be weak and the adjustment will be off, or so I believe.

Partial success. I used the composite from my DVD player through a 75ohm resistor across the leads and a single 47uf cap on signal side with negative towards dvd and positive towards C1 in set. With tuner attached to IF, nothing. With tuner disconnected from IF, I got the attached. So it is now synching, but the colors are washed out and I don't know where to go from here. Am happy I got this far, though.

Very likely that first video stage is getting some bias from the IF. Not unusual.
Attached is the schematic for the video input circuit I used for an RCA CTC92. I found this circuit on the web and modified it some.
The pot labeled DC sets DC level which equates to black level.
I would inject the video signal after the sound trap as the sound trap rolls off everything over 4.5 MHz.
Maxim used to give out free samples. But I think that's now only available to Asian EE students.
Another source of video chips is Linear Technologies (now a part of Analog Devices). Not sure what their sample policy is.
Something to remember, if you go the op amp route, you will most likely need a bipolar power supply. Whereas the transistor circuit requires only one.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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/threads/on-the-bench.237493/page-15#post-2110545 Small images are in the post below

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

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.

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.

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.

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.

Reason for the sync sep chip is so I can use non standard or poor sources and maintain a good lock, I also run everything on a master BB source and an external/internal sync switch was in the works.
Shading seems to defiantly be in the XZ demod section. I thought it was input video related but noticed on some scenes things appeared to have this sort of trailing color smear along with very intense reds and blues. The 6MK8 checks out and I have several new examples of this tube along with the 6HS8 demod. I can turn the color control all the way to zero and even short the pot to ground and I still get a shading that varies with the tint control. If I pull the 6HS8 or 6GH8 3.58 osc tube the shading goes away, where or how this is leaking in has me stumped.

Defiantly, you say? That shading is definitely something coming across on your supply rail. Most likely your demod and 3.58 oscillator tubes share the same supply source. Scope it out. You most likely have a bunch of electrolytics that are weak in that set.
I want to see a scan of the schematic of the video detector and the first video stage. Can you post that?

Defiantly, you say? That shading is definitely something coming across on your supply rail. Most likely your demod and 3.58 oscillator tubes share the same supply source. Scope it out. You most likely have a bunch of electrolytics that are weak in that set.
I want to see a scan of the schematic of the video detector and the first video stage. Can you post that?

All the old 'lytics were disconnected and subbed with radial leads of the same or higher values but maybe it's a case of ESR, I will add some .005uf bypass caps on the boards where the B+ comes in and see how that goes.

I can post the schematic sure but will have to take hi-res pics to do it, would you like an email?

You can try that. Those call letters at Gmail.Com would be good. But if its a very large file, it won't make it thru the email system. Can you convert it to a pdf? I think Linux can do that.

I will snap several detailed sections so you can see what's going on here. Give me a few days to get this all together. This morning was our official fishing season opener and a total bust for me. Cool & windy across Knife Lake.

Added some bypass caps to the DC feeds going to the various sections but the shading remains, going to put another day or two into this before I sideline things for a while as I'm getting burned out and need a break. I am starting to wonder if my add-on caps are bad/leaky/high ESR as I'm getting a very well defined 120hz hum, the hum bars are very square and distinct not rounded like a bad capacitor normally shows so later this evening I'm going to cliplead a couple of known good caps in and see where that goes. Never noticed this until the AGC issue was ironed out as it was masking things with its own issues. Set uses a full wave bridge not a doubler and a capacitor input choke so maybe there's some resonance with a failing cap, the original "cans" are out of the circuits and only plugging the holes and I'm using what was supposed to be low impedance Rubycon 450V photoflash caps (EC series) but they're now 20 years old having been installed the summer of 2006. I don't see more than a slight bit of ripple on the post choke side on the scope, manual shows 2V of ripple on the main 410V source to be acceptable so what I have is far below this. A possibility is heater/cathode leakage in V703 which is the AGC/sync sep 6GH8 so I will also sub this with another... I have over 50 new & used pulls to choose from.

On the plus side of all this the IF sections appears to be on the money and the picture is about as good as I remember it minus the 120hz bending. Vertical is running a bit short despite swapping new tubes but overall I'm happy with the progress and may stay with the RF distribution using the CATV head-end Blonder Tongue modulator, its a CAMS-60 MTS modulator with an IF loopthru and I was able to use the IF from this to verify the Magnavox. Not exactly outlined in the service procedures but it works. I feed the modulator directly from a Microtime T100 TBC so everything is about as clean as an actual broadcast.

Those were some very large files. Surprised they made it thru the email system.
Attached is a drawing of the connection I would use. You should go in right after the video diode. Disconnect the anode of the video detector diode and connect a 2.2 uf capacitor. Then connect your video source to that. Be sure to terminate into a 75 ohm resistor.
That's a very old set. You probably have some other problems. I hope you have a good supply of tubes as they are drying up!

I measure my tube inventory by the 25 gallon tote however my stock of 6LQ6's are getting thin. I was a Motorola radio tech for the better part of 25 years and accumulated a lot of RF service tubes along with the usual 6GH8s and related low signal TV tubes which go back to my youth messing with TV's.

The problem with the Mag is it's looking for greater than 8 volts P-P on the grid of the 6LM8 pentode, the schematic calls out a volt or so but so far experience says otherwise. I made an op-amp based input interface with 10db of gain using an LT1252 video op-amp using the mfr example schematic however it was hitting the rails at +/-12v driving the 1st video amp crushing the peaks.

I may pick up the project again over the weekend if the replacement tuner shows up, USPS promised a friday delivery however that was pushed back to saturday. I used to be a rural route carrier and know low priority packages are not delivered on the weekends around here.
The big picture plan is to get the replacement tuner in place, run thru the IF once more with the marker gen and bring everything back to "stock" as I've made so many mods over the 40+ years I can't remember them all. It's an original family set and I inherited it in early 85 when it was replaced in favor of a 27" Zenith S-3 however this one died from a shorted K/G1 short on the red gun and ran in this condition overnight. It had the A/V inputs and was left on with only black/burst and no one noticed the set was on, Magnavox won the longevity battle.

Yes, V5a grid (pin2) is looking for a volt of video. Why do you think the schematic is not correct? Did you see that circuit that goes off to the Brightness control? I would investigate that. Check the plate and screen voltages of V5a. Verify the peaking coil is not open. That would screw up the bias on V5a.
Maybe a problem further down the road.
How many tubes are in that set? Looks like a well designed set to me.
I once worked at a radio shop. Did mostly installation into cop cars. At that time, it was Mocom70, Micor, and Maxar radios. Syntors had not yet come along.

No the schematic doesn't reflect the same voltages as the T933 uses the 6LM8 for a video amp and my T940 uses a 6MU8. The DC bias from the brightness pot is present. My T940 print isn't nearly good enough for a photograph.

So the replacement tuner arrived late Saturday afternoon and installed without a problem but now I'm chasing down another that really has me scratching my head. Tuner has two B+ sources, #1 is the +130V source derived from R240 which is an 8200 ohm 1W on the supply divider board and this feeds the plate circuit of the 6HQ5 RF amp, the other is B+ #2 which is 140V supply that feeds the 6HB7 mixer/osc and gets dropped further to run the UHF tuner when selected. I'm finding +235 volts on the #1 B+ source and it seems to be from a lack of loading, the resistor itself checks good on the meter at 8225 ohms. I've swapped three known good 6HQ5 triodes and nothing changes, these are the ones that have the cathode tied to pins 2 & 7 via thin ribbon inside and are easily damaged by careless testing. I know everyone is passing a signal as I'm feeding in a known good IF signal into the UHF port of the VHF tuner and selecting UHF and if I feed from the UHF tuner I get plenty of mixer noise which is not present on any of the VHF channels.
This is a known good tuner so I' at a loss with this. My test IF signal is from a 80's vintage Panasonic 11" color on an iso trans and what I see on this set is perfect, I tapped the IF out of this tuner and feeding a dedicated jack I use for testing.

The original issue with the tuner was an accidental contact of the AC terminal of the little fuse block near the IF input with the tuner IF cable and that popped a number of parts in the tuner, thankfully another member was able to send me a spare replacement. The IF cable is good showing continuity and no shorts, it's passing the test IF to the chassis IF strip and all is well but it's like the RF osc is dead or way off frequency due to the elevated voltage which itself doesn't make sense. All the grounds are good and the AGC bias is right on the mark at 0.45v. According to the schematic the signal path in UHF starts at the little RCA jack on the side and is then coupled thru the RF amp thru the turret, from there it's coupled to the mixer (pentode side) of the 6HB7 which is acting as a second IF amp before it makes its way down the IF strip to the detector. IF AGC action is good and I can easily push that to an overload or cutoff so there's plenty of signal making its way thru and I know the Panasonic IF is spot on as they use a SAW filter to set the response and the magnavox matches this.

I'm at a total loss to why the tuner B+ one isn't being loaded down by the RF amp. Gone thru three known good 6HQ5 triodes and no changes.

Check for a broken pin in the socket or an open cathode circuit.
If it worked before, it can work again. An old CE once told me that. One time I pulled a fried rat out of his TT25BL transmitter. He came into the control room and looked at me and said, it's a dirty job, but somebody has to do it.
Probably wasn't a good idea getting him around all that high voltage after he had just gotten back from the Gold Bug Room.

Been in a few FM transmitters in my day, had a rookie tech get mad at one and throw a wrench into the power supply not expecting it to come back and hit him in the face, took his teeth out. Yes anything can be made to work again and my philosophy is if it worked from the factory no reason it won't work again today.

So that being said the replacement tuner has resulted in little progress, it does pass a signal from the little UHF input RCA on the side thru the tubes and down to the IF input on the chassis however that's about it. All the solder connections in the tuner itself are good, the 6HQ5 RF amp and 6HB7 mix/osc check good on the tube tester but I bought some NOS tubes on the 'bay and will give them a try upon arrival. My supply of these is limited to a few in a storage tote and while they do check good on a Heathkit IT-17 something may very well be "off" enough to cause a problem. I've had the IT-17 since middle school (1982) and trust it however being little more than a "diode test" of the tube there could be something like a burnt grid wire or a spot on the cathode that wouldn't show on the meter yet could kill the local oscillator. I got the IT-17 new in the box unassembled at the Medina MN hamfest for $10.00

Until then the remaining repairs will be done using an outside IF source provided by one of those tuner units that was part of a Panasonic portable VCR combo, still need to iron out the weird phasing and chroma shading issues that I suspect might be coming from the TAC board. If you remember these had the "Total Automatic Color" feature that basically added more red & yellow giving everything an orange cast, great if you like burnt orange shag carpeting but it's not my favorite and I may just bypass the board all together.

Will post more when I can.

The RF amp seems very simple. 8200 ohm resistor from 270 B+ into tuner, then 1200 ohm resistor to the wiper contacts for the tuner drum, then the 6HQ5 plate. If B+ is not being dropped I would suspect those wiper contacts not making connection. Without the 6HQ5 plugged in, is there B+ on pin 5, measured form the top of the tuner? With set turned off, what resistance is there between pin 5 and B+ 1 source ?

I get the 1275 ohms between B+1 and pin-5 of the 6HQ5 and without the tube in place I get the full +285 volts of the power supply so there is a path to the plate. The AGC here is about 0.45V and that's consistent with the schematic, both pin-2 and pin-7 show continuity and I get about 55% on the tube tester, not stellar but it should pull some current.

According to the schematic when tuned to UHF the input from the UHF tuner is coupled directly to the grid of the 6HQ5 and from there gets passed to grid-1 of the pentode section of the 6HB7 osc/mix and that plate is coupled to the tuner IF output coil and out thru a .001uf cap. Feeding in a weak IF from an external source gets amplified and sent to the IF strip so I know things are doing what they should which doesn't make much sense considering how little current the RF amp tube is taking.

I have two NOS 6HQ5 and 6HB7 tubes on the way and once verified in the tester they're destined for the sockets, until then I have a color issue to iron out in the 3.58 chroma bandpass amp and this isn't the first time for the problem.

If tube subs don't do much and voltage checks don't give an obvious issue (if you have a SS socket saver with test points that usually is the best Best way to verify the socket) then my next step would be to look for carbon comp resistors that have drifted high enough to be out of tolerance (low out of tolerance is usually something else in the circuit in parallel with it confusing the meter).

Yeah the one plate resistor has only drifted a few ohms off the mark so I'm not to worried about that, the 8200 ohm metal film on the supply board is a few ohms high as well and the only unknowns at the moment are the tubes themselves and tracking shows their arrival on Thursday... we'll see about that.
Over the years most of my servicing was done on the RCA and Zenith sets so I have very little for Magnavox specific parts like the 6HQ5 despite being a family TV for 56 years, too bad Magnavox didn't use the Nuvisters as I have plenty of them.

Friday the 5th and the tuner is once again alive with a new RF amp and osc/mixer. My theory is they were somehow damaged internally when the IF connector brushed against the 120VAC terminal of the fuse holder and the replacement confirms this. Everything is now on the mark up to the video amp and now I have to sort out the chroma problems.

To back up some 40 years the set took on some lightning damage when a tree was hit and this killed both the phase detect and killer diodes, I replaced them with modern silicon small signal diodes but it was never quite the same and this was the real reason the set was replaced with a Zenith S-III and I ended up with the albatross in my bedroom. Now I want to get the set running like it used to and this has been quite the uphill battle. I found a set of what I thought were good NOS 1N270's and 1N34A's but they're all leaky passing tens of mv in reverse with no two quite the same. Plan for the time being is to "soft wire" in a 6JU8 and see how that goes, not the most elegant but they worked before in this service.

I will post my findings here so as it's been said in the past, Stay Tuned.

Try 1N914 or 1N4148 diodes. They're good to about 100 MHz. and are cheap. Radio Shack used to sell them for $1.99 for a hundred diodes.

Had the 914's in there and the drop is too high for the magnavox circuit so I wired up a 6JU8 in their places and ran thru the entire IF and chroma alignment and everything came together. Yes we have success. A few loose ends to tie up, one seems to be an issue with horiz blanking that's causing the right side of the screen to essentially go into cutoff. Bias and guns are set correctly and the cutoff remains when I run the horiz gun adjustments. Other issue is a touchy AGC and this seems to be a bad AGC pot on the rear apron, have not put an analog meter and run the control thru its range as of this evening.

Ended up replacing all of the tubes on the IF strip with new-old-stock. I sourced the tubes from AxMan Surplus in St. Paul MN which is this bazaar of electronics and other things that's been in business for almost 60 years and they have everything... and I mean everything including thousands of new-old-stock tubes in their original cartons so for $2.50/ea so I invested a good hour scanning thru them and got the everything I needed.

IF strip took a good three hours to get the response back into shape, I'm working with a Heathkit IG-57 sweep/marker generator that has seen better days. The IF sweep works correctly but for some reason the markers themselves are not so I had to basically eyeball the response curve and work from there, I used my Motorola R2670 service monitor to sweep the response of the chroma section as this has calibrated sweep/track and spectrum analyzer functions.

Once I clean things up a little I will snap a few realtime pics of the set in operation, source will be from a Sony VO-2860A editor and whatever tape I happen to grab.

Great. I'm glad things are coming together for you. I would think a silicon diode would have better performance specs than a germanium diode or... a tube.
AxMan sounds a little like WierdStuff out in San Jose, Ca.
I've got a Singer FM10CS service monitor but I use it mostly for radio work. Goes to 512MHz.
Looking forward to the pics of this Franken-set!

I'm sure the 1N914 is better overall but the problem seems to be riding the crest of the waveform with the small signals and its higher forward voltage. Once I went to the 6JU8 the chroma response was far better. This set never had what I'd call "stellar" color, it was good but maybe I'm spoiled by the sheer number of broadcast monitors. Dunno, it uses the 6HS8 as an X-Y demod and from what I remember this design was an "acceptable" lower cost design albeit not as accurate as the R-Y B-Y or I/Q with what appears to be a slower phase response. Good enough for the 1970's I guess when everything was burnt orange with splashes of primary colors here and there.

So everything was working well last night when I shut things down around 0400 central, just before this I dribbled a little hot wax down the IF coils to lock the slugs in place then checked the response curve and picture once again and everything was on the mark.
This afternoon I resumed my work as the outside temps went to the low 90s with mid-60 dewpoints, fired up the set and noticed a really bad picture, checked the IF response and found it had a very sharp peak going just above the picture carrier that wasn't there last night after I dropped the wax.
Ran thru the IF again and locked the slugs in place and everything is holding at the moment, I'm wondering if this might be a moisture issue as the humidity went from roughly 55% to almost 80% over the past day. Not much for air conditioning unless it's oppressive like this afternoon.

Did verify the AGC pot with an analog Triplett 630-NA meter and found it has several "scratchy" points along the travel so it's another trip on the motorcycle to AxMan Surplus in St. Paul as they have bins of these. Minneapolis/St. Paul area used to be a big tech hub during the early computer days with the likes of Sperry-UNIVAC, Honeywell, Control Data, ADC Telecom and IBM in Rochester and had two dozen surplus stores full of this stuff. By the mid-80's only a few remained and today AxMan is the last one standing after almost 60 years in business, they have enough inventory in that building to go another 60 years and it's always busy.

I got the Motorola R2670 monitor during my days at the regional service depot, employees could buy their test equipment at cost and pay for it out of salary. I optioned this one with the everything I could and it's served me well. Other bench test equipment includes a Tek 475 scope, Motorola R1011B 40 amp power supply, along with the various Bird RF meters. TV service is done with a late 60's Tek 453 scope originally from IBM, it has nuvisters on the front end and can survive a kilovolt hit to the input jacks. I also have the Heathkit IG-57A IF sweep/marker and B&K 1246 dot/bar generator plus 466 and 467 pix tube analyzers. Yes I'm on the trailing edge of 1970s technology.

Okay so an update on the Bench Queen; it's running. I still have to iron out a few ripples in the IF section but it's a happy set after 20 years of idle time. Going to run thru the Heathkit manual for the IG-57A sweep/marker generator again and see if I can't find why the markers themselves are dead.

The big issue came down to a bum resistor on the horiz keying pulse, schematic called for a 270K and by color codes that's what I replaced the original with, turned out to be 27K on the meter. Red-Violet-Orange and 5% gold stripe so I went thru and verified ALL the replacements. Guess that's why Ohmite calls them "Little Devils". So once the correct value was in place everything started to act normally, AGC is good but still getting the horizontal bends and trails on high contrast scene details, vertical size is a little challenged despite a new tube and known correct resistor values. I suspect the Boost B+ is low.
Convergence was a challenge that had me stumped for several hours, replaced all the selenium diodes on the board but wasn't getting the correct horizontal actions until I bumped the input connector and discovered a number of traces had broken right where the Molex pins were soldered to the board and lifted the pads. Servicing the set I'd run it with the chassis out of the cabinet sitting on some tape cases then when it was time for the bench would just disconnect everything at the Molex connectors, the sheer number of cycles broke the traces. Once that was rectified the convergence fell into place for the most part, still doesn't respond to the R/G diff tilt so that's on the to-do list.

So overall it's getting there, not quite up to where it was back in the day but now an incentive to keep moving forward. One thing it still has is RF sensitivity. Growing up I remember the DX football games with this massive antenna and Alliance U-100 rotator, there were tape markings for the station positions all around the face and every fall we'd have to check all the flat cables. In the house I can run the modulator on a 1/4 wave antenna with good results.

Got loads of detail pictures of the chassis and operation, give me some time to smash them down to fit the server limitations and I will post them

I thought there might some wierdnesses going on, You should not have to do a complete IF realignment. I suspected broken grounds. Many times you can just resolder the board and that fixes it. Another thing I thought of was perhaps whiskers growing in your IF cans. I know there is at least one IF can from the schematic you sent me. And then there is the problem of carbon resistors going off value due to its age. I'd replace them with metal film.

The weirdness was the wrong value resister in disguise causing the Chroma, blanking and AGC issues and this is where I mistook the problem as being a misaligned IF strip that came from a T933. The AGC overloading was causing the strange response so I attempted to counter these and made things worse. I then went to replace all the resistors on the IF board along with by pass caps then during the alignment phase grazed the IF cable from the tuner against the little fuse block next to the input jack on the chassis and smoked the tuner. Two months later I have the front end out of a T933 and while double checking things came across the rogue resistor, once that was replaced along with the damaged AGC pot everything started to work correctly leaving me with the task of cleaning up the mess I made.

This all started years ago when I attempted to resurrect the old family set using NOS parts I had cleaned out of a storage room working at a Motorola service center. We had hundreds of those resistor kits in the little file drawers along with more in little jars and on tape & reel, I was the service manager at the time and tasked with the decision making... the decision was made to bypass the dumpster and load most everything in my truck. We had just a few weeks to clear out some 40+ years of history as the depot was closing and lease was done, once the test equipment and other capital assets were removed I got the word out to the ham radio community and they picked the place clean over a weekend.

So during the attempt at replacing all of the resistors in the set I managed to break the original IF strip thru sheer clumsiness and made the decision to go with composite video, with the push to digital and impending demise of analog broadcast this seemed like a good alternative and I worked out a few circuits but a few bad real estate moves followed by a divorce that traded me in for a younger model forced the set into a large refrigerator box and into the barn for long term storage. That was 20 years ago.

Now the goal is to un-modify the set and bring everything back to the way it was in 1970... well the TV chassis anyway. I replaced the original amp many years ago with a Heathkit AA-100 I picked up at a hamfest around 1985 and modded the receiver section for stereo TV audio adding one of those stand alone Recoton "FRED" MTS decoders picking the 4.5 SIF up using a loop.

As it stands at the moment I have a good response curve on the IF strip, not stellar and the task for the afternoon is to get this squared away and move onto the lack of vertical height. The 1K-3W resistor for the plate side of the 6GF7 is on the mark and DC values are within tolerance, the tube itself is new from the carton and I've sub'd in a few others with the same results. I have not checked the boost B+ as of yet but it might be a bad boost rectifier as it still has the original selenium on the board.
Once this gets straightened out it's onto a convergence board issue that has the R/G tilt not responding, I've only replaced the diodes on the board so far as I don't need perfect convergence at the moment. We'll get there.

What you could do is add a switch (or relay controlled by a switch) and you could use both video and RF inputs on that set.

I could and have given thought to that but honestly I'm quite pleased with the performance using the RF input, I did leave a provision to add a composite input by soldering a two post tag strip to the top of the IF section cover shield and ran a wire to this for testing right off of TP2 and this goes directly to the grid of the 1st video amp.

The video amp is responding well and I get roughly 300 or so lines of horizontal resolution that's actually limited by the dot trio & aperture of the CRT itself, chroma response is good without the shading or fading that I was getting earlier. Convergence is what it used to be "back in the day" which is to say acceptable. It always had a slight bit of blue error on the right side that dipped down slightly and the only time it was annoying was when I was doing the ATARI 800 BASIC thing back in the early 80's.

The one annoyance is with the slight horizontal sync bending and the dark shading following the highlights and sometimes the vertical wants to follow the black bar used to adapt the 16:9 aspect ratio to the 4:3 frame in the modern DTV receiver boxes, wondering if video is sneaking into the sync somewhere but the scope didn't show anything during the last probe. I've gone thru the AGC section several times and everything is as it should be in terms of values, the 6GH8 is new from the carton and my signal strength is a measures -86db/m at the 75/300 ohm matching transformer coming from the test port of a Blonder-Tongue CAMS-60 MTS modulator with the RF drive all the way down. Thought it was an overload so I tried a different consumer grade modulator and got the same results so I'm out of ideas as I'm totally missing something here.

Vertical is still a little challenged and if I pull the height any further it loses lock and goes into this cycle of collapse then startup again only to collapse a second or so later never locking onto the sync. Supply line shows a constant +394V after the 1K/3W resistor (R112) and C5 is known good. Boost B+ is running 1196V give or take a few and I'm not seeing any horiz ripple on the supply side so my conclusion is one of the "new" parts isn't doing so well here. Plenty to keep me busy for a while.


I do have to give a big thanks to all the members who have put up with my long-o tell-o posts and given me the pointers and directions, also want to thank damen and Danny S. for their help with the replacement pieces. Without their help I'd still be at square one.

Danny if you're reading this I have not forgotten about you, just a little short on the spare change at the moment. Thanks again for all you have done.

Going to watch the set for a few more days while taking notes before I pull the back off once again, I'd leave it open but I do have three curious kitties in the house and they don't take no for an answer.

I will post a short video of the set warm-up and picture itself showing the bending and shading, this will be done in HDCAM so everything can be genlock'd together to eliminate the flicker effects.

If you don't like Letterbox from your DTV converter get s Zenith DTT-900 (or 901, or the LG or insignia rebadged versions of it)... Those boxes have a zoom button on the remote (also accessible through the menus) that lets you pick whether to down convert 16x9 to 4x3 cropped (pan and scan but with pan fixed in the center), 4x3 with letterbox, or 16x9 horizontally squeezed to fit.

I have one of those Apex receivers that also has the zoom/crop/squeeze/fluff buttons on the remote, annoying in that the vertical wants to lock onto this black and not the actual blanking bar forcing me to use the ZOOM setting and cropping the sides or FILL which squeezes everything into the 4:3 box... fluff is reserved for my cats.

Just bought up a complete Magnavox T940 SAMS Set-1144 Folder-1 from stealbay for $14.00 that's going to get scanned in complete as a pdf if anyone wants a copy. My original copy is too far gone to work with anymore and for the money I couldn't go down to the main library and copy it for this price.

Give it a few days to arrive and I will get things together. It also has the (incorrect) schematic for the R228 receiver and the 8-function remote. I say incorrect as the Astro Sonic Imperial uses the R220 receiver so I will include both models plus the original Magnavox service pages making one large folder.

That's great. Show us some pictures of what you're doing. One thing to remember about using the RF input versus the a video input is that the video response is limited thru the IF strip. There's a sound trap at 4.5 MHz. that you would not have in a video input. Also there is some group delay thru the IF transformers. The video amplifier response is probably flat out to maybe 6 MHz. The sets I have converted look really sharp using the video input. Good luck!

I will get some pictures cropped & chopped to fit the vintage videokarma servers in the coming days as there's still some remaining work to be done on the chassis. Right now it looks like some video is making its way thru the sync sep and giving things the wiggles and I also have some horizontal shading of the highlights that need to be worked out, as of this post I have not verified which side of the video out amp this is on.

So for the moment things are going to stay on an RF distribution as this gives me the flexibility to run my other vintage sets without modifications. I have an early 80's Panasonic CT-1110D portable, a 70's vintage Philco B&W and Sony Watchman FD-40A plus one of those 16:9 CRT Panasonic CT34WX54 sets however that one normally runs on the Betacam inputs.

When I did have the Magnavox on composite the detail was a trife better overall so I will revisit the video input in the near future however it was more important to bring everything back to factory stock again both for technical and historical aspects as this set has been in the family since new. We were not the couch potatoes so much but shows like Dallas, this music thing in the late 70's and the seemingly endless football games were never missed and the TV got its use along with the stereo which was almost a constant growing up.

So another update to this one, June 17th.

Got the shading issue ironed out, turned out to be a bad new capacitor that was across the contrast control so that got replaced again and now all is well there. Other issue I'm still dealing with is horizontal sync bending that would normally be attributed to borderline AGC overload, AGC is set on the conservative side and there's a fixed bend to the right about 1/4 way down the raster from the top. Can't see it on the scope and it's there regardless of the source be it straight from a TBC or consumer grade DTV converter on CH-3 or 4.

Still fighting a vertical issue and this might be from well baked caps near the vert tube, resistors have all been replaced and a subsequent check shows them on the mark, Boost B+ is +1160 volts and supply is at +413V so those are good.

Did inject some raw composite video into the 1st video amp after lifting the 3rd IF amp tube out and got excellent results. When I replaced the IF strip I added a two lug terminal strip near the 1st video amp tube with the wire connecting directly to test point 2 so I could do the IF alignments with all the covers & shields in place, this is the junction of L14 and pin-2 of the 6LM8 video amp on the T933 schematic. I'm working with a T940 which is very similar however it uses a 6MU8 for the 1st video and calls for what looks like over 2V P-P signal on that grid. So the plan is to address the few remaining issues then add a tube based composite video amplifier to feed the test lug via DPDT relay which will also open the B+ line to the IF amp effectively isolating it, this way the DC integrity of the grid is maintained and the signal will be behind the 4.5 trap.

More to follow shortly.

What source are you using when you see bending. Three quarter and half inch machines that don't have a TBC output can cause horizontal bending. What are you using for a signal source?
I use Paint (in W7) to crop and squeeze my pictures down. Generally I have to go down to 640x480 to get them to post.
Do you think if I donate to the site, they'll let me have more space? Hmm.

The bending happens regardless of the source be it from digital or TBC analog, all of my analog tape has dedicated TBC's and everything is locked to a master BB source thru a Kramer VM-1045 distro amp.

Sources are a Sony VO2860A U-Matic that's on a NOVA-800 full frame TBC. The DigiBeta doesn't need any TBC and the Type-C Sony BVH2000/3 has the internal TBC option boards. I also source from a one of those DTV converter boxes and get the same results.

I'm seeing two types of bending, one related to the black subjects and the other bend is fixed at roughly 1/4 down from the top that doesn't move except when I push the horiz hold to the edge before lock is lost.

Going to sub in some capacitors on test probes and see how things are affected.

Having bending related to black subjects seems to indicate sync clipping at the wrong level, possibly video signal is too small peak-to-peak or at wrong DC level; or, the sync clipper itself is at fault.

That's what I'm thinking as well, going to run thru the IF alignment again and keep a close eye on the sync & burst using both the sweep/marker and actual video to see how things are interacting. I'm sure it's something I'm not doing right but can't put my finger on it just yet.

Going direct composite gives similar results however I can't get my DA to go much higher than 1.5V P-P from an unloaded output. It does however make for a very nice picture without all of the lumped bumps from the coils along the way.

You should see what my cat did to the set earlier today, it's posted here on Audiokarma... and I'm not too happy about it. https://audiokarma.org/forums/threads/any-cat-lovers-here.888946/page-437#post-18377210

It's 0200 central and I'm out of ideas. A picture can be worth a thousand words so I did my best to grab a few and crunch them down, if you want larger originals they're available. I named the files to be self explanatory, if something doesn't turn out please let me know.

So what I have is endless frustration with the horizontal and to a lesser degree vertical sync. I've run thru the IF response several times tweaking here and checking for symmetry there, replaced the AGC/Sync sep 6GH8 with several NOS tubes fresh from the cartons and checked resistor values along with probing in capacitors on top of the originals with no changes to the bending.

The signal into the 1st video amp is strong, over 2.5V P-P from the detector and I'm seeing signal levels on the various test points that are very near the spec'd levels in the schematics, horizontal lock is very strong but the vertical is dependent on the size control settings, stretch the vertical to fill the screen and it wants to lock on the black band within the picture itself unless I run a 4:3 image in which case it attempts to lock onto a something near black. Tube is new and I've sub'd in several different NOS tubes with the same results.

Power supply voltages are within 5 to 10 volts of spec and I can't pick out any ripple on the scope. AGC is set conservatively however going for overload doesn't affect the bending as much as it does the overall black and contrast as expected.

Horiz out cathode is shy of 200ma at medium brightness and the 6LQ6 tests at 90% give or take.

In the group of pictures I've included the test pattern I'm using to demonstrate running in as both composite and CH-3 RF. The scope screen shot is direct from the test point TP2 using a 10:1 100 MHz probe and the test pattern itself comes from my Newtek Tricaster TC-550 which is essentially a studio control room in a box. All the output levels conform to NTSC standards and verified using a Leader waveform monitor and vectorscope.

So what this all comes down to really is what am I missing?

Just for those who want to see what the set looks like here are a few gems to safely drool over.

Sorry if I missed it, but if you haven't yet replaced C511 in the vertical circuit, do try a .03, or .033 there.

Complete success.

Found R728 had carbon tracked from what I suspect was an overvolt being it's a carbon film subjected to some 400-plus volts during the initial power on and that particular tube warming up rather quickly. My theory anyway. What I do know is there's a clear carbon burn going from end to end and being on the bottom I didn't see it right away, a carbon comp has since replaced it and the wiggles along with the nagging vertical issue are gone.

Picture quality? Absolutely stunning for 1970 and I've known this set for the past 56 years. Remaining issues? None. A couple of loose ends with the add-on MTS decoder I use in place of the sound discriminator and a mountain of cat hair that's trapped in the cooling fan is really all that remains. What a ride this has been!

So at this stage I'm going to run it for a while and construct the Phil Nelson CT-100 triode based video input adapter that uses the 12AT7 and go from there. It's not a need so much as I have the IF working quite well but now that all the variables have been worked out I can move to the next chapter.

Give me a day or two and I will post more pictures of actual operation and some of what I feed the set with.

Again want to thank everyone for all their help and support. A special thank you to damen and Danny S. for help with the IF board and tuner. Without you I would still be at square one.

Okay so as I promised here's a short clip of the Magnavox in action with playback from my U-Matic of a tape from 1985. Contrast is a little crummy from the ambient lighting but I wanted to do all of this as in the moment at the moment. Anyone know how to repair or restore the woodgrain on that left side slider?

see for yourself.

https://youtu.be/5nUg7hVFQmo

:thmbsp:

Looks pretty good! Is that a 25 or a 23 inch CRT?

It's a 23 inch (25BHP22) and the third tube so far. First was a frosty with bad cataracts and low on emissions, second one I swapped in the summer of 83 that came from a C-II and it went another 20+ years before the emissions died off, this was around 2006 and was replaced with an RCA Hi-Lite from an unknown CTC before the set was parked in a big refrigerator box and stored in my parents barn.

It's been quite the journey but now it's back to its former glory and working like it did, most of the resistors and a number of capacitors were replaced along with the tuner after the accidental shorting of the IF cable to the 120V primary fuse. The picture is still vertically challenged but I'm going to let that go for now while I focus on the composite video input interface. One idea right now is to use a DPDT relay in conjunction with the rear audio selector switch that will cut the B+ feeding the IF amp itself and at the same time switch in the composite to that test terminal strip I soldered to the cover tin. Years ago when I added the Recoton MTS decoder I also wired in a 3P3T rotary which selects between the internal MTS and external R/L "simulcast" TV audio input so that way I could have the stereo audio from my U-Matic deck or the satellite we still had going, the 3rd contact set or position was never used so this could handle the switching of a relay as I need to keep the lead length to the 1st video amp very short to avoid smear.


I also received the SAMS 1144 folder this morning and will scan in the Magnavox as a complete set including the TV chassis, stereo receiver & amp chassis and the remote if you want to add these to your collection. Everything will be in a high-res pdf format and will overlap the large pages so things don't get cut off. Give me a few days and I will email it, if anyone else wants a copy drop me a private message and I will get it out to ya.

Here are a few of the audio details. The 45_Coil is exactly how I did it back in 1986 and it feeds a "probe" input on the Recoton FRED MTS decoder, originally they included this adhesive loop to grab some stray 4.5 MHz but that didn't work so I made the pickup instead and have it set to grab some from the 4.5 MHz trap.
Input switch selects between the MTS decoder and external R/L feeding the TV position of the receiver selector.

The hairy fan needs no explanation, the set doubles as a vacuum cleaner of cat hair.

Hi to all,
Hi ARC Tech-109,

The Phil Nelson composite video adapter for the CT-100 is here :
Original RCA design + Pete Deksnis improved version.

https://antiqueradio.org/VideoAdapterForRCACT-100Television.htm

I purchased the Pete Deksnis modified version of the original RCA design, it works well and is easy to install (on the CT-100) on other TVs, i don't know.

Best Regards
jhalphen
Paris/France

Hi to all,
Hi ARC Tech-109,

The Phil Nelson composite video adapter for the CT-100 is here :
Original RCA design + Pete Deksnis improved version.

https://antiqueradio.org/VideoAdapterForRCACT-100Television.htm

I purchased the Pete Deksnis modified version of the original RCA design, it works well and is easy to install (on the CT-100) on other TVs, i don't know.

Best Regards
jhalphen
Paris/France

A few years ago I built an improved version for a restoration customer. We wanted to still be able to use the tuner in a switchable capacity. Running the detector video through coax to a switch degraded it so I put a mini double throw relay on the socket extender so the tuner signal path lead dress would be virtually unaltered when not in composite video input mode, I added an audio injection circuit too and copied the preamp circuit from an RCA 45RPM record player for the audio line input... It worked great and I need to build a couple more for me.

That's what I had in mind, DPDT relay to feed the 1st video amp while cutting the DC to the IF amp, composite goes in after the 4.5 sound trap so I still maintain the DC integrity of the grid, did some tests just poking a 1V Hi-Z video signal in and got excellent results.

That's what I had in mind, DPDT relay to feed the 1st video amp while cutting the DC to the IF amp, composite goes in after the 4.5 sound trap so I still maintain the DC integrity of the grid, did some tests just poking a 1V Hi-Z video signal in and got excellent results.

I've found it unnecessary to cut power to the IF, it doesn't really change the opperation of the Deknis circuit on the CT-100 or 21CT55... If you build it with the socket adapter and use a clip lead to harvest the B+ that goes to one side of the rear most contrast pot next to the HV cage you can make the video mod a completely bolt on/plug in mod...
...With audio that becomes more involved. On the customer set I added a stereo headphone jack to a bracket that screws to the tuner... the jack is configured to short the detector output to the audio input (return the circuit to stock) if the plug from my preamp module is pulled out, but pass both lines to that module in isolation if the plug is in.

If life wasn't so busy I'd build a few more and sell them. I think I did a really good job with it.

The reason for doing it this way is so I can mechanically isolate the video input line from the grid of the 1st video amp when using the tuner and then cut the IF amp off when using the video input so I don't get swamped with mixer noise. Not cutting the grid out of the existing circuit as I see no need to and this way the modifications are kept to a minimum.

The reason for doing it this way is so I can mechanically isolate the video input line from the grid of the 1st video amp when using the tuner and then cut the IF amp off when using the video input so I don't get swamped with mixer noise. Not cutting the grid out of the existing circuit as I see no need to and this way the modifications are kept to a minimum.

I haven't seen mixer noise doing it the way I have. Neither circuit noticably interferes with the other when the video amp grid is switched.