Adding Composite Input (Capacitor Question)
 

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.

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.

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 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.

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?

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.