Video inputs Tube TV

[kf4rca]

Video AGC??? Most monitors don't have that.
The best video AGC I ever saw was a TA-19 proc amp used in NTSC signal processing at TV stations.
It would make chicken salad out of chicken $#!+.

[old_tv_nut]

Video AGC shoould not be needed if you just set the gain at a fixed value for a 1 Vpp input (terminated).

[ARC Tech-109]

Input value is 1V P-P confirmed by a Leader 5860C waveform monitor and it's terminated at the input amp itself. Problem I'm chasing is a lack of sync and high whites that's throwing off the 1st video amp of the set causing what appeared to be an overload condition thus my attempt to use the AGC to stabilize the input amp. Problem I seem to be having is a lack of DC for black ref and this is killing the sync. Picture one is the clean signal coming from the Phil Nelson 12AT7 input amp feeding directly into the pentode grid of the 6MU8 1st video amp. White is clean & sharp but the black is lacking, picture two is the output of the 1st video amp at the plate, the little overshoots are products of the sharpness coil on the end of the cathode. Picture three is the 1st video amp of the Magnavox itself, this has been completly isolated from the detector & 4.5 MHz trap and has a 470K grid resistor tied to ground. The Phil Nelson DC restoration circuit I'm using has the 22K resistor and 10uF capacitor that was described in the modifications. The big question I have is: what am I doing wrong here?

1st Image:
https://i.imgur.com/3jeOfbw.jpg

2nd Image:
https://i.imgur.com/Jvlb1kX.jpg

3rd Image:
https://i.imgur.com/85tGtdD.jpg

[old_tv_nut]

The input circuit with 1N34, 470k, lytic, and voltage divider to negative supply should supply the correct DC restoration level.

You should not add a resistor from the first video amp to ground.
Instead, adjust the value of the asterisked resistor to get a reasonable grid and plate voltage similar to what you get when the set is operating from its own video detector.

[old_tv_nut]

Also check that the p-p value of the signals is correct. If not, you may have to adjust the gain of your input circuit.

[ARC Tech-109]

Would anyone here have any info on an RCA model JM-897W? I don't have a chassis number but it was used as a "professional monitor" during the 1970s

Thanks

[kf4rca]

Arc-tec, are you using AC coupling or DC coupling to go from your circuit to the TV?
That RCA model, I vaguely remember a set like that at a station in SC. Its rectangular and solid state if I recall correctly.

[ARC Tech-109]

Everything is AC coupled via 0.22uF caps from the input to the circuit, between the stages and output before the DC restoration and that is directly coupled to the grid of the 1st video amp pin-2.

[ARC Tech-109]

Someone slap me upside the head?!
I'm chasing my tail over the DC restoration totally neglecting the lost connection between the 4.5 sound trap and the input point of the 1st video amplifier... the brightness control is basically running wide open without a reference on the backside to provide the negative voltage it needs. I lifted R217 & R218 to bring the negative value back and everything fell into place.

Yes I shall humbly accept the dumb@ss of the year award and host the rubber chicken dinner.

[Alex KL-1]

Hehe, ouch, but s*&%@ happens everywhere, with anyone...

[kf4rca]

Congratulations! Yeah, I used to work on Micors and Mocom 70s converting them to 2M and 440. My Iwatsu 5702 scope was good enough for me. Why don't you post the final schematic for future reference?
Yeah, you'll like video AGC. Its like watching a movie at the theater. It's the same brightness all the time. It takes the highest value of luminance and makes it 100%. Doesn't do anything to the chroma.
You can probably build one with an optocoupler. You have to filter out the 3.58 before going to the luminance detector.
Probably some circuits on this here web.
When I was at the SC station with the TA19, the cable TV guys always said we had the best looking picture on the air.
And we always had beer in the transmitter building refrigerator.

[ARC Tech-109]

I will and the mods needed for the T-940 to make things work happily together... once I figure out that secret formula. As it is now there's still some instability in the overall black reference but I'm confidant this will be overcome, I'm going to add the LM-1881 sync sep to the mix for a number of reasons one is to get a cleaner sharper timing signal for overall stability and the other is to use the composite sync output to trigger a black reference gate, this has turned into quite the project.

The biggest gain was changing out the existing 150 ohm cathode resistors on the input amp to 200 ohm pots in lieu of padding the grids, was able to vary the gain without saturating. Far too much drive from the input amp going into the 1st video of the 6MU8.
I will post more as time allows.

[Electronic M]

Originally Posted by ARC Tech-109

Someone slap me upside the head?!

Yes I shall humbly accept the dumb@ss of the year award and host the rubber chicken dinner.

Don't feel too bad. At least you didn't break anything...a couple of weeks ago I was quickly restoring a Sylvania Halolite so I could sell a second place holder model Halolite. I got it working great, then decided to transplant into it the remote control parts from the place holder. In the course of doing so I lost the series heaters, tried a bunch of stuff, including bypassing the heater thermistor (which made a new lytic pop and vent), gave up for the night then next day discovered that I connected the ground strap that connects the 2 chassis to the one large piece of metal that's isolated from the chassis...A connection point swap and a new lytic later and I was back in business but I sure felt stupid... Especially in light of observing that part was isolated about an hour before I inadvertently clipped to it...

[ARC Tech-109]

So some progress has been made on the T-940, I cleared off the remains of the video IF strip back to the 1st video amp leaving a single 9-pin socket behind and built the input amp on the bottom of the board. the result is far less ringing and slightly better HF response, I also replaced numerous resistors on the video/chroma board as many were outside their tolerance. A 680 ohm measured 44 ohms and a 330K measured over 500K.

Going to AxMan surplus today for a handful of 7812's and other pieces to make the LM1881 sync separator, if anyone is familiar with the cities surplus scene they know what it's all about, rows of parts much of them NOS going back to the 1960's plus hundreds of new vacuum tubes for $2.50 each. They've been around since the 1960's and have most everything to support the older electronics.

[kf4rca]

Whoa! I found 2 video AGC chips you might be interested in. They are the MAX7452 and the AN3916. Never used either of these chips so I don't know much about them. The Maxim chip is available only in an SO-EP package- which, I think, is quite small. The AN3916 is a sip package. Both operate off +5. Both could be used for future video input mods for analog TV's. The AN3916 has an H OUT pin.
Maxim used to be quite liberal for free samples but they are now owned by Analog Devices who might be anal about it. I saw both chips on Ebay.
An interesting PDF: https://www.analog.com/media/en/tech...tes/an57fa.pdf