Okay time for a much delayed update on the T940 video input progress.
I won't say the set is complete by any means but it's good enough to watch. I completed the LM1881 sync separator board and have successfully interfaced it to the Magnavox. Original vertical feed resistor (R-515) was removed and this is where I feed the separator via 0.047uf 400V capacitor reduced from the 0.1Uf, the multivibrator triggers off the edge anyway so it's not super critical but too large a cap caused a retrace and foldover at the top, vertical output tube is a 6GF7 twin triode. This arrangement gets me almost 40 degrees of swing on the control before jitter and will not fully unlock going low, it also maintains a full lock with a macrovision signal.
Horizontal uses the composite sync out of the LM1881 via NPN inverter 2SC945 and feeds the original 6GH8 sync separator via emitter thru a 0.1Uf cap. The LM1881 follows the recommended layout from Texas Instruments and uses a 1-meg pot to set the pulse widths. Horizontal stability is very good and it's tolerant of timebase errors of home grade VHS sources. Everything I have is slaved to a master Videotek STG-6000 reference, playback formats are DigiBeta, BetaSP, DVCPRO Type-C and Newtek TC-550 Tricaster so timebase errors are not an issue.
Once I had the sync separator in place a few new issues came up with the chroma, color killer and 3.58 osc itself and I ended up replacing all of the 1/2 watt carbon comps on both the video and sweep boards as most were well outside of their 10% and 20% tolerance. I also cleared off the video IF board and it was repurposed for the 12AT7 input amplifier that was described earlier. Response is far more linear without the ringing I had earlier and it's passing better than 5.5 MHz of luma, still having issues with the chroma washing out on the highlights and I'm not sure what is responsible for this. I spent over 6 hours working with the bandpass and other coils for the chroma circuit but I'm still at a loss. At first it appeared to be a saturation issue with the input amp so I added 500 ohm pots to the cathodes of the 12AT7 to soften the gain but the less saturated areas seem to wash out of any color regardless of the luma. Plenty of room with the color level control but even at full drive the less saturated areas wash out to B&W. Burst amp and the related are all keyed correctly as the horizontal is timed ahead of the input amp and the pulse width is allowing the full burst thru without opening the bandpass amp late.
Other issues. Convergence has been a pain with what appears to be a harmonic of the horizontal leaking into the blue, on setup the blue has a distinct wave over the red & green which are flat and I'm at a loss for where it's coming from. Everything will converge with the controls but that wave or warp leaks into the blue horizontal line. Diodes all check good and everything responds well when adjusted.
Overall it looks good enough to watch without complaint and I've buttoned the back on for the time being, I have countless hours into replacing resistors and scrubbing off the rosin flux from the board in between the recent tornadoes we had, over a dozen trees damaged or completely uprooted plus a pine that broke the trunk and ended up upside down in the V of a nearby mature oak. It took over a gallon of gas and burned up two chains on a Stihl 024 chainsaw to slice this one up. No injuries or property damage but that pine was a priority removal.
So here are the links to the images for your entertainment, I've kept the resolution high as I don't feel like spending hours in GIMP to squeeze them down. They're large and detailed.
LM1881 sync board
https://imgur.com/QY3eEc6
Repurposed Video IF board
https://imgur.com/QXyJRES
Image in operation
https://imgur.com/NWea574
Front Perspective
https://imgur.com/PuzCXkv
I have numerous other images of the set & progress posted on imgur.com, search for images by brhatweed to see them all... I'm being lazy tonight. The link:
https://imgur.com/gallery/magnavox-t...onsole-shuTJoT