Okay. I have done a number of different things.
I checked the cathodes, and they have a +50VDC offset. Not entirely sure why, but I think it might have something to do with the multisync nature of the monitor in that it operates the cathodes at a floating voltage. Perhaps it has something to do with the way that it overlays the video on top of the raster. Not sure, it's a little above my head at the moment.
But for whatever reason, it does float the voltage and it relies on the G2 to G1 to raise the pedestal of the image to the right level.
That means I could potentially push the whole voltage level down a bit and get more headroom.
I also bought a slightly higher bandwidth version of the video amp IC (it can do 100MHz vs 70MHz), and tried it out.
There were a couple takeaways from that. First, I had to install a green drive pot to match the red and blue because the new video amp has slightly different gain characteristics that don't work well with the existing circuit. I just matched the values of the other two and that seems to be okay.
Secondly, I started messing with the 33 ohm resistors on the TE pins of the video amp inputs. I understand that this is a HF compensation method and that it will alter how fast the video amp can function. However, changing these values also seems to have an effect on the clipping of the video amps.
You can see them on the right side of the video amp IC:
Raising the values of the red and blue channels, I am able to get more total brightness out of the raster without clipping from the video amp. I think there's something besides the total swing of the video signal on the amp that is contributing to the clipping. It seems like the TE pins are effecting the biasing of the IC as well.
The internal schematic of the Video Amp IC:
Unfortunately, the new amp doesn't seem to play nicely with the current circuit in other ways as it seems to have a different DC offset, and it ends up too dim when I adjust the G2 for proper black levels. Basically, I think it ends up with the pedestal too high up, and then the G2 has to be lowered a lot to compensate, which lowers the G1 G2 relationship and the overall luminance goes down.
I think after that, I realized something interesting the service manual. The color balance procedure has you max out the bias controls to start, and then to pull them down to balance. That means that you are effectively pulling the pedestal all the way down and then selectively pulling guns up to reduce their effect on the background. This IIRC is not how it's done in other monitors and TVs I've worked on; typically, they have you start in the middle or at the bottom and then work your way up. I have to assume that they are considering zero volts as the brightest possible bright, and this monitor expects that to be between 45-50V. Black levels are supposed to be about 90V and it's supposed to swing around 45V in the middle.
In any case, I'd love to figure out how to redesign the bias network to use the VPS10 (100MHz part) in this monitor. From my testing, it was much, much crisper of an image (at a similar level of brightness to the old video amp), even though it turned out dimmer with the present circuit.
This is the bias network circuit if anyone has any suggestions. I guess I would assume I would have to mess with these resistors in the voltage divider for the 180V to the bias controls. The new video amp looks like it maxes out at like 106V instead of 90V or something, so I'd need to lower the DC offset of the video signal by about that much. I'd still have the bias controls to do the fine tune, but it's apparently too far out.
