Below is a shot from the block diagram of the video processing board taken from the HTM-1550R/2050R service manual. This is the board to which RGB/YPbPr and EXT sync are connected, and it handles things like contrast/brightness adjusting for all sources before passing processed signals to the RGB amp board.
Spoiler
Outlined in the service manual is a procedure for calibrating this video processing board, and the most critical step in it is where you do fine-tuning so that YPbPr converted from RGB winds up the same as an equivalent native YPbPr signal. Here is that step from the service manual:
Spoiler
1. The correct test points for this first step are actually different from what the manual states. It must be a typo or something. You literally cannot use TP101, TP301 and TP501 for what it says to do. Instead, you should use TP103, TP303 and TP503.
2. All three of my boards, which came from different sources, were badly screwing up the RGB -> YPbPr conversion. Could it have something to do with the error in the service manual? I don't know. Regardless, improper conversion basically means that your colors wind up being out of wack. With the buttons on the monitor's front panel that isolate the RGB guns and the 240p Test Suite's Color Bar screen, you could see colors appearing in places where they shouldn't, and also being out of proportion with white.
3. Other test-points related to things like DC offsets tended not to show so much trouble, although they did often show some. Across the three boards I calibrated, I probably wound up needing to make adjustments on around 40% of the total test-point/potentiometer pairs outlined in the procedure.
4. While the buttons in the drawer don't give you a way to adjust red gain, there is a potentiometer for it on the video processing board. Cranking that up was quite the game-changer in terms of the total amount of contrast I could get from my monitors.
I would strongly recommend to anybody with an Ikegami HTM monitor that you go through the calibration procedure. The difference it made for me was quite dramatic.
In order to do it, you'll need a few things:
1. An oscilloscope with at least several MHz bandwidth
2. A signal source that can output the EBU color bars in both RGB and YPbPr. I used a Shibasoku TG19CC signal generator, but I think a PS2 with DVD playback in RGB unlocked would work fine.
3. Small hook leads to attach to the test points on the board, since typical oscilloscope probes will be too big to fit.
PM me if you want the service manual. As long as you have all the equipment, there is really nothing difficult about the calibration procedure. (Just make sure the cable that connects the video processing board to the RGB amp is unplugged and out of the way every time you pull the board out!)