Thanks to both of you for the tips and measurements. I'm not sure why, but my CRT seems to need some extra help to produce a good picture. I wonder if it has just a touch of gas in it?
I was able to make quite a bit of progress after reading your posts above, and taking some time to understand the DC restoration and brightness circuitry in this set. Bottom line is that I had to make a modification to get the brightness into a usable range. I changed the value of R282 on the Sams schematic from 1.8 K to 900 ohms (put another 1.8 K in parallel) and this brought the brightness control into a normal range (or perhaps it even goes a little too bright at the top now).
As you can see, the brightness control is at the bottom of a big voltage divider that starts with the screen controls connected to B+. The CRT cathodes are connected to the junction of the 1.8 K and 4K resistors, and were previously running about +135 volts. The brightness control couldn't bring the grids high enough. Changing the 1.8 K to 900 ohms brings the top of the brightness control closer to the cathode voltage (and lowers the cathodes to +124 V, more or less what the schematic says it should be).
This allows me to run with normal brightness and not need excessive contrast just to make the picture bright. More on that in the next post on the function of the DC restorer.
Other mods I previously made are shown here:
To coax enough red out of this CRT and to tone down the green and blue, the blue and green gain control ranges have been modified (to allow further reduction) and the green background range has been modified. This provides good grayscale from black to white, as long as the contrast is not set too high. Not sure if the green background mod is really still needed at this point (I should try removing it).