Adjusting the repaired 3178 terminal monitor - now have three viewable monochrome monitors

 SCREEN FACE COATED WITH GUNK

The glass covering of the CRT face was dirty, with a layer of grime that made the image even worse. I used soap, water and alcohol wipes until I had it clean. That contributed to a better image but still not bright enough.

OBSERVED THAT VERTICAL RANGE WAS NOT ADJUSTED PROPERLY

In the images from the last post you can see that the height of the image is compressed, the top line should be higher on the tube face and the bottom status line closer to the bottom edge. That can contribute to character legibility problems.

I found the vertical height adjustment potentiometer and rotated it until the image was properly sized. I was pleased with the results.

WORK ON FOCUS ISSUE

There are two potentiometers on the board that adjust the focus - a larger main focus pot and a small 'dynamic focus' trimmer. Rotating the main pot to either end had very little effect on the image, which led me to check the resistance readings of this pot. It seems fine and the image is marginally better at one extreme. 

The dynamic focus circuit compensates for differences in beam distance at edges versus the center; since I don't see any discernable difference in focus across the tube face, I declare this pot to be set correctly. 

There is a potentiometer labeled Sub-Bright that is in line with the brightness control and seems to limit current flow. I cranked this fully out of circuit and saw a bit of green glow across the entire screen, but with just a tiny rotation back it gave me acceptably bright images. 

If I had a schematic with voltages noted, I could check the correctness of all the HV for the various grids and anode. Since I don't, all I know is that a sticker declares the anode to be at 14KV. I could at least check that voltage. If the anode is too low it would affect tube brightness and potentially focus. 

Some internet posts suggested that this monitor is identical to the IBM 5151 used with the original 5150 PC. The circuit is similar but the parts numbers differ enough that it can't be used directly. It does show me the voltages to expect at various points related to the CRT pins, which is enough for me to do some voltage tests.  

I checked the ends of the Focus pot and saw about 600V on one end and about 100V on the other - this is reasonable given the types of focus voltage that a CRT like this would receive. If it were easy I would test the anode voltage, given as 13.2KV in the 5151 schematic and 14K listed inside the monitor, but the anode connection has a rubber cap that I can't slip the probe underneath. 

Methods exist to deal with the cap, but since the adjustment of the Sub Bright pot, the display is the equal of my best 3178. I applied the same adjustment to the remaining 3178, thus I have three monitors at about the same (acceptable) brightness. 

ALMOST THREE COMPLETE WORKING 3178 TERMINALS

I have three keyboards and three monitors, but only two logic elements (the base that connects the other parts and the 3174 controller.) If I ever needed to complete the third, logic elements can be had for about $50 including shipping. At the present, my supply of two fully functional 3178 and the soon to be working 3179 exceeds my known requirements. I don't feel the need to buy another keyboard for my second 3179 nor a logic element for the third 3178.