Looking into the overly dim 3179 monitors

 3179 WITH EXCESSIVELY DIM TUBE

The first color terminal I bought, a 3179, came with the monitor and logic element but was missing a keyboard. When I power it up, I can barely see the dim line and error symbol that declares the keyboard is missing, but only at nighttime when it is dark.

There are a number of reasons this may be so dim, but the most difficult to solve is if the cathode surface is oxidized inside the CRT such that the electron emission rate is very low. However, there may be issues with the voltages on the anode and various grids of the tube, which would be far easier to repair. 

I have schematics for the 3179 so that I know the pins to check and the expected voltages for the various tube elements. The anode is charged to 25,000V, the focus grid is at a max of 8,500V, and grid 2 is at approximately 1,500V. Deflection is done magnetically. The tube actually has one heater but three independent cathodes, one for each of the primary colors. 

The existence of three different cathodes would like mean that once I can generate other colors besides the blue that is displayed sans keyboard, I may find variations in brightness of the three colors. It is unlikely that all three cathodes would oxidize at exactly the same rate, making the likelihood higher that the problem lays in the grid or anode voltages. 

EXPERIENCE WITH SECOND 3179 TUBE

My second 3179 is similarly dim, maybe a bit brighter, and after I hooked it to the logic base and provided a keyboard, I found that all three colors were equally dim. Two tubes in a row, I guess a bit unlikely that the issue is six independent cathodes oxidized but it is certainly possible.

To account for both of these having almost identical characteristics I would need to identify some component(s) that would have failed with age. The monitor is too old to have been afflicted with the 'capacitor plague', where capacitors from the turn of the century had a bad electrolyte formula and widely failed after a few years of operation. 

CHALLENGE - MEASURING EXTREMELY HIGH VOLTAGES 

I have a Vacuum Tube Voltmeter (VTVM) with a long insulated probe for checking high voltages. The meter itself, a Heathkit V7A, can measure DC voltages up to 1.5KV which is far less than the levels I must verify. 

Heathkit V7A VTVM

Fortunately, I also have the companion high voltage probe for this VTVM, which is a long wand of plastic and has internal resistance that divides the voltage being measured by 100. That means that the 25KV anode voltage will read 250V on the meter. 

High voltage probe