Checking out my 3178 terminal components

INVENTORY AFTER DIGGING OUT ALL MY TERMINAL PARTS

Using IBM nomenclature, the 3178 C terminal consists of a monitor element, a logic element and a keyboard element. The logic element is what I have referred to as a controller box. 

I have two logic elements, thus can have a maximum of two working terminals. I have three monitor elements and three keyboards.

Two of the keyboards are 75 key units, marked C1 underneath, while the remaining keyboard is marked C3 and has 87 keys. 

TESTING LOGIC ELEMENTS

Both logic elements power up and appear to work fine, given that I don't have a coax connected to a control unit and thus see only local functions. It goes into Test mode and paints characters across the entire screen. In Normal mode I have the horizontal green line between lines 24 and 25, with all nulls visible in the remaining locations.

TESTING MONITORS

One monitor is intact and provides adequate but not strong brightness. The second monitor is intact but has a quite dim level, only suitable for a darkened room. The third monitor has a hole punched in the top of the case and when operated has a decently bright horizontal line right across the middle at about line 12 position. 

TESTING KEYBOARDS

All three keyboards seem to operate, as I can type and see some characters show up on the screen when the terminal is in TEST mode. In Normal mode, nothing happens since the terminal does not see the control unit on the other end of the coax. 

The behavior is the same for all three keyboards and across both logic elements. Pressing the alpha character keys yields the capital letter on screen that matches that keycap. However, the special character keys above the alpha top row (QWERTY etc) show up as decimal digits rather than the value on the keycap.

Further, the cursor arrow keys don't move the cursor, they just put certain special characters on screen. Shifting to upper and lower case on the keyboard still generates the same capital alpha letter. 

I don't know if this is the behavior that would occur on a fully functional 3178 while it had no coax connection and was in test mode, or if this signifies some mismatch between the keyboard elements I have and the logic elements. I won't really know until I have the 3174 control unit project cabled up and active with the terminals in Normal mode. 

INVESTIGATING THE BROKEN MONITOR

I opened the monitor element that had a broken top and no apparent vertical deflection. Right away I astutely noticed the electrolytic capacitor laying inside, disconnected from any PCB. The base had oozing electrolyte. 

I examined the PCB inside the monitor and see the spot where this capacitor (C312) had corroded off the board. It is in the Vertical section of the board, quite consistent with the behavior I observed. 

I intend to buy a replacement cap and install it, after cleaning the corrosion off the PCB. If the traces and connectivity check out okay, I will power up and see if the vertical scan resumes operation. At worst case, I have a fully functional PCB on the other monitor, the one that is excessively dim. I also have a good plastic top that I can swap onto this unit. Therefore I expect I can restore this to working order and have two sufficiently bright, working 3178 terminals.