Tracing SLT card pins to components, part 2

CONNECTION TRACING STRATEGIES

Inputs usually go to a diode either in an Add-Invert type of gate or to additional diodes that are housed in AOXb (And-Or-Extender) or FDD (Four Dual Diode) modules. Some inputs are edge triggers, so these will go to a capacitor which in turn feeds a gate.

High fan-out signals will be produced by HPD and AI-3V circuits, so I will check back from these to the pins most likely to have those fanouts. 

Once the external signals are tied to specific SLT modules, I can look at the appropriate other pins of those modules and figure out where they run. 

TESTING A BIT SLOW DUE TO FLAKY BANANA PLUG JUMPER CABLE

I was having difficulty finding the signals using the continuity tester, at first thinking I had oxidation on the banana sockets of my test setup, but I realized that the jumper cable was undoubtedly a cheap piece of junk that used aluminum spiral around cloth rather than copper wire. These overseas sourced cables will often fail intermittently or open, as this one did. 

THE SLT MODULE I LABELED SS IS ACTUALLY AN FTX MODULE

FTX stands for four transistors, which is what this module contains. A single shot requires two transistors, which are picked up in many cases from an FTX module. In fact, the two 6213 boards differ in the module number used in one position, but both are FTX with the same pin assignments so it appears that it was a manufacturing convenience which one they installed.

MAIN STEPDOWN TRANSFORMER ARRIVED

As previously reported, the small capacity stepdown transformers that were installed in this system, supporting only 300VA or less than 3A at 120V, are insufficient to support the fused convenience outlets and the other 115V loads inside the machine.

Transformer T2 steps the 230V (or 208 if that is how the machine is configured) down to 115V to power the seven cooling fans, the incandescent light driver power supply, the motor in the Selectric based console printer and other 115 loads that may be connected. 

T3 powers only the convenience outlets in the processor chassis and other peripheral boxes, but the fuse installed in the primary circuit determines the maximum draw that can be asked of transformer T3. 

The first of two transformers arrived - a beefy 2,500VA unit that can provide more than 20A of 115V power. The second will arrive in a couple of days and is a 750VA unit good for a bit more than 6A of load. 

Powerful stepdown transformer - 2.5KVA