Checked out magnet firing board in 1132 printer

MAGNETS (SOLENOIDS) DRIVE PRINT WHEELS TO STRIKE PAPER FOR EACH COLUMN

120 spinning print wheels ride a common axle, driven by the main motor. These have 48 characters arranged around their periphery and all wheels are locked so the same character is ready for printing on all 120 columns. A magnet will pivot a print wheel forward so that it strikes the print ribbon, making an impression onto the printer paper behind the ribbon which is held in place by a platen. 

The 1132 printer generates a clock pulse when each of the 48 characters reaches the correct position to be printed, during the rotation of the wheels. The 1132 has previously fetched a string of bits from the 1130 memory that indicate any columns that contain this character and thus need to have the magnet fired. The board we are testing takes the clock pulse and delivers +48V to all the magnets. The selected magnets (columns to be printed) are pulled to ground so that only those magnets conduct. 

This circuit accepts the +Print Disk Clock pulse which is generated from a photocell that detects the positioning holes on the timing disk that rotates on the end of the axle on which all 120 print wheels are mounted. The amplified pulse is cleaned up and output as +CB Clock back to the print controller logic inside the 1130 CPU. 

In the CPU, when the +CB Clock signal falls, the printer control logic requests a cycle steal (direct memory access) to begin fetching the bits that indicate which hammer magnet(s) to fire. When the cycle steal is done fetching eight words that define all 120 columns, it raises an interrupt on level 1. Software must read the value of the next character coming up on the print wheels, which are also encoded by holes in the timing disk and read by photocells. 

I can do a rudimentary check of the condition of the six transistors and diode on the board just to validate the diode action but with them in-circuit that won't be good enough. I don't want to burn out the hammers, if the +48V stays on steady, so I feel that I have to test this further.

I will power the components through the TB2 terminals, supplying +6, -3, +48 and ground for power. I will hook up a resistor divider load across TB2-14 and TB2-18 where the scope can observe the positive voltage that would be supplied to the magnets. I will also put a scope trace on TB2-19 to observe the generated +CB Clock signal. As input, I will connect a transistor to TB2-15, set up as an open collector, driven by short pulses that should cause the input to be mostly pulled to ground with the short intervals allowing the line to be pulled up to indicate the print disk signal. 

RESULTS OF TESTING

This circuitry performed perfectly. Both the switched 48V and the 6V signal logic level output moved up and down with the input that will come from the photocell on the clock portion of the timing disk.