Interim method won't work; assembling new PCB

FASTER 2N2369A TRANSISTORS TRIED

The same setup with the square wave generator was used with the faster 2N2369A transistor and I did see a substantial improvement, with the pair of transistors operating to about 4.4MHz although the waveform was very distorted. The first transistor signal was better than the second, where irregularities were multiplied. 

The circuit on the new PCB has a single transistor driven by an AND gate but the interim method has a NAND gate on the old board so I needed to invert its output with a first transistor before driving the second one. The other issue is that I need the special board to intercept the signals on the ribbon cable, wires tacked on and a breadboard to host the transistors. 

Turns out that the capacitance and other consequences of the all the above will cut down on the max frequency I could handle. The new PCB with its short controlled traces will undoubtedly operate faster - hopefully up to the 10-12MHz rate of the memory system. 

BUILDING NEW PCB IN STAGES

I received the new PCBs, the stencil, and the parts while I was on a roadtrip to pick up a 3179 color terminal in Atlanta. I decided to assemble only the components I need to determine if my approach will stop the spurious retriggering issues. Since this occurs when many bits of a word have a 1 value, causing many of the sense output lines to be pulsed at the same time, I prepared the PCB to produce that situation when the 1130 attempts a read. 

I had to install the connector pins, the power regulator, the five AND gates, the 18 fast transistors, the two timer chips and the logic IC, plus resistors and capacitors. These will produce the timer pulse when the +Storage Read signal has a rising edge, causing the transistors to pull the output line down to ground for any bit position where the input appears to be a logic high (1). 

I am hoping that the AND gate will treat the floating input pins as a logic high, thus generating an output pulse when the timer pulse occurs. The previous NAND chips acted that way. If it does. then the partially assembled board will emit a pulse on all eighteen output pins, the maximum stressing condition for retriggering. 

The IBM 1130 can be connected to the PCB by the three ribbon cables. When the 1130 issues a read (+Storage Read goes high) and +Storage Use is high (floating pin is treated as a high), the board will produce 80-100 nanosecond pulses about 800 ns after the rising edge of the read request. 

BIG FAN OF USING A STENCIL

The stencil is a thin bit of aluminum with holes cut into it in the shape of every solder pad. It is held on the PCB and solder paste is forced into the holes with a putty spreader. The stencil is pulled up, the components are placed on the board and hot air is used to melt the solder and bond the pins to the pads. 

I used tin shears to but the stencil, removing each section as I soldered down components. Otherwise the previously installed parts would block the stencil from laying flat on the PCB. This means I can't use it for additional PCBs - a one time use - but I will happily order another once this board is fully tested and worth duplicating.