Curve tracer step generator repaired - next issue found

FOUND THE CAUSE OF THE STEP GENERATOR PROBLEM

A binary  counter applies voltages through a ladder of resistors to produce up to seven even steps in voltage as the 1-bit, 2-bit and 4-bit outputs of the counter turn on in the sequence 000 001 010 011 100 101 110 and 111. The patterns I saw had a step when the count would be 001, then nothing for 010, then the same single step for 011, a larger step for 100, etc. This strongly suggested that the 2-bit line was not getting to the resistor ladder. 

Examining the chip on the board carefully, I found the pin for 2-bit had a blob of solder but it did not extend down to the pad on the PCB. Thus, an open circuit. I reflowed the connections to the chip and put everything back together.

The stairstep output was perfect - varied from 0 to 7 steps as I turned the rotary 'steps' switch. The problem was resolved. 

CONTINUING CHECKOUT

Next up, the checkout had me install a 1K resistor across base and emitter connections and do some verifications to show that the polarity of the applied voltage is correct and that the offset controls perform properly, moving the start of the signal above or below ground. 

In BJT mode, the offset should range to + or - 14V as the offset control is increased until it begins to clip. In FET mode, the voltage should range up to + or - 2V as the offset control is increased. This checked out, so we were ready to move on to calibration. 

ONE ISSUE FOUND AND CORRECTED

I had no signal at all on the right device test connector, but the left worked. This was an issue with the connectivity within the connector, which required that I solder down both pairs of pins for each position although the assembly manual had suggested I only do one of each pair. That was a quick fix.

FAILURE OF THE OFFSET CIRCUITRY TO DO ANYTHING

The offset had no effect at all, either positive or negative. It is a pretty simple circuit, a potentiometer tied into an op amp that will generate the offset voltage chosen on the pot and the polarity switch. I began testing connectivity from the pot and switch back and quickly found a culprit. A resistor between the controls and the op amp did not have a good solder joint on one side. It looked just fine visually but was electrically isolated. I reflowed that joint, restoring the connection. 

That was all the time I had to play with this device today, as I had to invest about four hours of work digging into the 1053 typewriter print escapement mechanism to get it set up properly. Next visit to the shop, I can determine whether the fixed joint was the last bug in the device or not. 

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