Curve tracer offset repaired and beginning calibration

FOUND THE ISSUE WITH THE OFFSET OPERATION

After tracing the circuit on the back side which was readily accessible, I found a short across capacitor C52, but removal of that capacitor did not relieve the short. The schematic shows a resistor in parallel but mounted on the front side of the board which meant disassembly of the sandwich to gain access.

I lifted the resistor and still did not clear the short. At that point, I referred to the additional PCB I have to look for potential shorts since this signal just runs to the output of the TL071 op-amp that produces the negative or positive bias. 

I spotted a via for our signal that was just to the side of an unrelated capacitor and the solder on that component was shorting to the via. Usually vias that are possible to short are part of the same net, but in this case it was our op-amp output net instead. I removed the short, reassembled the sandwich, put all the cables on the sandwich and retested. 

Via and adjacent capacitor

The circuit behaved exactly as it should. It moved the voltage of the base steps in a positive or negative offset over a decent range. Then, in FET mode, it changed the voltage only within the +/- 2V range designed into the circuit. Problem fixed, checkout complete and onward to the calibration. 

CALIBRATION BEGUN

Calibration involves adjusting trimmer potentiometers to set various behaviors, as well as some testing of the limiters and fault detectors. 

The first adjustment was for the 10V reference voltage. Rotating the pot to its extreme only brought the voltage down to 10.2V but our target is as close to exactly 10.00V as we can achieve. The manual mentions paralleling another resistor on R12 if the adjustment doesn't reach the target. I added the same value, to halve the resistance, but the result was no different. Time to study the circuit.

FLAW IN DESIGN MAKES IT TOUGH TO HIT TARGET WITH THE SPECIFIED RESISTORS

This circuit employs an LM317 regulator whose output voltage is set by a resistor divider. The adjustment terminal should be set to a voltage 1.25V less than the output target, thus 8.75V through the action of the divider. 

The divider has a 127 ohm resistor between output and adjustment terminals, then an 887 ohm resistor and the potentiometer below that going to ground. 

The potentiometer is a 100 ohm trimmer with a 100 ohm resistor in parallel, with the tap of the pot connected to ground. Thus, the maximum resistance is 50 ohms in one direction and 0 ohms in the other. Since I was already at the 0 ohms position, changing the resistor in parallel had zero effect. 

With the potentiometer adjusted to zero ohms, the divider takes the 10V output to 887/1014 of 10V or 8.748 V. With the pot rotated to the other direction is sets the adjustment terminal at 937/1064 or 8.8V. Theoretically the circuit should just get down to 10V (9.88V to 10.05V) but it appears that resistance variations put us over the top. 

I will measure the actual in circuit resistances and decide what changes to apply to cause the range of adjustment to overlap our target. Once this is done and the voltage adjusted very precisely I can continue with the rest of the calibration process. 

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