Hi,
A while ago I began to look at a bench power supply. It has a selection of faults, and the first I think I should tackle is the voltage regulator in the power supply for the internal amplifiers etc of the unit.
The supply in question consists of a current source (transformer secondary) a full wave rectifier (4 silicon diodes), and thence two regulators in a kind of mirror image arrangement. One regulator is a LM317, the other a LM320. The output should then be +15 volts from the LM317, and -15V from the LM320 about a common 0V ground.
The usual divider networks are used to set the output voltages of the regulators, and in this case for the LM320 includes a 820R fixed value resistor and a 1K trimpot. This network *should* be able to be set so -15VDC is output from the regulator, but the 1K trimpot can't seem to quite make it happen. The highest it can achieve is -17.97V.
My arithmetic tells me that if the trimpot is set fairly close to centre, then the regulator ought to be outputting close to -15 Volts so something drifted significantly. The thing is, I can't seem to be able to determine what. The resistors in the circuit all measure well within tolerance. I have tried swapping the regulator for another. Input voltages are within acceptable ranges.
Attached is a schematic of the circuit. Appended are expected voltages, and actual in (brackets).
Suggestions for why the voltage cannot be brought up to -15 Volts would be welcome.