Tandy 1000 HX Technical Reference Manual page 342

5V Output Voltage Detecting Circuit
The circuit detects the change of output load current compared
with the output voltage and AC line input voltage, which feeds
back to the control circuit through a photo coupler to keep the
output voltage stable. The Photo coupler isolates the primary
and secondary circuits.
Over-Voltage Protection
When the +5 output voltage rises, between 5.8V to 6.8V, the +12V
circuit will be shorted by the Thyristor SCRl under the control
of zener diode D14, and then the supply shuts down because of the
over current protection. In the above case, correct the cause of
the failure, and reinput the power. The overvoltage protection
circuit will reset automatically under good conditions.