Assuming a CRT monitor:
If the whole picture "expands" and "contracts", then you should suspect the EHT circuitry in the monitor.
If the EHT (normally about 25,000 volts) goes low, the electron beam spends longer in the deflection field as it travels slower, and gets deflected further. The result is an enlarged display area.
Not a "user serviceable" issue unless you have extra-high-voltage (EHT) test gear and are experienced.
If the whole picture "expands" and "contracts", then you should suspect the EHT circuitry in the monitor.
If the EHT (normally about 25,000 volts) goes low, the electron beam spends longer in the deflection field as it travels slower, and gets deflected further. The result is an enlarged display area.
Not a "user serviceable" issue unless you have extra-high-voltage (EHT) test gear and are experienced.