The purpose of voltage conversion from 100v to 120v is to give the amplifier circuitry correct operation voltages.
https://1.bp.blogspot.com/-Yn62ubWAJB8/XZBPQoN1Z8I/AAAAAAADXY8/Fa5bof2bzGs7ECGh9lAp1qj-ddhNSw6dwCLcB...
To measure voltages across large filter cap, use a DMM set to DC voltage measure voltage on 60000uF capacitor (RED probe on red wire connection, BLACK probe on the aluminum plate connection), you should get same or very close voltage reading by power up the A-48 with 100V mains voltage before conversion and power up the A-48 with 120V mains voltage after conversion.
BTW, I strongly recommend to use a Dim Bulb Tester to power up the amp at the first time after voltage conversion.
If anything goes wrong the light bulb will simply light up instead of dimming and save the amplifier from any damage.
Although the Dim Bulb Tester is a low tech simple circuit connecting an incandescent light bulb in series from the mains to the amplifier, it is an amazing device works as a current limiter. Every one should have one in their tool box!
https://1.bp.blogspot.com/-Yn62ubWAJB8/XZBPQoN1Z8I/AAAAAAADXY8/Fa5bof2bzGs7ECGh9lAp1qj-ddhNSw6dwCLcB...
To measure voltages across large filter cap, use a DMM set to DC voltage measure voltage on 60000uF capacitor (RED probe on red wire connection, BLACK probe on the aluminum plate connection), you should get same or very close voltage reading by power up the A-48 with 100V mains voltage before conversion and power up the A-48 with 120V mains voltage after conversion.
BTW, I strongly recommend to use a Dim Bulb Tester to power up the amp at the first time after voltage conversion.
If anything goes wrong the light bulb will simply light up instead of dimming and save the amplifier from any damage.
Although the Dim Bulb Tester is a low tech simple circuit connecting an incandescent light bulb in series from the mains to the amplifier, it is an amazing device works as a current limiter. Every one should have one in their tool box!