Hello Lee,
Not sure how helpful this is, but occasionally I look at the website "Amp Repair Studio", a specialist audio amplifier repair shop located in Gunma Prefecture, Japan.
http://www.amp8.com/Recently I noticed they worked on a customer's Accuphase P-4500 Power Amp, coincidentally it was about changing the AC supply voltage.
Apparently, the customer was relocating from Japan to the United States and requested his Accuphase P-4500 be changed from 100 VAC to 110 VAC operation.
http://amp8.com/tr-amp/accuphase/p4500.htmAs a rough translation of the pictures.
A1 shows the area around the power transformer.
A2 shows the voltage selection board wired for 100 VAC operation.
A3 shows the voltage selection board wired for 100 VAC operation, with the cabling detached from the multi-pin connectors.
A4 is of the back side of the voltage selection board showing the 100 VAC soldered connections.
A5 is of the back side of the voltage selection board with the 100 VAC connections desoldered.
Noted with pictures A2 and A3, the required fuse ratings are labelled on the voltage selection board, i.e., 6.3A/250V for 100-120V, 3.15A/250V for 220-240V, which would indicate this same board is used for all common voltage operations.
Being a commercial business there's no step-by-step instructions on how to do it or make the changes, likely the business intention is to show the variety of equipment models this repair shop has successfully repaired and the scope of work they can perform.
Nevertheless, it demonstrates the original board can be configured to work with different AC voltages.
Without having a circuit diagram, presume the power transformer would have two primary windings connected in parallel for 110-120V, or connected in series for 220-240V.
Typically for 100V operation the primary winding has a tap ~10% in, using ~90% of the 110-120V winding, but you would need to carefully check the primary windings yourself to be sure.
I'm guessing it's the multi-pin connectors that bring the power transformer primaries to this board, and depending which jumpers are soldered connect the desired power transformer primary windings.
As suggestion, with your current setup using the external step down transformer for 100V, very carefully measure and record the voltages at each of the Accuphase power transformer windings. Then repeat this with the secondary windings disconnected from the downstream power supply rectifier circuit. That way, when you reconfigure the voltage selector board for your desired operation you can confirm the secondary voltages are the same values as before without risking damage to the power supply rectifier circuit.
Please always be aware of and respect the high voltages present in a power amplifier. If not familiar with working on high voltages or don't feel safe, it would be best to have a qualified service shop perform this work.
Although these are just some suggestions, hope this helps.
Good luck!
Thank you,
John
