STATUS: Everything working (Nov 12, 2015). All in all a pretty easy mod – but always be careful with high voltage stuff! There are some unused holes I need to plug, bu that is purely cosmetic.
The original transformer in my Pro-One hummed a LOT (I mean physically, not from the Audio jack).
I replaced it with a toroid, which is supposedly better for audio applications. I also ditched the 110/220 voltage selector, and installed a standard IEC power cable chassis connector so I could ditch the annoying and potentially dangerous permanent power cord.
(It seems someone has already done some work on it, the original fuse holder is not connected and there is a new additional one mounted).
Step 2 – mounting a standard IEC power connector
I modified the unused square hole from the old fuse holder.
Haven’t decided how to cover the remaining unused holes yet.
All soldering done and heat shrink tubing in place.
The “middle tap” from the transformer should supposedly be connected to ground somewhere on the circuit board and NOT the metal case to avoid ground loops.
I measured the voltage once more to be sure everything was ok, and it seemed fine: around 20V on each pin against the middle tap/ground and 40 V between the two pins in the white connector. (The transformer is a 2*18V 30 VA model)
Hooked it up to the main PCB and fired it up… sweet music – no hum!
Transformer size: I was curious about the data for the original transformer and found that the company still exists. I emailed them and got a data sheet back in less than an hour – amazing! The transformer model is DP-241-7-36, and it is a 56 VA model. That’s a lot of juice.. but does the Pro-One need it?
No. Syntaur is selling Pro-One replacement transformers that are only 12 VA! I guess Sequential got a good deal on these bigger ones and just used them.Considering that the first 1500 Pro-Ones had a small transformer mounted on the PCB, there is no way that was a 56 VA model. The voltage regulators on the PCB also have very little heat sinking, and a seasoned tech told me the currents were then probably close to 200 mA or so. My 30 VA model can deliver 830 mA. So even that is bigger than needed.
Nov 16, 2015: Wrong fuse? My Pro-One didn’t have the original fuse holder, a new one had been added. There was also a sticker that said ‘250 mA’. Looking at the Pro-One schematics, it says “1/4A” (250 mA) when running it at 110V and “1/8A” (125 mA) when running at 220V. So I think the person who fixed this before missed that info.. Will change it to a 125 mA ASAP.