More power?

The MIDI interface needs a power supply between 8 and 35 volts. The supply current needs to be 15 mA for the interface itself plus whatever is drawn by the output loads.

The loads are connected to a common positive supply switched to ground by the darlington pair drivers.

If the driver voltage is 15V I'd need the 24V rated Coto reed relay which has a coil resistance of 2KOhm. That would be a current draw of (15 / 2000) 7.5 mA each. Assuming worst case of all 49 relays being on at once that would be approx 368 mA.

That's worst case but the question is this: Can the existing SS30 PSU be used to power the interface?

The SS30 PSU has a +15V rail (this is all carefully worked out, you understand) which is rated up to 500 mA. 

What's not clear, is how much of that 500mA is used by the SS30. I know that the +15V rail is not used for the K boards (-7V) or the G boards (all voltages are derived form the -26V rail) but it is used on the LF, OR and F boards. If the interface took the best part of 400mA (absolute maximum) is it likely the SS30 is using under 100mA?

I'm going to have to get the whole thing working and then measure the current on that rail before I can decide whether I need another power supply.