Could also be simple pragmatics or the economy of scale. If your business makes both 5V and 6.2V devices it could be a decent cost saving to make them both use 6.2V adapters, unless production number are low burning off that extra 1.2V in the 5V devices will generally be cheaper than separate power adapters for each and if nothing else it is one less inventory item to deal with.
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Thanks. That makes sense.
The rhythm machine takes AC at 120 and 220 through two separate figure eight female jacks. It's from India.
Then there is a transformer that outputs ~6.2v from my 120v AC (don't know what it would be from 220). It would make sense if the transformer was sourced off a shelf at the cost of a voltage regulator which there appears to be.
That is an odd layout*, is the schematic online somewhere?
Since this is older gear, it is probably a 6.3V transformer, 6.3 and 12.6 volt transformers were some of the most common and cheapest voltages before the rise of cheap switcher supplies. 6.3 and 12.6 were the optimum voltages for powering the filaments of vacuum tubes and became standard voltages in transistor gear since they were cheap and available, even today transformers in these voltages are easy to find cheaply on the surplus market.
*Edit: Nevermind, I read that as you had 120 and 220VAC transformers followed by a 6.3VAC transformer, you have one transformer with 120/220 primaries and a 6.3VAC secondary. Not odd at all.
That's a pretty typical design for power supplies. Transformers output depends on the mains input. Where I am this is nominally 230V, but in reality ranges from 235-245V. So they often use a linear regulator to obtain the required voltage, but these need to drop from a higher voltage so they choose a range that would require the least dissipation.
6.2V AC? That's 9V peak-to-peak, so after rectification it would be 9VDC less diode drops.
I had a little problem with my desktop computer (Intel I7-6700 CPU) a couple of months ago. It would start up and run OK, but eventually the file system would switch itself into the read-only state, causing it to lock up. After a few weeks of trying everything to fix it, I noticed that the slider switch on the power supply was in the 230 volt position, even though I'm sitting here in the USA and the thing has been plugged into the 115 volt mains for years. That it could just about run on half the voltage it required(?) makes me think that the rightmost digits on voltages are there for historical or marketing reasons only, like the old 101 mm cigarettes, the 81 mg aspirins, and the guitar amp that goes up to 11.
Those digits are fairly important to EEs.
Common switch mode power supplies are not picky about input voltages and generally have a wide range of operating voltages, having the switch on 230 likely just knocked 20 volts or so off the input voltage to keep the 230 below the max voltage, not half it. The reason this eventually started causing problems is probably because your wall voltage has dropped and/or components in the supply aging and drifting from spec meant it could no longer maintain good clean output which triggered a safe guard in the computer.