Quoted from wpmcnamara:

In reading through this thread, it's obvious there are several different causes of hum and noise. Noise especially. However, as has been noted, there appears to be a fundamental problem with Data East machines and hum in the audio. This is 120Hz hum and will show up in the audio output when the audio board is completely disconnected from anything but power. My Secret Service suffered from this hum.
My machine has an XPin wms8345 power board in place of the original Data East power board. This will actually exacerbate the hum (makes it louder), though I would fully expect it to be there at a lower level with the original Data East power board.
My problem, and a fundamental issue with the Data East power supply board, exacerbated by the design of the XPin replacement is ripple on the +12V line.
The +12V supply comes directly from the bridge rectifier, and the only conditioning is a single bulk filter cap. The Data East schematics specify this as 0.18F (yes, Farad). All the pictures on original power boards I have seen have 18,000uF (0.018F), or 1/10th the specified capacitance. The XPin board has 1000uf (0.001F), or about 1/200th of the specified capacitance.
The +12V supply goes two places. It feeds the +5V regulator, which powers all the logic. It also goes to the audio board, where it goes through an inductor to filter high frequency noise, though a couple of small bulk filter caps, and directly into the audio output amplifiers. Because of the large load of the derived +5V supply, there is very significant ripple on the +12V supply. On my machine, with an XPin supply, the ripple is about 400mV.
Note that the -12V supply also has significant ripple, something around 200mv. However, it feeds the -5V regulator on the audio board and itself used directly anywhere. The -5V regulator does its job and there is no ripple on its output.
So, my solution? I bought an adjustable buck/boost DC/DC converted and wired it into the +12V feed to the audio board. I adjusted it for 12V output before powering it it. The result is night and day.
The yellow trace is the ripple on the +12V from the power supply board. The red is the output of the DC/DC converter. In both cases, the scope inputs were capacitively coupled, so all you are seeing is the ripple/noise. The hum is completely gone from the audio output.
Note this only fixes hum. It doesn't fix the "hiss" from the amplifier design. It achieves the same thing that putting in a separate power supply for the audio board, in a slightly more targeted fashion, addressing just the problem power feed.[quoted image][quoted image]

Quoted from NYP:

Been about a year, got rid of the hum, holding up fine. Small enough to hide in the backbox, no complaints.