The PSU holds both the +5V and +12v supplies as well as the RF modulator (Channel 36) with a cable that carries the power to the Coupé and a composite video and monaural audio back to the PSU for the modulator. A flying RF cable connects back to the television from the PSU.
The PSU is rated as follows:
5V DC 2A
12V DC 100mA
The 12v is only required internally by the PAL/NTSC video encoder so if you use an RGB Scart lead with RGB there is no need to provide a 12V supply at all to the Sam.
With the connector to the Sam carrying the 5V and 12V down and the composite video and mono audio back up to the modulator this was a recipe for a lot of interference on the RF signal. One fix was to allegedly remove the modulator and box it up separate from the PSU however more popular was to just use RGB/Composite via the RGB Scart.
One issue is that the 5V is a little anemic if trying to internally power a had disc with the Atom.
The External PSU was an Amstrad CPC MP2 http://www.cpcwiki.eu/index.php/AmstradMP1/MP2modulator but sprayed (not very well) white.
The Amstrad CPC’s were normally powered from their dedicated monitors which meant that the MP1/MP2 units were an after-sales unit and according to Bob Brenchley when Bruce Gordon was looking for a PSU for the Sam one supplier had a lot of these lying around in a warehouse unsold and so were snapped up by MGT.
There were modifications made by a 3rd Party company called Hi-Tech, according to Nev Young these were to fit the MGT-made modulator board and remove a resistor from the 5V regulator to boost the output.
Also, because the Sam takes so little from the 12V supply for the Video chip they also tend to burn out the 12V zener diode as it had to sink too much of the excess current meaning the 12V series resistor often had to be changed for a higher value.
Apart from that there seems to be little difference in the power board from the Amstrad layout but apparently there was still a high failure rate of the units.