SONY D-50 power supply mod (also applies to AC930A)

Given the common problem with D-50s having burnt DC-DC converters I decided to do a mod on the power supply of my unit.
As with the AC930A adapters used in later Discmans, these linear unregulated supplies output a voltage much higher than the 9V required by the unit.
I measured as high as 13.4V when unit was OFF and as high as 10.4V when unit was in play with headphones connected.
The higher the difference between input and output voltage of the convertor, the higher the maximum inductor current will be.
Given the DC-DC is already enclosed in a can with no ventilation for shielding purposes, these inductors inside can get pretty hot.
If they get hot enough, the enamel from th windings will break and create short-circuits between the windings.
More current through the inductors also means more current through the switching transistors, which are not on heatsinks.

The mod is very simple requiring only 3 components: a voltage regulator, a heatsink and a capacitor.
The regulator I chose is LM2940-9 because it has a dropout of only 500mV at the maximum current of 1A.
So even if the situation where the mains voltage goes down, the regulator will still be able to regulate, as at 400mA (which is close to the operating current of 440mA) the dropout is only ~220mV.

NOTE: This regulator requires an output capacitor of at least 22μF for stability.
Despite the device already has a 330μF inside in parallel with the input, you still need the 22μF one as the internal one is too far away from the regulator connected with wires/traces that have parasitic inductance.

NOTE2: The popular LM7809 will not work as it will require a minimum voltage drop of 2V, so even in a best case scenario there is not enough input voltage under load to make it work.

Heatsink I used is ATS-PCB1073, which despite not very large maintains the regulator at a reasonable temperature (well within the datasheet spec).
I recommend doing this mod on your D-50 supply or AC930A for peace of mind. I did it on my 2 x AC930A as well that I use on my D-150 and D-250 Discmans so I don't worry anymore about leaving them plugged in.

 

So this could/should be done to the MZ-1 adapter?

 

This can be done to any linear supply which does not have a regulator (and most do not), because all these supplies will have a significantly higher voltage when there is no load.
Even with load, the tolerance of the output voltage is pretty high since since all the components are calculated for a worst case scenario: minimum mains voltage, maximum rated load.

Now, not all devices will necessarily have a problem with a voltage that's a bit high, so for the "should" part the individual device must be analyzed to asses the necessity of such a mod.
And of course the regulator must be dimensioned so it has at least double the current capability compared to the actual current being drawn (the maximum figure).
Depending on scenario, a different regulator might be needed or an external transistor added to increase its current capability.

As for the MZ-1, does it use the AC-MZ1 supply (10.5V @ 1A) ? For that an adjustable regulator like LM317 is needed as there is no fixed voltage one to output 10.5V that I'm aware of.
I can provide the details necessary for that mod since this thread is meant to be more generic in regard to those linear supplies.