I have a 12V 6A PSU that was from a defunct camping cooler… Do you think using a 12 to 5V stepdown can I use that to power the Zynthian or risk the stepdown heating up and melting my 3D printed case? 
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If you use a buck converter it should be okay. This is a type of switched mode PSU that converts dc to dc, stepping down the voltage whilst stepping up the current (so you will have bags of current available - although most converters are limited in their current delivery!). It is more efficient than a traditional regulator that dissipates a lot of heat in the regulation process. The main thing to watch for is hf interference. These are SMP tech so switch the input at high frequency then filter to give the lower d.c. output. But most mains powered PSU work that way so you have the same concern anyway.
[Edit] If you look at buck converters on Amazon you will see that anything that handles more than about 3A has a heatsink so the switching transistors will dissipate more heat at higher currents.
I figured that somewhere the current must be dissipated… The heat won’t be low… better to find a 3A power supply like I put on the first Zynthian which works great.
Thank you Riban
My step down is this. It has a trimmer for voltage regulation (flying for now but then it has its seat.).
The mpc socket is still empty because I’m waiting for it
A traditional regulator turns a transistor partially on to limit the current and hence set the voltage. This results in a lot of heat when limiting higher voltage and / or current. A buck converter chops the supply turning it fully on and fully off so there is very little time when the switching transistor is partially conducting. This massively reduces the heat dissipation. There is very little energy wasted (through heat) when the transistor is fully on or fully off. This means that a switching supply (like a buck converter) can step down a voltage and pass a fairly high current with limited heat rise. The efficiency of the switch operation affects the amount of heat generated as well as the voltage drop and the current draw.
Very clear! I learned a new thing. Thank you
P.S.
In the “forgotten items” box… I found this buck converter 12/5V (when I bought the one for my Zynthian, I ordered two). I’m done with that…
One last question Riban… If I connect a 12V fan on the 12V 6A side… are the 6A too many?
I can answer that, even though my names not Riban. It will be ok - the fan will only draw whatever current it needs, so having ‘too many’ amps is not a problem, only too few. That is NOT the case for voltage!
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Many thanks… even if your name is not Riban… 
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Absolutely! Voltage (actually potential difference) is the difference between two voltage levels. Current is what flows between different voltage levels (potentials). The load defines how much current is drawn. You must ensure the voltage is correct and the supply is capable of delivering the required current. In this case you have a 12V supply that is capable of supplying up to 6A before the voltage starts to collapse (or something gets hot, goes ‘pop’ or lets the magic smoke out).
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Riban docet! 