Can you see how the part is "named" according to silkscreen marking on the board?
Also: do you have a voltmeter? If so: find a 0V (Ground) point, and measure voltage on each of the pins of that part. Results may give us some clues what we're looking at, for example whether it's a plain 78xx 7805 regulator, or a power transistor.
No problem, will do that this weekend. 
@tvalenca
Let me just add more details here and correct some info, because otherwise people might damage their machines.
- Toshiba's MSX models have some of the worst power supplies I have yet seen on MSX computers.
- Toshiba's Japanese models can only be run at 100V and that's it. Do not try to run them at 110V, and even less at 127V. It can both melt the transformer or literally toast the power supply PCB.
- The heatsinks are way too small, or even missing.
- Because of that the power supply heats up a lot. It's even worse when you plug extensions. And even worser when you plug modern FPGA/CPLD based cartridges.
- For those who want to preserve their beloved machines better and minimize the chance of damage, I published some tips on the Brazilian mailing list that I'll prescribe here too
So, here are my tips to preserve your Toshiba MSX machine and avoid to damage it. But I don't take any responsibilities for such changes. Try them at your own risk:
1) For those who want to keep the machine original, without any mods
- Only use it at the native specified voltage. That is: 100V for Japanese models, and 220V for European models. Do not go over that, or it can melt the transformer and toast the power supply PCB. The MSX2 models have the power supply circuit integrated to the main PCB, so this mean that the machine will be kaput.
- Do not use slot-expanders that don't have their own power supply
- Do not use combo cartridges with an internal slot expander
- Do not use cartridges that have a too high power consumption, like OPL4, V9990, Franky, Playsonic etc
IOW, if you want to keep the machine all original, maybe it's better to just place it in a glass box.
2) For those who want to fix the problem and be able to use the machine without so many restrictions
- Add heatsinks to all regulators that don't have them
- Screw a 4x4cm 12V fan over the biggest original heatsink of the power supply. This will be the +5V regulator. Try to position the fan in a way that the air flow also goes to the other regulators around. There are easy points in the PCB where you can solder the wires for the fan.
(I.e.: In the Japanese HX-22, I had to fix it with only one screw, rotated in 45 degrees in the Z axis. In the HX-23F I had to fix the fan with 2 screws, leaving the "south" part of the fan suspended over the other heatsinks)
- Swap the old Z80A NMOS for a Z80A CMOS, like the Zilog Z84C0004 or the Toshiba TMPZ84C00AP
With these mods you'll be able to use modern cartridges and even slot expanders without restrictions, but will still be restricted to the specified voltage for what it was built, namely 100V or 220V. *Do not use them over these original voltages*. I can't stress this enough.
For 220V machines, the mods explained until this point are sufficient.
3) for those who want to be able to use a Toshiba 100V MSX machine on 110V or 127V
- The transformer *must* to be rewinded. Find a good professional/company who's specialized in such work, and give them the voltage specs you want them to adjust the transformer too. Maybe it's better to ask them to rewind the transformer to 127V, so it'll have a bit of margin in case of any unexpected oscillations in the mains.
- AFAIK, it's not possible to replace the transformer with a new one, because the original transformer is slim and it's absolutely not easy to find one that fits on its place. This is why you have to rewind the original.
4) For those who don't care to completely change the design and have much more power, then for the MSX1 models there's always the possibility to:
- Remove the slim transformer and the power supply PCB
- Insert a new switched power supply, preferably one that can take 100V to 240V and output at least 2A
With this mod you won't need to replace the Z80 NMOS with the Z80 CMOS
But for the all-in-one MSX2 models such mod isn't that easy, because the power supply circuit is integrated in the motherboard.
Swap the old Z80A NMOS for a Z80A CMOS, like the Zilog Z84C0004 or the Toshiba TMPZ84C00AP
Second that.. can take almost 1W + associated losses in power supply off the table. NMOS Z80: max. 200 mA (=1W at 5V, a lot of heat for a 40-pin plastic DIP). CMOS Z80: max. 5 mA (=25 mW at 5V) per MHz, so 0.1W at 4 MHz.
4) For those who don't care to completely change the design and have much more power, then for the MSX1 models there's always the possibility to:
- Remove the slim transformer and the power supply PCB
- Insert a new switched power supply, preferably one that can take 100V to 240V and output at least 2A
Would be a good option in this case. But requires changes to the mainboard, probably removing some components, cutting/re-routing some pcb tracks etc. Kinda difficult to figure out what needs doing, when schematic is missing. 
I dislike machines with power circuit integrated on mainboard, for this very reason. 
Makes it unnecessarily difficult to cut out the power supply as a whole & replace with something else.
