Acorn
Prestel
Adapter
This is the PCB in a bit more detail. Unusually for a cheese wedge, the mains power connects straight onto the board. Usually it goes via a transformer, but in the case of the Prestel Adapter, the space normally occupied by the transformer is taken up by a speaker, so we can hear what is happening when dialling up.
There's quite a number of fuses on here in what is essentially an early modem.