To the trained eye, there are loads of differences between it and the first Issue 1 boards – capacitor location and size, ULA markings etc. – but one of the most obvious changes was that the prototype Spectrum used a 3.5mm mono jack socket for the power supply input, as did the ZX81.
This was a ‘feature’ that never made it to production before being replaced with the familiar 2.1mm barrel type socket, but did you know that provision for both types remained on the PCB right up until the Issue 4B?
Let’s take a look at an Issue 4A PCB. Flipping it over and examining the socket pads, we can indeed see that we have more pads than are necessary for the standard barrel socket, and the pads do fit the footprint of the EAR/MIC sockets used on the Spectrum. In the second and third pictures, I show the standard socket in place, and then its replacement with a 3.5mm socket.
And just to prove I’m not cheating, I’ve powered the machine on using a Spectrum to ZX81 PSU adapter.
It’s obvious the machine case was designed with the ZX81 type PSU in mind, given the look of the 3.5mm connector with the case fitted. Also considering that the ZX Printer PSU’s were rated at 1.2A as were the original Issue 1 PSU’s, the only difference being the connector on the end differing.
The intent was presumably to continue with the 1.2A ZX Printer PSU as the standard Spectrum PSU, but concerns about the fragility of the DC-DC converter circuit may have forced the switch to the barrel type connector, which is much less prone to causing a short in the circuit when inserted.
The Issue 6A PCB finally removed the provision to use the 3.5mm socket for DC input.