idlemarvel said:
I'm sure there is no such a thing as a 7 pin decoder. It may be that some decoders don't use all the pins but the socket for fitting the decoder will have 8 sockets (in general, leaving aside n-gauge 6 pin and some
other manufacturer's 21 pin). I have a Hornby Pendolino which came "DCC ready", I just removed the "dummy" (blanking plate) and inserted the ESU sound chip and it worked. Follow the instructions (carefully!) that came with the Pendolino (or download the maintenance
sheet from the Hornby site, link below) to open the Pendolino and install the decoder. If it came "DCC fitted" then I assume you just remove the supplied basic decoder and replace with ESU. Look at step 4 on the instruction sheet.
https://www.hornby.com/filelibrary/download/?FileID=244
Hope this helps.
If you look at the service sheet for the DCC fitted version ( https://www.hornby.com/FileLibrary/Files/Download/?FileID=83 ) you will see that it really does have a seven pin socket which makes decoder replacement more difficult
but not impossible.
BTW the two locos in the old Mixed Goods set used a 4 pin socket, I don't know if the replacement Mixed Freight set uses the same 4 pin socket.