Having now got my 8048 working I thought my observations on RS232 leads may be of help.
I made my lead up following the info elsewhere with only pins 3, 4, 5, 7, 8 & 9 connected.
I also made sure that I made up the shortest lead that I needed since I am suspicious that too long a cable could cause data problems because of possible voltage drop.
I also found that I did still have a couple of old all “pin to pin” RS232 leads that I had from years ago. These would not work for some reason.
They were fully wired cables with all 9 pins connected 1-1, 2-2 3-3 etc. They were also screened cables so the plug housings and bodies were connected to the computer ground and the programmer ground.
They were also in excess of 1.5M long.
Why they don’t work I have no idea but I am suspicious of earthing loops between the PC and the programmer.
However using the shortest version of the ‘6 pins only’ set up has worked fine for me and might be worth a try if you’ve got odd problems with the K8048.