More on my HP7245A problem

I've been doing some more fiddling with that HP7245A printer/plotter. On the ROM/RAM board there are 8 9216s, arranged to give an 8K*16 program storage space. And also 4 off 82S123 PROMs arranged to give 64words*16, probably for some kind of parameter storage. Those last 4 chips are socketed. Test 02 does appear to be the ROM test (as on the -B model). On my machine it seems to access all the program ROMs and the first pair of 82S123s. It then gives up with error 54(8). I have rpelaced the sockets (with nice turned-piu ones), no change. By swapping the ROMs around, I think I've discovered that the test is a simple cheacksum, independant of the address.In particular, if I swap round the 2 sets of PROMs (in the test mode), it fails with error 56(8). And if I put them back in the right sockets, and then swap round the 2 ROMs in the low bank, it fails with code 53(8) This leads me to guess that the error codes (in octal) are : 53 : U15 (PROM A MSB) 54 : U10 (PROM A LSB) 55 : U16 (PROM B MSB) 56 : U11 (PROM B LSB). And that U10 is genuinely the problem in my machine (i.e. not the socket, PCB traces, or address decoder, all of which I have checked anyway). Curiously, all 4 PROMs have the same HP part number, but I suspect that's the code for a blank chip, and that the contents of each chip is different. OK, it's a long shot, but does anyone have a dead 7245A that they can pull these ROMs from? Or any idea as to what the contents should be? -tony