Urs Koenig (QL) wrote, on 19/Jul/11 11:11 | Jul19:
Earlier I wrote:
I will investigate the "very early" standard QL next week.
First I will check the hardware (e.g. with loopback cable as
Tony suggests), then I will do several tries (e.g. swap ROM
chips with JM, swap ZX8302/IPC). I'll report my findings.
I swapped each socketed chip (first ROM's, then IPC, ZX8302, ZX8301 one
after another) and tested with Thorn-EMI Ser1toSer2 Test-Rig. Result: The
Serial ports are not working in this early QL.
It is quite hard getting broken ports to work.
Did you try the 8302/8049 from the dodgy QL in the working QL? You need
to establish they are OK first.
Illogically, adding the flying resistors onto the 8302 (if not there)
does sometimes cure.
Check the +12/-12 lines on the output chip - 1488 I think.
Check paths of signals in and out with scope when doing a loopback test.
A test *without* handshake is best first, in case one of the handshake
lines is dead.
It is a hard job and possibly too much trouble. I remember in my QL
repair days if I had to do that sort of thing, it was a charity repair (8-)#
Tony
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