A500 REV 5 MOTHEBOARD
In earlier A500 REV 5 assembly versions CPU PIN 20 is connected to ODD
CIA PIN 25. The track goes through via to the bottom side of the
motheboard near DENISE and comes back to the top side though another
via near ODD CIA.
In later A500 REV 5 assembly versions the bottom side track was cut off
by a drilling and on the top side 68 ohm resistor was soldered to via
holes. Also 37mm plastic cover was added under the resistor to prevent
surface contact. Due to this CPU PIN 20 is connected to 68 ohm resistor
which is connected to ODD CIA PIN 25.
I have not found out why this was done, but I know ODD CIA was often
damaged because people connected PARALLEL PORT devices without powering
off first, so adding 68ohm resistor is there could protect CPU from
receiving same damage through ODD CIA. Well this is just my quick
theory without more research. Let me know if you know better.
In all A500 REV 5 assembly versions there is a 150 ohm resistor
soldered between MONOCHROME VIDEO connector center feet and PCB ground
(in the middle of EMI404 silk screen marking). When measured it shows
75 ohms and when desoldered and measures it shows 150 ohms, so there is
already another 150 ohms in there even if that resistor is removed
(parallel connection of two
150 ohm resistors is 75 ohms when measured). Also there is no reason
why this resitor should be on the bottom side, it can be moved to the
similar positio on the top side when taken care that it cannot short
circuit with anything.