Page Updated 28.5.2010
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.

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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.


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