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SpeedGeek · 06 December 2010, 09:50

On the A3000 the syncronized 20Mhz clock is obtained by disconnecting the 50 Mhz oscillator and connecting the 40 Mhz clock from the A3640 delay line to the A3000 74F74 input (R120). The 10 ns tap works good for me. There are 2 optional mods to U208 on the A3640 needed for Ramsey to run a 4 clock cycle. All motherboard jumpers should be set to "Internal" and "16 Mhz". Also, you need to disconnect the A3640 cpu clock and clock 90 from R102 & R101 on the A3640.

On the A4000 you have the 74F74 and delay line available at different locations (see A4000 schematics) than the A3000. A diagram is provided. Good Luck! PS. You can use 1 or 2 unused pins on the 200 pin cpu connector to connect the jumper(s) to the A3640 so you can remove the A3640 without unsoldering jumper wires.

** NEWS UPDATE ** I have added as a second option another way to do the U208 part of the mod. It eliminates the possible problem with U208 timing from the motherboard delay line and 1 of the motherboard jumpers to the A3640. The only trick is finding the trace to pin 9.

06 December 2010, 09:50	#27
SpeedGeek Moderator Join Date: Dec 2010 Location: Wisconsin USA Age: 60 Posts: 839	On the A3000 the syncronized 20Mhz clock is obtained by disconnecting the 50 Mhz oscillator and connecting the 40 Mhz clock from the A3640 delay line to the A3000 74F74 input (R120). The 10 ns tap works good for me. There are 2 optional mods to U208 on the A3640 needed for Ramsey to run a 4 clock cycle. All motherboard jumpers should be set to "Internal" and "16 Mhz". Also, you need to disconnect the A3640 cpu clock and clock 90 from R102 & R101 on the A3640. On the A4000 you have the 74F74 and delay line available at different locations (see A4000 schematics) than the A3000. A diagram is provided. Good Luck! PS. You can use 1 or 2 unused pins on the 200 pin cpu connector to connect the jumper(s) to the A3640 so you can remove the A3640 without unsoldering jumper wires. NEWS UPDATE I have added as a second option another way to do the U208 part of the mod. It eliminates the possible problem with U208 timing from the motherboard delay line and 1 of the motherboard jumpers to the A3640. The only trick is finding the trace to pin 9. Attached Thumbnails Last edited by SpeedGeek; 15 October 2020 at 00:14.