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Speeding Up an LC475

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Hi All, as promised I've been working on speeding up my LC475.  I have two motherboards but one case.  I found the documents online for speeding up an LC475 to 33 or even 40 MHz.  So I ordered the switches, wires, resistors, heat shrink, etc., to add the mod.  I had already purchased a couple of 33 MHz 68040s along with heatsinks as they got pretty hot.  So, I modded the first board successfully and it ran fine at 33 MHz.  My Apple IIe card because a little finicky intermittently reporting an error #4 but a software restart would fix the problem.  


I was going to perform the same mod on the other MB but I got curious if it would run at 40 MHz.  Modding the second MB to switch between 25 and 40 MHz was even easier than the mod to 33 MHz.  I put the newly moded board in and it ran fine at 25 MHz.  But when switching to 40 MHz the board never did it's pleasing startup chime or boot.  I swapped the 33 MHz 68040 processors but that made no difference.  It was definitely the MB.  So I rewired the mod to switch between 25 and 33 MHz like the first one and it runs fine now at either 25 or 33 MHz.


So how successful was anyone else on the 40 MHz mod on the LC475?  Prior to giving up on the 40 MHz mod, I was able to boot the LC475 at 40 MHz if I switched from 25 to 40 MHz right after the start up chime.  But I got some strange graphic issues like mouse pointer trails on the desktop and some of the startup screens had a small amount of corruption throughout the picture.  I suspect the higher processor speed was too fast for the 100 ns, 1 MB display RAM.  When I ran the apple personal diagnostics at 25 or 33 MHz it reported the correct speed.  But at 40 MHz it still reported the processor was running at 25 MHz.


So it's been fun playing with my LC475 but now I need to figure out what to do with it.


Thank you for your time and suggestions,


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Hi Gerry,


I assume you've read the below (old) guides on clock chipping your LC475/Q605?  Both suggest getting to 40Mhz might be a tall order, I'd stick to the tried and true 33Mhz or something in between, eg 36 - 38 Mhz.  Max out the RAM, put in a fast HD and you're set to throw pretty much any 68K application and game at it.







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WAG: you're pounding the snot outta everything on the board with a system bus clock of 40MHz, so be sure your SIMMs have a response time rated for use with the 840AV. If not, that'll all but guarantee stopping the RAM check dead in its tracks, dunno if you'd even get to the point of a memory error chime in that scenario. Is ROM response time up to the task? Disabling that and installing a socket for BMOW's ROM-inator II would probably be that fix.



edit: re-read your post, inadequate SIMM memory response times and switching speeds after the memory check could explain those flaky results. It also reminded me about your mucus-challenged VRAM SIMMs, make sure those are 840AV compatible as well. ;)



edit: re-read it again, saw you noted the VRAM response time issue, hope the other suggestions prove fruitful.

Edited by Trash80toHP_Mini
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