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MMU address line resistors changed to 68ohms. Why?

Posted: Sat May 26, 2018 7:14 pm
by Atarian Computing
I started working on a 520STM, without floppy. Everything seems to work but I noticed that someone had replaced the original 33ohm resistors on the address lines to 68ohms. The memory is the standard 512K that I plan to upgrade to 1MB via piggyback method.

Question is, why would one need to replace these resistors and what implications are there for having 68ohms?
InkedResistors_LI.jpg
InkedResistors_LI.jpg (2.28 MiB) Viewed 803 times

Re: MMU address line resistors changed to 68ohms. Why?

Posted: Sat May 26, 2018 8:45 pm
by exxos
Probably someone didn't know what they were doing and put wrong ones back in.

Re: MMU address line resistors changed to 68ohms. Why?

Posted: Sat May 26, 2018 9:08 pm
by Atarian Computing
exxos wrote:
Sat May 26, 2018 8:45 pm
Probably someone didn't know what they were doing and put wrong ones back in.
Wonder why they would do that... You can't see from the picture but they actually snipped the original resistors off and soldered these on the stumps. I guess I'll just put 33 ohm ones back in.

Re: MMU address line resistors changed to 68ohms. Why?

Posted: Sun May 27, 2018 7:46 am
by Petari
I'm not sure that reason is some mistake or lack of 33 Ohm resistors. Little larger values causing better noise suppression.
Not to mention that people who doesn't know what doing normally changes not resistors just like that. Even less likely is that resistors were ever removed - with plan to put same ones back.