OBERHEIM SYNTH

Hi,
     I am guessing that you are curious about the clock frequency of the main 
controlling CPU, which would be the Z80 (A101 on the upper control board)I checked both Rev 1 and Rev 3 schematics and in both cases the main crystaloscillator was 4.9152 MHz.   That signal is divided by two and then fed to the Z80.Since it's not a Z80A (4 MHz limit) but a plain Z80, the clock limit was 2.5 MHz,so they would have been just under that.
    The main clock frequency is divided down by other circuits to generate interruptrates, rates for updating the voices, and clocks for serial communication (MIDI).Changing the main clock is not as simple as it might sound as all of those derived ones
change too, and that hardware was designed to work properly with those speeds in mind.If you speed up the main clock and put in a faster CPU that can handle it, the CPU maybe OK, but the RAM's and EPROMs may not be fast enough and some of the other 
hw may become flaky as well.
    You can see  the main CPU clock on pin 6 of the Z80.  Be careful not to short thatpin to pin 5 or pin 7 when you are probing.

     Bob

      From: "richtrix@... [oberheim]" <oberheim@yahoogroups.com>
 To: oberheim@yahoogroups.com 
 Sent: Monday, February 23, 2015 2:55 AM
 Subject: [oberheim] Finding Clock frequency of my OBXa
   
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Random question. Anyone know what pins on which component I can find the main clock source for the whole unit? I have access to an oscilloscope to read it with just don't know where to look. I know it will probably be on the main processor on the UCB though. Mine is a late model OBXa and suspect the clock may be a different speed than earlier units. 

Most Sincerely,Rich Clarke