Clock (SJ)

  This is the most versatile of the clock boxes (beaten only in friendliness by the SJ Bridge which allows you to set the clock rates by pushing buttons).

It is so versatile because it allows you to fine-tune the MARK duration down to 0.125µS and the PERIOD down to 0.25µS.

The clock speed settings are two banks of 6 DIL switches up the middle of the circuit. The upper bank controls the MARK duration (some early clocks have this incorrectly marked as SPACE). The lower bank controls the SPACE duration.

Setting this clock involves moving some switches and thinking...

Firstly, there is no direct way to set the SPACE. What you must do is set the PERIOD to the result of (MARK + SPACE). Personally, I find this easier to think of than Acorn's style.
Secondly, the switches are additive. If you wish for a MARK of 1.375 then you want to set the switches for 1µS, 0.25µS, and 0.125µS.

You set a switch by moving it to the right.

            mark

   [***  ]  0.125
   [***  ]  0.25
   [***  ]  0.5
   [  ***]  1
   [***  ]  2
   [***  ]  4 µS


            period

   [***  ]  0.25
   [***  ]  0.5
   [  ***]  1
   [***  ]  2
   [  ***]  4
   [***  ]  8 µS