The hw design of the Enterprise is originally ready for 6MHz model. The most important evidence about it is the option bit in the Dave (at port BFh), set the Dave sound frequency divider for the 4 or 6 Mhz system.
Very strange - why was 6Mhz machine not released?
Originally only 3 modifications needed:
-Z80B instead of Z80A
-12MHz crystal instead of 8MHz
-modify one bit in the EXOS code, for setting right frq for Dave
Unfortunately the Enterprise company made later a big mistake with the EXDOS: the WD disk controller does not have an own clock generator, it uses the system clock from the machine instead. So with 6MHz model the EXDOS cannot work...
I think this is because of cost reduction... but make some additional work when want to make a Turbo Enterprise.
See the schematic.The 8MHz System Clock come from U18 Pin 12, need to cut this output line. Unfortunately it is hard to access, the best way is removing U18 and restoring it after cutting.
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Next: cut the System Clock from the Expansion Connector then connect the A23 pin to original 8MHz clock at the U18 pin12.
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Then a new clock source is needed, 5V TTL compatible oscillator is the easiest solution, can be piggyback on U8, GND and VCC pins connected to the IC GND and VCC pins.
The clock output goes to the U18 pin 1 which are the input for the new System Clock.
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Replacing CPU to Z80B is optional, most of Z80A CPUs work on 6MHz. Try it and if it doesn't work then replace it. (I have never replaced it for 6MHz!)
The 7.12MHz turbo is a cheaper trick because no new clock generator is needed, using the 14M Video Clock!
All same as above, just instead of the oscillator output, connect the U37 Pin 10 (Video Clock) to U18 Pin 1 as System Clock. Many Z80A work at 7.12MHz but it is more possible that it needs to be replaced.
On some motherboards some problem can be found with the video memory on turbo speed, then short circuit R12. On higher speed (10MHz
) also needs removing C7.
In 128K machines there also might be a problem with the internal 64K expansion, then short circuit R151. It will be good up to 7.12MHz CPU, but for 10MHz new super fast SRAM expansion should be bought from Saint
It is a good idea to add Turbo Switch because not all the games are better at faster
Very ugly solution
just add directly a switch to the clock lines: one input from U18 pin 12 (8MHz) another from the new clock generator or the Video Clock, and output go U18 pin 1. It is worked to me for many years in many machines
The next lesson will be: how to build electronic switch with 74LS157 IC
Final words about the replacement of Z80: buy least 10MHz model (Z84C0010), but don't order it from China! Previously I wrote my experiences, the Chinese sellers sold relabeled old and slow Z80 CPUs
