This is a response to the comment from an earlier post on modifying a Roland DR-110 drum machine for a clock input. Here is the PCB layout from the service manual. I added the section of the schematic that shows the clock.
To modify the clock, I simply broke the connection between the clock generator and the rest of the circuit (in the DR-110 schematic and PCB above it is shown in red), then I replace it with a switch and a jack. The switch can reconnect the clock circuit to the board so the tempo knob works again, or it can switch to a jack to be connected to an external clock. If you try to attempt a modifications like this, make SURE that you understand which node is the clock_source and which node is the clock_destination of the broken link - you don't want to miswire this and accidentally feed an external clock into a clock generator circuit! In the above example, the side closest to U2 is the clock_source. My master clock is coming from a Garfield Mini Doc so I have lots of clock options to choose from. Some divisions work better than others. For the DR-110, the "12" division output (12 pulses per quarter note, I believe but I have never verified this) seems to be in perfect sync with all of my other gear.
This method can be used with other drum machines or sequencers that don't have a clock input, provided that they have an internal analog clock generator. A word of caution though: not all clocks work at the same voltage level: Do NOT try to directly clock something with an external clock without first understanding what speed and voltage levels it toggles between! Use an oscilloscope with a x10 probe (if available) to tell you what the clock needs to look like in order to sync up to the device, then try and match the external clock signal as close as possible to the measured internal clock. For some machines, an additional transistor may have to be added to change the voltage level or clock pulse width, but I will not go into details on using a transistor as a switch in this post - there are plenty of examples elsewhere. For a machine like the DR-55, I find the clock source that works the best to be sending a 16th note clock into a decay/release envelope circuit, then into the external clock input of my DR-55. I did not need to do this for the DR-110 because the clock was close to the same levels that the Mini Doc generates.
This being said, modify your equipment at your own risk. I have done modifications that work fine, then you plug the wrong type of signal into the jack (Voltage could be too high, or you create a current path that should not be there) and because the internal circuitry was not designed to handle this, components fail. Without being too crazy or obsessive, treat equipment and circuitry with caution, respect, and understanding and your modifications will work fine.