After completing a large project upgrading and refurbishing an MKS70 to fully modified spec, a direct audio comparison with a JX10 was disappointing. The hum level was much higher on the MKS70 than the JX10.
Wiring layout is not optimal, however it suits speed of manufacturing because the loom is built up in one complete unit.
The wires running over the module boards carry power and ground return. The ground return is also at same potential as the casing, so the gap between the casing and the power return wires create a large inductive pickup loop….and there are two of them. It means that any hum or digital noise emitting from the transformer or PCBs get caught up and induced into the ground of the Module Boards eventually reaching the audio outputs.
The picture below shows how the diagonal cable running over the module boards CPU sections. The worst pickup loop is between the bottom case and Module Board B as it resides over all CPU areas and the transformer.
I was inspired to investigate more deeply after reading a post created by scottrod on an online forum at gearsz.com. A couple of the techniques from that post are used here but in conjunction with tests, cable routing was perfected to get the very best out of the instrument. In tests, placing the audio wires up against the power filter made little difference, everything is primarily about ground path loop area to casing.
Top 3 Wire Routing Issues
- Loop area of power leads (case to power return path) to the module boards is huge as it runs around the transformer past the assigner, then over the CPU sections of the Module Boards.
- Module Board digital control signals should be routed as close to metalwork as possible and not over more digital circuits that add more noise.
- Mains wires could be routed better away from audio sections of Module Boards, however, the difference here is minor.
- After Module Board removal, separate the Module Board power looms away from the assigner Module Board control signal looms.
- Remove the large cable tie base, adhesive pad remains and secure to the bottom left heatsink securing nut.
- Route the module power wires first running them as close to the metalwork as possible up and away from anything else.
- Run the assigner power loom diagonally across, under the midi signal loom (to keep close to metalwork) and secure with a new cable tie base.
- Finally tidy the mains routing by placing it outbound of the fixing post before pushing all other wires closer to the casing.
Handy Tip: For best possible results, mess about with the wire bundles so that the black wires end up being as close to the metal base plate as possible.
- Fit the Bottom “C” Module Board.
- Route the audio wires about 15mm inbound along the PCB, ideally away from power wires and close to the ground plane (wide copper area) that runs along the top of the longest length of the PCB. Original routing is actually not bad for audio.
- Route the assigner control signals as close to metalwork as possible and away from the CPU section.
- Just let the Module Board Power loom “hang out” for now.
- Fit Module Board C.
- Again route the audio wires in same way as for B.
- Route assigner control signals close to the metalwork and secure both “B” and “C” looms with a new cable tie base secured to the metalwork.
- Loop but do not “coil” (ie. do not make another single turn pickup coil!) the power wires to the Module Boards. Secure them adjacent to metalwork to a new large cable tie base over the top of the “Caution” sticker. Do not be tempted to use the adjacent cable tie base for the audio.
- Finally, using a new small cable tie base, secure the Switch Board wires that go to the assigner away from the audio headphone loom.
Handy Tip: If the old wires wont budge because they have been in same position for so long, use a hairdryer to warm them up before letting them cool down in the new position.
Close Up Pictures
The collection of close up pictures below summarises the key areas.
If You Are Really Fussy
In tests, moving the mains wire tie point away from the module board didn’t seem to make any perceptible difference, the fridge running in the kitchen caused more noise coming out of the speakers!
However, here is an alternative routing for the AC mains and AC analogue power. Be careful not to cover the bridge rectifier heatsink as it gets quite hot. The red and black wires (analogue AC power) have to be disconnected, shortened and twisting them together gives optimum results.
Copyright © 2017 Super Synth Projects, Guy Wilkinson but note that this work is based and inspired by scottrod on an online forum at gearsz.com. Details can be found on the JXZone Info page under “Hum Modification” here.