MKS70 Noise Reduction

After completing a large project upgrading and refurbishing a Roland 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.

The technically “sharp eyed” will also notice that in the picture below, the PWM upgraded ROM B is incorrectly installed and should be ROM C if the CN1 cable pin 10 has no black wire.

MKS70Rewire-Slide0

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

  1. 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.
  2. Module Board digital control signals should be routed as close to metalwork as possible and not over more digital circuits that add more noise.
  3. Mains wires could be routed better away from audio sections of Module Boards, however, the difference here is minor.

MKS70Rewire-Slide1

Stage 1

The MKS70 Board Removal page shows how the carefully dismantle the internals as there can be issues when doing so.

  1. After Module Board removal, separate the Module Board power looms away from the assigner Module Board control signal looms.
  2. Remove the large cable tie base, adhesive pad remains and secure to the bottom left heatsink securing nut.
  3. Route the module power wires first running them as close to the metalwork as possible up and away from anything else.
  4. 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.
  5. 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.

MKS70Rewire-Slide2

Stage 2

Module A that outputs tone A, uses ROM marked B. Module B that outputs tone B, uses ROM marked C. Hence the “B” or “A” pen markings occasionally present on these PCBs.

Note that it doesn’t matter if C or B ROM’d module boards are on top or bottom:

  • The Module A board with the “B” ROM has a black wire (MIDI out) on CN1 cable pin 10 and it has Orange, Blue, Purple and Grey wires on the CN3 audio cable that end up on Jack Board CN2 pins 3,6,7,8.
  • The Module B board with the “C” ROM doesn’t have a black wire on CN1 cable and it has Brown, Red, Yellow and Green wires on the CN3 cable that end up on Jack Board CN2 pins 1,2,4,5.

Steps below assume Module A board with ROM “B”  on bottom:

  1. Fit the Bottom Module A Board that has ROM “B” and first checking that cables to assigner CN7 connection are as flat as possible. It is a good idea to add insulation on the base of the PCB in this area to stop IC legs penetrating the wires and causing issues!
  2. Route the CN3 audio cable 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. CN3 Audio cable has Orange, Blue, Purple and Grey wires (Module A ROM B board) that end up on Jack Board CN2 pins 8,3,6,7.
  3. Route the assigner control signals as close to metalwork as possible and away from the CPU section. It will be necessary to warm the wires with a hairdryer before bending so that they route towards the case and “set” in position.
  4. Connect the Power loom but let the “loop” just “hang out” for now.
  5. Fit Top Module B Board that has ROM marked “C”.
  6. Again route the audio CN3 wires in same way as for B, Brown, Red, Yellow and Green wires that end up on Jack Board CN2 Pins 1,2,4,5.
  7. Route assigner control signals close to the metalwork and secure both “B” and “C” power looms with a new cable tie base secured to the metalwork. The control signal cable bundle for Module A with ROM B has a black wire (MIDI out) connecting the pin 10, Module B with ROM C bundle does not.
  8. 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.
  9. 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.

MKS70Rewire-Slide3

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.

MKSRewire-UberFussy

 

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.

 

 

 

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