Roland MKS80 Power Supply

Introduction This is a feasibility study into a replacement power supply for the Roland MKS80, and it is not a project that AMSynths are aiming to deliver, as Guy Wilkinson owns the rights to the MKS70/JX10 re-engineered power supply which this thinking is based on. Thoughts and ideas are welcomed!

Overview I am fortunate to have a near mint Roland MKS80 (March 1985) and MPG80 (August 1984). This is the second time I have owned one, as I had to sell my first MKS80 in 2001. It is a REV5 with a 110V power supply and therefore requires an external converter, which is all a bit bulky and messy. The transformer in the MKS80 is not tapped for both 240 and 110V operation, so a NOS 240V replacement transformer is needed.

Rather than going down that route I have was inspired by the work done by Guy Wilkinson in replacing the Roland JX10 power supply with a modern switched power supply. Details are here. This approach greatly reduces the heat from the old linear regulators as a bonus, as well as having 240V operation, and is a great long term solution with no capacitors to replace.

Roland are good at designing high quality power supplies with precision rails in the right places for analog circuits. The MKS80 is a completely analog voice circuit, where the MKS70 is using DCO’s and then an analog circuit path. We do not want to fall short of the Roland design specs.

Roland MKS80 PSU

The New JX10 PSU Guy’s design provides +5V and +/-15V rails with low ripple after careful filtering of a set of three Meanwell power converters. Unfortunately this PCB cannot be directly reused in the MKS80 as it requires a +10V precision reference voltage for the VCO/DAC and requires a more power to the 15V rails than a MKS70, even though it has 8 rather than 12 voices and the overall power consumption is similar at 35 W.

The low voltage connections are also in a different location, and the mains filter is on board the power PCB. A new PCB needs to be designed with slightly different Meanwell units to meet the demands of the MKS80. There is an Unregulated +7VDC in the MKS80 to power the MPG80, just like the MKS70 design which also powers the display. Guys design uses a Vigortronix power converter to deliver +9V DC at 0.55A which can be used for the MKS80.

+5V Power
MKS80 Rectifier = 2D4B41 (2A) Main Capacitor = 4700uF 800mA at 8.5VAC
MKS70 Rectifier = 4D4B41 (4A) Main Capacitor = 6800uF
Guys Replacement PSU delivers +5V at 1.5A

+/-15V Power
MKS80 Rectifier = 2B4B41 (2A) Main Capacitor = 3300uF 450mAx2 at 21.5VAC
MKS70 Rectifier = 1B4B1 (1.5A) Main Capacitor = 4700uF
Guys Replacement PSU delivers +/-15V at 600mA x2

Replacement MKS80 Power Supply The MKS80 service manual states that the +5V rail should be within +/-30 mV, which is a tighter tolerance than the IRM Meanwell converter delivers and the actual +4.94 volts measured in Guys design. The +5V does not get used as a reference voltage, but potentially a more accurate converter/solution is needed as AC/DC Converters typically have 2% initial voltage accuracy, and Roland are stating 0.6%.  In the MKS80 the +5V rail is using less power than in the MKS70, so it is possible to retain the Meanwell IRM-10-5 which is rated at 10W (provided we can accept the voltage accuracy).

For the 15V rails the more powerful 20W IRM-20-15, which delivers 33% more current at 800mA per rail, is needed. The 15V rails are at a much lower tolerance of +/-100 and 400 mV, so the Meanwell IRM can easily achieve this accuracy. Other Roland synths require very accurate 15V rails, such as the Juno 60 where the tolerance is +/-10mV which really demands a linear or LDO design.

The +9V supply remains the same as in Guy’s design, using the 5W Vigortronix power converter to deliver +9V DC at 0.55A, more than enough to power the MPG80.

The original Roland +10V power is provided by a 1SZ59 6.5V zener diode, with an Op Amp and trimmer to set this to an accurate +10.00V, the 1SZ59 has a temperature co-efficient of 2ppm/C which is very good! A modern LT1027 precision regulator can deliver the same 2ppm/C. It needs to be buffered with a precision Op Amp on the power supply PCB, although in the voice boards the +10V is further buffered by average performance M5218L Op Amps. The reason to take care here is that this voltage is used in the VCO’s as a reference voltage, as well as the DAC reference voltage.

The rest of the MKS80 replacement power supply can follow Guys design but with a layout and pin out that fits the MKS80 PCB dimensions. The mains power conditioner on the top right of the Roland PCB can be dispensed with and its taking up valuable space at the rear of the MKS80. The power socket needs replacing with a grounded 3 pin IEC socket with power leads going straight to the front panel ON/OFF switch and then running back to the bottom right hand corner of the new PCB.

MKS80 Power Connectors There are 7 low voltage power connectors on the PCB to supply the various MKS80 modules:

• P1 Analog +15V 10 pin
• P2 Analog GND 12 pin
• P3 Analog -15V 8 pin
• P4 Digital GND 6 pin
• P5 Digital +5V  6 pin
• P6 Reference Voltage Analog 2 Pin
• P7 Unregulated and Digital GND 4 Pin

These need to remain in the same position on the new PCB to ensure the cables and connectors fit.

PCB Design The PCB size can be as large as a rectangular 100 x 200 mm, with the same mounting holes as the original. The removal of the large internal heat sink means there is more space at the rear of the case. I have mocked up a high level design with the 4 converters and the power input at the front corner.

Next step is to do a PCB layout mock up in Eagle CAD to see if the components and the required isolated tracks can fit. A solution to the +5V DC initial voltage accuracy needs to be found as the best AC/DC converter I can find is 1% accurate.