Roland MKS-80 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!

UPDATE Plasma Music have taken the idea of a replacement MKS80 power supply and launched a commercial product here. Its a bit expensive at £400+ but well designed and proven. However its too expensive for me, as I can design and build one for £100.

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 an MKS-80, the first one I had to sell in 2001 due to hard times. 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 MKS-80 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 JX-10 power supply with a modern switched power supply. Details of the JX-10 PSU 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 MKS-80 is an all analog voice design, where-as the MKS-70 is using DCO’s and then an analog circuit path. The MKS-80 therefore draws more current from the 15V rails and I do not want to fall short of the Roland design specs.

Roland MKS-80 PSU

The New JX-10 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 MKS-80 Power Supply The MKS-80 service manual states that the +5V rail should be within +/-30 mV, and 15V rails at a tolerance of +/-100 and 400 mV. This is more accurate than a switched mode power supply can deliver (typically 2%)  and tighter than the IRM Meanwell used in Guys design which delivered +4.94 volts.

However checking the schematics the +5V and +/-15V rails are not used for critical reference voltages, so the loss of voltage accuracy from the switched power supplies in not an issue. I have used Vigortronic VTX modules that supply more than enough current:

• VTX 214-025-115 delivers +15V at 1.6A
• VTX 214-025-115 delivers -15V at 1.6A
• VTX 214-015-205 delivers +5V at 3A

Guys MKS-70 design uses Pi filters to reduce power supply ripple down to a tested 25mV from the 200mV which the switched power modules generate. I have re-used this design in my MKS-80 design and followed the same careful PCB trace layout. There is a nominal +8.5V supply to the MPG-80 from the MKS-80, and I have used a 2W Vigortronix VTX-214-002-109 to deliver +9V DC at 222mA, which is more than enough to power the MPG-80.

Precision Voltages 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! Carefully checking the schematics revealed that the precision power rails needed by the DAC and VCO’s are generated from the +10V rail and not from the digital or main analog rails.

• The +10V rail drives the +10V and -5V needed by the VCO’s
• +/-7.3V is generated from the +10V rail for the VCA and HPF
• The DAC uses +10.67V generated from the +10V rail.

A modern REF102CP precision regulator can deliver 2.5ppm/C and can be configured to be adjustable, so that I can set the reference voltage to exactly the same as I measure on my original MKS-80 power supply. The voltage needs to be buffered with a precision Op Amp to provide more current (20mA), although the Roland main and voice boards buffer the +10V with rather average performance M5218L Op Amps.

The rest of the MKS-80 replacement power supply can follow Guys design but with a layout and pin out that fits the MKS80 PCB dimensions. The power socket needs replacing with a grounded 3 pin IEC socket with power leads going to the power switch and then power conditioning PCB.

MKS-80 Power Connectors There are seven 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 main PCB size can be as large as a rectangular 120 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. A second smaller PCB holds the power input conditioning. 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.