Power Supply

Many requests have been submitted for a guide on refurbishing the SuperJX power supply. It is considered as a weak point because when it goes wrong, the instrument shows no sign of life or behaves erratically. It also suffers from age and power rails can begin to drift out.

If only a partial rebuild is undertaken it is a good idea to check the health of the power supply by measuring the voltages shown in the “test” section at the end of the rebuild guide.

The power supply boards for each instrument are shown below, the JX10 on left has the mains input

filter populated, whereas the MKS70 version has a separate filter mounted near the mains input connector.

The web is full of myths around component selection and what can go wrong, this page attempts to explain the reasons why it goes wrong and what components are needed to get it back in perfect working order.

New Parts

The parts specification is shown in this xls spreadsheet: SuperJX-PSU-Parts

Parts are suggested for all components in the power supply but the guide doesn’t explain how to replace all of them, simply follow the same techniques to replace others if wanted. Part suggestions are usually modern equivalents that give better performance or are more robust when handling so that the DIY’er isn’t disappointed!

Please note that the spreadsheet contains parts suggestions only and specifications must be checked when ordering. These have not all been ordered or tested in a power supply. Please feedback if you have successfully used the parts or prefer alternatives to help other SuperJX users!

Rebuild

See this page for a detailed technical rebuild guide: SuperJX Power Supply Rebuild

Many SuperJX instruments are configured incorrectly for the country of use and subsequently running hot or not delivering stable rails to the PCBs, check out Mains Input configuration page for full details of how it should be wired for each country.

If you are going to rip your MKS70 to bits to get to the Power Supply you may as well be brave and consider this noise reduction modification that rearranges the wiring: Roland MKS70 Noise Reduction

The same rebuild techniques can also be applied to the JX10 display board, JX10 switch board and the Jack board. All single sided PCBs can have similar issues.

If you use this guide to refurbish a power supply, please tell us how you got on, we would like to know.

Solder Joints

This is the number one issue with the SuperJX power supply. If the board is not being rebuilt then at minimum this issue will very likely need fixing.

The PCB is single sided FR2 and as the copper traces are single sided without plated through holes, the joints on the underside are vulnerable to fracturing often known as “dry joints”. Symptoms usually manifest themselves as intermittent audio, display loss, resetting and crashing. The worst affected areas are the power transistors and bridge rectifier terminations.

The picture below is from an MKS70 Power Supply Board showing the transistor legs almost completely fractured from the pads and solder. Looking under magnification you would see a crack running all around the leg to solder interface. With the MKS70 this is extremely common due to mechanical stress on the legs when the instrument is moved. JX10 is vulnerable too but less so.

DryJoints-15VTransistor

The picture below shows a “burn out” with a dry joint. A user had fitted a PWM board using adhesive tape. Eventually it moved and shorted against the casing, the 5V rail was catastrophically damaged and overheated the transistor.

Dry&Burnt-5VTransistorPads

To repair dry joints simply use a desoldering tool to remove the old solder then replace with new solder. Many people fixing this issue just “reflow” by reheating with an iron but this can fail again because oxidation and dirt are put into the joint and it makes it weak.

If replacing the transistors, see the rebuild page for a method of making the MKS70 power supply more robust by removing solder resist and folding the legs to make a more secure joint.

Smoothing Capacitors

Capacitors in this type of “low stress” linear supply don’t usually fail unless they have been extremely hot.

Replacing them after 30 years is recommended because they don’t cost much and you may as well if you are fixing dry solder joints. Issues with capacitors result in extreme hum on audio or no function with maybe strange characters on the display. When they do fail they are likely to bulge.

Huge Range of Choice

Replacing them with “Audio Grade” or “Low ESR” types provide no benefit because the linear regulators have wide bandwidth and regulate the voltage rails to the instrument very well. Audio grade capacitors have particular characteristics that can improve performance if they are supplying a power amplifier output stage that doesn’t have pre-regulation and big currents flow at high audio volume. In the SuperJX the characteristics of the audio grade capacitor are unimportant as they are only smoothing rectified 50Hz AC so the response of the audio grade type is wasted.

Choosing a “low ESR” type will be good for delivering current spikes to the linear regulators, but there isn’t that kind of demand by the circuitry.

Some types have a lifetime figure quoted at 2000 or 3000 hours. This is the lifetime at the rated ripple current and maximum temperature, typically most electrolytic capacitors are 1000 hours. No need to be alarmed though, the ripple current is very low in the SuperJX power supply and maximum temperature is approximately 60 degrees Celsius, hence why they last so long. Datasheets give lifetime vs temperature and ripple current charts if it is of interest.

What Is Best?

As the PSU puts very little demand on the capacitors, the only attribute worth considering is finding ones for longevity and reliability so that they will last another 30 years or more. Higher voltage rating doesn’t help reliability but temperature rating and choosing a reputable manufacturer does.

Experiments during product design has revealed that high temperature rated capacitors (For example +105 degree C) give marked improvement. The simple reason is that the construction of these types are geared towards keeping the electrolyte from evaporating, so there is usually a double seal in the base. They are slightly larger and more expensive than the standard 85 degree C types too, but with modern technology the size is smaller anyway and usually they are the same size as the 85 degree types from 1986….so they will fit.

A smaller capacitor for C11 is less affected by heat from the rectifier heat-sink (D4) that  is adjacent to the capacitor that smooths the “7V Unreg” power rail. This means that a modern 85 degree type is more than suitable and will give a long life.

The standard 85 degree types are fit for purpose and they will last at least another 30 years if a reputable manufacturer is chosen. Pictures below show the relative sizes of old and new smoothing capacitors:

  • Left: A JX10 using 85 degree C types
  • Right: An MKS70 using 105 degree C types that are thinner but 5mm taller (fit satisfactorily inside an MKS70 as there is more space available).

In summary, the very best capacitor for this application will be a standard 105 degree C type that will outlive the instrument, but an 85 degree type will last a very long time too.

Fitting New

When replacing a capacitor it is very important not to excessively over heat the leads during soldering because the heat can damage the seal and they could fail relatively quickly. Using a temperature controlled iron achieves a good soldered joint quickly.

It is also a good idea to secure the base of the capacitor with a blob of sealant to reduce vibration stress on the legs if the instrument is transported. There is a saying in the automotive electronic industry, if it is not secured, it “waves you goodbye”. Another benefit is that the seal around the legs is further protected. Silicone sealant is best because it can be removed in the future, superglue or epoxy will make the capacitors difficult to remove without cracking the brittle PCB.

Decoupling Capacitors

The smaller electrolytic and film capacitors can be replaced readily by modern equivalents and only have to be general purpose types.

The critical part in this area is C4, if this becomes resistive, because electrolytic capacitors do that over time, all power rails will droop.

510-DecouplingCaps

For electrolytics, technology has moved on dramatically such that for given capacitance, the voltage rating is very much higher. It won’t be long before 1uF electrolytic capacitors will be discontinued and a 50V moulded radial ceramic MLCC should be substituted instead.

The green coloured film capacitors are extremely reliable but if suspect, could be replaced by a metal film equivalent or better still, a modern MLCC ceramic.

They are for decoupling purposes, therefore a dipped or moulded ceramic leaded MLCC capacitor improves high frequency rejection. Note that the leads will have to be bent to fit the capacitors on the board and ceramic MLCC types are vulnerable to stress. Care must be taken to bend the leads carefully without stressing the lead entry points at the body otherwise they crack internally and become resistors before overheating.

A moulded ceramic or film capacitor replacement is suggested in the parts list, these are not quite as vulnerable as the “dipped” ceramic MLCC types. The moulded ceramic suggested is a high reliability type for critical applications.

Bridge Rectifiers

The large bridge rectifier that has a heatsink takes a lot of punishment and runs warm. It is also near capacitor C11 and to improve the life of C11 one of the heatsink fins is angled slightly. Unfortunately, the heatsink is usually a bit battered, loose or bent at funny angles meaning that a good contact with the rectifier is not made. the PCB is usually “scorched” under the rectifier.

Rectifiers can be replaced with modern equivalents as long as they meet the peak and average current handling requirements. Note that they will usually have thinner legs and bodies.

Replacement part suggestions are shown in the parts spreadsheet.

Transistors

Failure of transistors seems to be rare unless there is a dry joint or a dodgy DIY modification has short circuited the power supply, in which case they will thankfully burn out before the transformer does but render your JX useless.

326-TransistorsLonely

Replacing them can be justified because if they fail short circuit between collector and emitter they will output high voltage onto the logic and analogue rails.

Note that replacing the transistors with same type means that you will be using “New Old Stock” so they could be as old as the instrument. They are readily available from UTSource but could easily be substituted with a modern part that has an insulated tab.

The MKS70 power transistor joints are vulnerable when the base of the enclosure flexes. The rebuild page shows how they can be made more robust when replacing the transistors that have untrimmed leads.

Diodes

Diodes, D1, D2, D3 (1SR-35-200) are fast recovery types that prevent positive power rails going negative and negative rails going positive during power up and down. They are very unlikely to need replacing.

Replacement is not shown in the rebuild guide but the parts list shows an equivalent that can drop in, if doing so, be careful to get the polarity correct otherwise the board will “smoke”.

IC

If after replacing all components and checking that they are the correct values, the regulator IC (M5230L) could be faulty. New old stock of this IC can be obtained from UTSource.

Resistors

Resistors change in value with time, some types by as much as 4% over 30 years depending on how much power it they have to dissipate.

Also preventative maintenance may be undertaken to prevent power rails from failing, for example, should R6 become open circuit or develop a dry joint then the 5V logic rail transistor Q1 will attempt to regulate at 15V instead, potentially causing damage.

600-Resistors

What Is Best?

When buying resistors, the price increases with lower tolerance and temperature drift. Note that the figure quoted is the “initial manufactured” tolerance, it can still change with time and “life time stability” is another parameter chosen in products requiring high accuracy over a long life.

Temperature drift, expressed in PPM/C (Parts per Million per Degree C) is another parameter that dictates how much the resistor will vary over temperature. A very low PPM also usually means that the lifetime stability is affected can be worse.

In the SuperJX power supply, Roland have chosen 1% Metal Film resistor types for critical areas, such as the resistors playing a part in voltage regulation. The rest of them are Carbon 5% that are very low cost, have poor tolerance and large temperature drift. If replacing the resistors, the power supply may as well have Metal Film 1% throughout as the impact of cost is minimal.

See the parts list for the resistor specification and example parts that can be sourced from Mouser.

Resistors R4, R6, R8, R9, R10, R14 are normally 1% Metal Film but if you have money to spare, then tighter analogue rail tracking and more accurate 5V rail can be achieved over temperature if they are upgraded with 0.1% <25 PPM types. Resistors like the TE Connectivity H8 series used in high end analogue audio, for example, R8 & R9 replaced with this one here.

 

Rebuild Information

See this page for a detailed technical rebuild guide: SuperJX Power Supply Rebuild

 

Copyright © 2017 Super Synth Projects, Guy Wilkinson

 

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