The Electroluminescent Display Replacement

A lot of older synths from the early 90’s had a form of Graphic display that was revolutionary when new but like the allegory from Bladerunner, the display that burns twice as bright burns half as long. The display used in a large range of synths at the time was made by a company called Optrex and was a 240 X 64 pixel array with a backlight that was made of a special plastic material that was wonderfully thin and for its time quite bright. A quick review of the synths that used this display includes the Yamaha SY77, the Roland W30, the Kurzweil K2000, the Korg 01/W and Wavestation, as well as several other synths of the era.

The Electroluminescent display backlight has a finite lifespan, 10,000 hours before it’s guaranteed to be dead, and in fairness it’s not a great display to view anyway in the HD Laptop screen generation. In the last few months we have had several Kurzweil K2000’s and a couple of Korg 01/W’s which all needed to have their displays replaced..

The backlight system in any of these synths uses a circuit which generates 100 Volts AC at a fairly high frequency because the backlight system requires it. It is analogous to a flourescent light in your home, there is no filament across the tube, but a starter generates the high voltage to ignite the gaseous contents of the tube.

In the electroluminescent display, the high voltage energises the chemicals in the plastic sheet that is the backlight panel, and they glow. Many colours are available, but the lifespan of the backlight panel is poor, after 1000 hours the brightness is halved, after 2000 hours halved again and so forth. Modern LED backlit displays offer excellent backlighting, much better LCD contrast than their older counterparts and the LED lifespan is over 100,000 Hours with barely noticeable reduction of brightness.

The hardest part of replacing the display is sourcing one with the same dimensions and mounting holes, any difference from the original leads to a lot of hassle and a possibly unsatisfactory result. Thankfully a perfectly sized replacement is available from RS components, it’s not cheap but it fits perfectly, and cost wise it is not that much more expensive than one of the many backlight replacement kits on the net, and it is a permanent solution, The downside is the amount of technical work that needs to be done, the transplanting of the Display Data cable from the old to the new is fiddly and needs great care and attention. Once this is done attention must be paid to the new backlight power system, 100 Volts AC at 400 Hz will kill it instantly, so the Inverter circuitry has to be removed (an inverter is any system that converts a low voltage to a high oneĀ  electronically) and a current limiting resistor put in to limit the current (obviously) to the LED’s in the backlight.

Our preference is for transmissive displays, this means that instead of black text on whatever colour the backlight is, the text is shown in the backlight colour (white in these cases) with the surrounding pixels switched off.

To envisage this, think Roland D50, which appears to have a yellow display, but the yellow colour is from the backlight, all the other pixels are switched to black.

The contrast and readability of the new displays is a quantum leap over the original Optrex displays, giving the instrument the quality of display found in modern synths and workstations.

k2k-display-smlNew Kurzweil K2000 Display

k2k-inverter-smlK2000 Inverter circuitry removed

In fact, the display is a much darker blue than the picture shows, the vagaries of the CCD’s in Digital cameras show here. Actually no Phone or Digital Camera we have seems to cope well with white on blue displays.

The LED current limiting resistor is on the underside of the PCB and we replaced the original connector as the original one was trashed by some hacking on somebody’s part.

The Recession Procession

It has been sad to see, but for the last several months (which have been extraordinarily busy) a large amount of our throughput has been equipment which has been dead for some time, or desireable items that need sorting before being sold on auction sites. Part of this scenario is no doubt due to harder times in Ireland but also the rapid move towards Plugins for DAWS which are smaller (obviously) and in some ways more convenient than their hardware counterparts. Probably in this computer digital age the only people that can
afford to hold onto their analogue gear are diehard homeowners with space for a studio and an understanding partner.

The Matrix Conundrum

We got a Matrix 12 in a couple of weeks ago which failed its autotuning system on 6 out of 12 voices.

This synth is in good shape and is one of the few American built ones, it also has aftertouch built in which is rare for an early model. We had an XPander last year which failed to calibrate 2 voices, and replacing the 2 CEM3374 Oscillator
chips did fix it, although several multiplexer chips were replaced on the way.

However this Matrix 12 has all its IC’s in sockets, so changing chips should be easy but which chips.The failed voices are not terminally failed, they are out of tune but not deceased, so logically the multiplexer IC’s would be suspect, but is it the chip, or is it the socket the chip sits in?

Replacing faulty sockets is far more difficult than replacing faulty IC’s. With a suspect IC that you have a replacement for, you simply cut all the pins close to the chip body, remove the chip body from the board and then one by one remove the pins with a soldering iron and small pliers or tweezers.

Cutting up a chip socket is far more difficult, great care has to be taken not to cut too hard and cut through tracks on the board, or cut so thickly that the plastic sections lever the adjacent pins out bringing their tracks with them.

The above scenario’s are only necessary for double sided circuit boards, but all Polysynths from about 1980 onward had them, so it includes pretty well everything you are likely to see.

It is good standard synth repair practice to put high quality sockets in the circuit board after a chip is cut out. The reason being that any PCB can only take so many rework attempts before pads break and tracks lift, so if a chip has failed once or is prone to failing, it’s a good idea to make it easily replaceable in the future.

The Matrix 12 and XPander both use a weird hybrid switch mode power supply. Most switchers convert raw mains into 400 Volts DC and then switch this energy at high speed through a small precision wound transformer which generates whatever voltage rails are necessary after being rectified and filtered. Voltage regualtion is done by feedback from the most important output back to the main switching transistor circuit at 400 Volts. Most modern equipment uses these supplies, including PC’s, most Samplers, most modern synths etc.

The Matrix differs from this slightly in that the main transformer is large, and produces about 9 Volts AC which is rectified and converted to 15 Volts DC which is then fed to the high speed switching system. This then goes through the high speed transformer and is rectified and fed to normal analog voltage regulators. It’s not as efficient as modern switchers but it is a lot safer to work on as all the switching circuitry runs at low voltage.

There are strange waveform artifacts on the Matrix 12’s power rails, so the PSU will have to be stripped, checked and rebuilt too see if this explains some of the Auto Calibration issues.

Our apologies for the Big Bang Theory headlines, they are deliberate, if Chuck needs someone else to help name episodes, we are available, not cheap but available.


Leave a Reply

Your email address will not be published. Required fields are marked *