Casio Guitar Synth

Over the past few years I have repaired some 45 of these guitar synths from the basic MIDI only MG510 series to the full blown PG380 with its synth module and MIDI too. There are a number of changes that can be made to the electronics to improve its response and its action under MIDI and whilst I can't do much to the typically 80's Casio sounds the changes do help a little. Since these are still going for some £800 plus on EBay and there are still several around that are broken, they are well worth the effort.

Some say that the notes are delayed from the synth but it depends so much on your playing style and if you are using a down stream MIDI voice box that I don't think it really matters. It is a consistent effect. Delays that do occur is purely an action of the A-D conversion that is shared across all 6 strings running in real time and shared between the note frequency and the envelope and the interpretation of the data by the VX chip set in the voice modules. The analogue input from the synth pickup requires amplification, filtering, limiting, clipping and envelope following and multiplexing. To get all this into a tiny board in the body of a guitar is no mean feat. There is a six note polyphonic synth built in the guitar in the PG380 series whilst the MG510 series has MIDI only which is better for battery use and of course the sounds are then much more diverse as you have to use an external box. There is a MIDI out and a voice card socket in addition to the 60 or so sounds already available. Add to this the fact that it is a complete Fender Strat copy in its own right and you have a great one man band solution. There is a lot of "The Kings New Clothes" attitude to musical hardware much as there is in hi-fi and computers. I always think this is a shame. Guitar synths are an excellent case in point. I have spoken to many musicians who tell me that, in the last 15 years they have tried all the new guitar synths both add on boxes, and dedicated units and none of them are any better than the Casio. Technology doesn't always get better only different
There are a few bugs with these old units. These can be split into two sections. The first is purely mechanical. With so many cut outs on the back of the guitar the screws come out and get lost; the sockets (two quarter inch jacks, midi out and power in) all deteriorate and the battery unit gets the AA cell "dead battery" corrosion. The second, electronic ones, can result in loss of a single output from one string; noise due to power supply break through as either hum or buzz or false triggering due to finger trouble. Considering their obvious complexity they are however very good and reliable devices. The MG series has one specific fault caused by the user. Often they arrive dead with the 5 volt regulator blown off the board due to an incorrect external PSU being used.

The analogue board needs careful repair as it is mostly surface mount. It also contains the multiplexor switching. Most faults start with the decay of the surface mount electrolytics. A much better option is a small conventional electrolytic as the boards will likely be damaged by the faulty caps. Faster op-amps for those that have failed also help performance. I use the 2068 as in the original but have also used LM series and Texas op-amps too.
Switch off the synth, then holding down the octave up and down buttons power up the synth. The display now shows the levels of each string as it is plucked. This is dependant on four things. The force of the string plucking, the level of the 6 way pick-up, i.e. how close it is to the string, the weight of the strings and the level of the pre-set. Individual strings plucked hard should give an initial read out of 75-95 on the display. They should all read about the same. Too much and the display shows oL meaning overload. Only small adjustment of the sensitivity is required and be aware that they are extremely delicate pre-sets on a surface mount board! One other thing about adjustment. There is inbuilt tuning on the guitar. Should it be substantially out of tune it will not only take longer to lock up on and sound a note but it will also note jump as the string vibration decays.

So a few specifics.
Missing notes are generally due to failure of caps, then surface mount op-amp IC's, on the analogue board.
Odd clicks, pops and thumps always disappear when you replace the caps on the ADC board i.e. the lower of the two in the two board sandwich with the analogue board.
Dead all round although the display is on can be due to either failure of the mid point voltage section on the power input socket board but can also be due to failure of the MUX circuitry formed around the CMOS 4066 surface chips on the analogue board. If the MUX stops then there is no feed into the ADC and hence no output.
Be aware that if the user uses an external power pack then there are a lot of SMPSU's that, under load produce far more than the 9volt the Casio needs despite them being supposedly regulated and high current. I prefer the old transformer and standard three pin regulator for this unit as it is more predictable. The protection diode can fail when using the SMPSU for no obvious reason even if the polarity is correct. I assume its harmonics and noise in the supply.
Output level on the mixed output can be increased by a simple change of resistor on the input output board to raise it to nearer a line level signal. This can be very useful!
Beware of trying to upgrade the guitar to active normal pickups and running the electronics from the same supply as you will have to change / upgrade the suppresion on the power lines by a significant amount. Also be aware that the toggle switch actually shorts out the outputs when in synth or guitar mode so that the opposing output is killed. This has the effect of loading any circuitry. If your new pickup circuitry doesn't like this it will be goodbye to the active bits! The synth circuitry is noisy and is easily picked up by active pickups. You will need screening on everything, supply line filtering, and careful consideration of wire routing. It does work though!
The main synth board has a number of surface mout capacitors on it that should all be changed. You may well see the remains of these capacitors when you take the lid off. The most troublesome seems to be located very close to the underside of the expansion slot which can be nothing more than a empty shell with the internal contents spread around the inside of the guitar.
There is no sensible upgrade to the sounds on the PG380. This is a shame but as there is MIDI there is little point in doing the research and reverse engineering to find out if it can be altered! I have though a spare non working unit so you may find at some point any modifications that are worth doing.

The simpler synth less MG range is slightly different. The same rules apply for the analogue board although its layout is slightly different it does the same thing. Biggest failure is the on board 5volt regulator which invariably blows up when the polarity is reversed. It also fails of its own accord.
The MG range is a little easier to work on as the boards are better arranged. Likewise there is less issue with noise if you want to go for active conventional pickups.

Further modifications include fitting a single multicore connection for all the IO on the end of the guitar. I use a camera type round locking connector with 10 pins. This is ideal for the PG range for power, MIDI and audio out and fits neatly in the now vacant MIDI out hole. A downstream breakout box can then be used at the other end. When this mod has been done it is then an easy matter to put in a further mod with rechargeable batteries, especially in place of the PG memory card slot which is usually redundant. This allows for better battery capacity and improves the function when used with the active pickup conventional guitar set up. So far I have only done this once as it is expensive and time consuming!


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