Both of these models are mini-keyed 4 octave PCM (Pulse Code Modulation) stereo keyboards sporting built-in drum pads. The main differences being that the 520 supports more drums and has internal memory.
The keyboards are intended to be used with DP-1 drumpads and/or SS-1 drum sticks which plug into the stereo 3.5 jacks. The 500 has 2 of these and the 520 has 4.
The PCM sound sources are clear. Currently I am using the 520,but have used the 500,I recall the sounds of the 500 outclass the 520 rather suprisingly. The 520,judging by its appearance on Ebay is less rare than the 500.
The 520 has 12 presets - two sets of 6, and the 500 has 20 - two sets of 10. The bells,synths sounds and human voice on the 500 were what impressed me,and neither of these machines has adjustable ADSR - you're stuck with the envelope shapes of the presets. The tones on the 520 are rather lacklustre and that is why it is currently stuck in my cupboard with its DP-1s,whilst its cousin the 540 is linked to my MIDI system.
The whole point of these two machines is the "SUPER DRUMS" facility - what this means is a series of switches which control whether a drum is in or out of the rhythm sequence played by any of the 12 rhythms.
The 4 position switches provide variants of the drums indicated by the legends and so even though there are 12 rhythms,the variations create variations on the theme depending on where you wish the emphasis to be placed on the beat. The 5th switch controls the same effect for the auto chording mechanism.Setting the selector to manual means that the drum will sound from the pad or from the plug in DP1/SS1.
[Note that it is possible to alter the DP1 pads for use with other equipment besides MT keyboards by adding a piezo sensor between the orange rubber layer rendering them into drum triggers for use with such items as the DZ1. ]
On the 500 switch selectors choose which pad sounds which drum - each pad being changed from 1 of 3 choices.
The autochords support the usual off/fingered/on - meaning that the chords will vamp as played using conventional chord fingering,whereas ON selects the CASIO method where each subsequent key depression above the root adds 5ths and 7ths and majors and minors after the pattern indicated in the manual.
The rhythm unit has tempo adjustment (with LED indicator) ,intro, fill and synchro facilities much as all these type of keyboards do.Seeing as drums were meant to be the speciality of these keyboards they are of higher quality than say the MT540,but there are few of them.
The 520 sports an internal memory which the manual says has 504 note capacity or 677 chords.The 500 has no internal memory.
Both have the ability to mix the rhythm and accompaniment with the main sound using sliders and send the output out of the 3.5 stereo jack or left and right line phono sockets. Both are tunable +/- 50 cents(1/4 tone) using a manual potentiometer.
Both operate from 9v -centre PSU and batteries.
Good Points: Both of these units have aesthetic appeal sporting their yellow drum pads,but for me,the 500 has the edge on sound.Being tunable enables them to fit in with other instruments you might have,this is not always available on such keyboards and is much better to have than not. The drumpad idea is obviously a bit gimmicky - but heck - at the time it sold me.As with all CASIO's of this era the "retro experience" of vintage technology is part of the appeal and being able to plug drum pads into a synth and do the kinds of things that used to take expensive Simmons pads is quite cool - with an MT and a DG10/20 you practically have all the instrumentation to form a band,but even if you cannot play the drums the fun aspect of being able to hit a big yellow pad and hear a drum is quite appealing. The extra phone sockets makes linking to mixers easy and all the variation switches mean there are enough combinations so that you don't lose interest quickly. The PCM drums are perhaps the forte given that the selling point was "super drums".
Niggles: The sound of the 520 is deficient when compared to the 500.As with all these models the net sound is a bit "tin pot",but I have used the 500 alongside other models and some of the presets are viable. I have very few niggles with these machines,they do what they are supposed to do and continue decades afterwards to keep doing it the same way they did when made.
These are well made keyboards and I have yet to find one that has had any massive problem on Ebay-well worth looking out for.