Just scored a cheap MFB-522 and post a couple of PCB pics and descriptions here, which might come handy in case you need to repair yours. Check also my Dimonion1 and my Synth Pro pages.
First of all, manuals:
https://medias.audiofanzine.com/files/manuel-simple-mfb-522-478076.pdf
https://archive.org/details/synthmanual-mfb-522-owners-manual
That frankensteinered capacitor is fat – and so is the sound of this little MFB (lol)…
Quick inventory
Power: PSU 15V DC, at least 300ma (best use 1000/1200ma); 3x voltage regulators: L78L05 (+5V), L78L12 (+12V), L78L06 (+6V) — if I get this right, there is no negative rail in this machine. I tested op-amps for supply and all were +12/0V…
Digital Control: ATMEL AT89C4051-24PU Microcontroller; ATTINY13A Microchip; TDA8444T Octuple DAC; LC549C ADC; 24C64WP EPROM; CNY17-1 opto coupler
2x HC505C shift registers; HEF4053 switch (switches the “B”s –> RS, Clave); 2x HEF40106 Hexinverter (one for the metal noise); Dual op-amps are TL062.
Transistors, diodes: BC487 NPN; BC587 PNP; BZX84C – ZENER DIODES (SOT23); single Schottky diodes too
Trimmers – adjusting sounds
Trimmer 1 (leftmost) – BD maximum length: Set decay to maximum and tune to minimum. Turning the trimmer anti-clockwise sets your max length. The latter can be near infinite.
Trimmer 2: Tom/Conga maximum length: turning this anti-clockwise increases the maximum decay of ALL toms/congas. Best play a midi/sequence loop going though all sounds and then adjust to taste while listening to that sequence.
Trimmer 3 – Hi Hat Oscillators Tune: While Cowbell D-tune and Cymbal Tune affect these potions of the metal noise that are prominent in CB and CY, this trimmer here tunes those metal noise oscillators more prominently featured in the hi hat sounds. The scope is not extreme, but you can nicely tune in some in/harmonic shimmer as per taste.
Trimmer 4 (rightmost) – CLAVE length. Tuning this trimmer anti-clockwise lengthens the clave sound.
Modifications
Instrument Panning
The instruments of the 522 have fixed panning, which is simply done by means of fixed resistors. Instruments in the middle pan position, such as the kick, simply have two identical resistor values for left/right. For instruments which are panned, you have different values per left/right. In order to shift their panning, simply change the resistors. As you can see in the pic above, tom/conga instruments share some resistors. If you find any of the instruments generally too low in volume, you can also change this easily by reducing the corresponding resistor values.
More snappy on the snare: the mfb snare is cool but I wanted some more snappy action, so I changed the snappy filter a little bit. On the picture you see a 3k3 resistor which I simply soldered over the 22k resistor that was originally there. No need to remove that 22k, just add your 3k3 on top.
sounds like this:
Longer snare snappy: simply put a 1uf capacitor across the cap marked “SNAPPY L” on the pic below…
Longer snare trigger: this affects snappy and also the underlying t-bridge plops. I like this quite a bit and put in a 1 uf cap between the cathode of teh marked diode above and GND. A/B demo here:
CLAP SWOOSH: if you put a 100nf or so cap across the diode marked in the above picture, your clap trigger gets longer, and the tone shifts a bit upward too. Sounds like this:
Longer Clap Reverb: If you want to prolong teh clap reverb, put a 1uf cap between the cathode of the diode marked “CP_REV” as per above pic and ground. The clap decay knob still works as advertised, yet max time is longer now. Similarly, you can implement a “reverb kill switch” of sorts by wiring an ON/OFF switch between ground and that very same diode cathode.
More clap body: the noise component of the clap is highpassed through a cap. By adding a cap on top (I used a 560nf cap) you can lower the fundament a bit. Can be done from the back of the pcb.
Sounds like this ( i tweak decay, filter and attack while playing)
Potential Trigger Inputs: Triggers (velocity sensitive!) are sent via the 8xDAC to individual sound components. A couple of these have diodes at the trigger entry point (I marked some of them yellow below). The cathodes of these diodes would be your pint for sending in external triggers (I’d stay below +5V and send these through diodes as well, so you have a nice AND-gate). Haven’t tested this though!
USE THE SWING TYPE VCAS: Like on the TR-808, all sounds with noise components (metal noise or white noise) have these little swing type VCAs (check my RD8 page or XBASE888 page for more info) that regulate the flow of noise component in the signal. You can send in CV (stay within zero to max +5V here!) to some of the diodes in the circuits and hear regulated noise action going on – here i simply use a LFO for noising…