A friend of mine bought this vintage keyboard: a Farfisa Matador (Mod. 611-E) coming directly from the ’70s. By the way the seller forgot to mention that, despite being very well conserved on the outside, it produced a strong and unpleasant hiss that made it almost unusable; it sounded like a out-of-tune chord, so the guy said it was just some dirt under the keys. False…
So I had the honour to put my hands inside this beautiful keyboard. I want to share this experience with you posting some photos I made during the troubleshoot process (se serve una traduzione in italiano chiedete nei commenti).
Serial n° 6338/147
The Matador is based on the subtractive sysnthesis in which 12 oscillators (one for every semi-tone: DO, RE#, RE, MI …) produce 12 square waves at fixed frequency. These signals are frequency multiplied and distributed across the octaves of the keyboard. When a key is pressed, the square signal is routed toward the the filters (treble and bass filters), the vibrato/percussion board and, finally, the output amplifier.
As I said, a continuous unpleasant noise was plaguing the output sound of the keyboard. So, with my oscilloscope, I started to trace back the noise from the output amplifier toward the oscillators, in order to find out its origin.
At last, I found the problem. The Manual Bass circuit has a small pre-amplifier, which takes its power supply from the 12 V line. In order to reduce the noise, a large electrolytic capacitor is used as pre-amp power decoupler (It’s called Cc31, in DWG4, if you downloaded the schematic).
By the way, it turns out that after almost 40 years of life, this capacitor has lost its ability to filter out the noise (and there’s a lot of noise in this keyboard: there are 12 square-wave oscillators and lot of wires everywhere).
The solution is quite simple: I replaced the old component with a new one of the same value (1000 uF, 16V).
A dust cleaning and minor adjustments were the only additional interventions needed by this awesome Farfisa to come back to rock !