Sunday, June 2, 2013

Another Farfisa Problem : hum in outputs, reverb tank, and bass tones

For those of you wondering how the bass tones are developed on your Farfisa compact (the black notes), they are simply the lowest octave notes sent through a filter. What I mean is, there is not another divider on the oscillator cards for those lowest octaves, those notes are just the same ones as the lowest octave but are routed through a filter before being sent to the outputs.The card with the filter is shown here:

The signal goes into that card from the contacts for the bass notes, and comes out and is routed to the output.

Another note:

 The old high voltage capacitors shown in this picture, below, were completely dead in a unit I recently worked on, and this added a lot of hum to the outputs. These caps need to be high voltage because they filter the power to the tube pre-amp. Apparently the Farfisa used this style preamp because they wanted it to work with their pizo-electric reverb tank, which is discussed on this blog .
This style of tank was used for reasons discussed here.

Since the tank was completely destroyed in the unit I was working on, I did manage to get a more traditional spring reverb tank working in the farfisa, but it required that I add a preamp in the circuit to drive it, not on the recovery side but on the driver side.I also replaced the rectifier, shown here. It is a more modern one and I was ultimately able to attach it to the inside of the chassis for the preamp.

1 comment:

  1. Hi, I cam across this blogpost searching for more info on the compact reverb tank. Mine was busted too; I replaced it with a high-impedance accutronics type but, as expected, it's pretty weak. I was thinking about boosting the signal a bit using some solid-state circuitry but haven't given it much thought yet. What kind of pre-amp arrangement did you settle on? Thanks