Friday, October 5, 2012
Farfisa Mini Compact Distorted Divider
The first step is dis-assembly. To get inside one of these, you remove the plastic cap on the top first (the red plastic thing). That is easy enough... just a few screws.
Then you get underneath it (have it standing on its legs) and remove all the thumb screws (5) that are hanging down. You can then lift the whole chassis up and out of the wooden case(but you can't lift it very far, just high enough to sit i on top of the wooden case, as shown below.
Oscillator boards, which are accessible through the back, when chassis is lifted from the wooden case...
Looking at the distortion with a scope revealed that the 'divider' circuits were producing a modulated oscillation, they were not cleanly dividing the incoming wave... they were erratically triggering and including erratic harmonics which didn't belong.
This was a difficult one to troubleshoot, because the problem was somewhat intermittent.
I made some progress by replacing the tuned caps for the effected divider, C3a and C3b, especially when I used appropriate polystyrene capacitors (good and stable for oscillator circuits). But this just made it a bit less intermittent, but did not solve the problem.
I replaced the germanium transistors for that divider, thinking they might be noisy but this did nothing.I have seen them fail too, but when they do they cause silence, or they cause the divider to output its input tone and not divide.
Ditto for replacing the 1uF coupling caps which output the divided signal to the key contacts (circled in yellow below). I have seen these fail as well, but they either cause the divider output to go silent, or distorted in a different way: they don't seem to interfere with the frequency of the divider's output when they fail, and the distortion I was witnessing was a modulated frequency.
I did notice that putting a high ohm resistor across the caps (okay I will admit it was actually my thumb) that couple each divider to the next one downstream would cause the distortion to go away. Ditto for a high ohm resistor coupling one divider output to another! I realized at that point that the caps which couple one divider to the next were the culprits.
Those I have circled in red in the schematic below:
You will notice that the schematic calls for different values for these caps, between the different dividers (470pf, 1nf).
I have on several occasions found these boards with ONLY 1nF caps installed as coupling caps between all the dividers, and they work fine. I am not sure if this is something Farfisa changed as time went on.
Anyway, adding a 1nf Polystyrene cap between the dividers fixed the problem.
Perhaps that original coupling cap, between the dividers, drifted over time and with heat... and did not feed the next divider downstream a good steady waveform to divide.
Anyway, I hope this helps anyone working on this issue!
Another blog by another guy who deals with this type of stuff and seems to have maintained his sanity is here.
Posted by chris rowland