How to fix the oscillators


At last I've now got decent photos of the oscillator board which means I can describe how to fix them. If you know your way round electronics then read on, it's quite a simple fix. Otherwise I would suggest you take this page along with the photos to your friendly organ fixer as it should help him enormously, saving time and your money.

I've found the faults fall into three categories:

     
  1. Faulty divider - accounts for 95% of the problems
  2. Faulty main oscillator - 15% needed attention
  3. Faulty smoothing capacitor - 15% needed replacing
The most (un)-popular fault, and the hardest to find, until now, are the divider faults. The good news is I have a easy fix for you.
The first check is to confirm that the top or main oscillator is running, this is best done with an oscilloscope on the fo tab (see photo). The quick way is to compare with adjacent oscillator boards, the absolute value is not critical at this point. Then identify which divider is faulty, again I used an oscilloscope to see where the divide-by-two stops, move from fo down through f/2, f/4, f/8 etc. noting where the divide by two stops. Sometimes the divider doesn't do the divide-by-two but passes the frequency without change, hence some notes on the keyboard will play the same pitch. Always start at the highest frequency first and work your way down.

 When you find a suspect divider the first thing to do is replace its two transistors, I use BC214L. If that hasn't done it then the magic trick is to add a 680k resistor in parallel with the respective 100k, see photo.
What seems to happen is that 100k resistor goes out of tolerance so effecting the transistors biassing to such an extent the thing ceases to operate. I found it easier to add a 680k than remove and replace the 100k. Either will do.

 On some boards I have replaced all the transistors, and added the 680k resistors, so be prepared to spend some time swapping components. I usually swap the two oscillator transistors as routine anyway, mostly because I've found the legs on the transistors rot away where they solder into the circuit board or where they enter the transistor package making them susceptible to breaking.

If some oscillator boards can't be tuned to the required pitch it is likely it needs capacitors added. I used a guitar tuner to measure pitch, adjusted the tuning core and on some oscillators I found it impossible to tune it to pitch. Use a plastic trimmer tool in the tuning core, don't use a metallic object such as a screwdriver as this effects the tuning, and easily breaks the ferrite core. The solution is somewhat iterative, I added a small capacitor in parallel with the tuning capacitors (refer to photo) until the oscillator tuned. Something in the order of 200pF, solder it on don't use your fingers as this effect the tuning.

On some boards once all operational seemed to produce a strange noise in the audio, I traced this to a 47uF capacitor which acts as a smoothing capacitor. It had simply 'dried out' - replacing it with a new 47uF 10V (or greater voltage) cured it.

In the photo below you will see that I've segregated each of the dividers and oscillator, this makes it easy to find the relevant components. The capacitor/diode link are the components that connect the dividers. These may seem like the most likeliest components when suffering faulty dividers but in fact I've not had to replace a single one.

As a note, I used the 0V to attach my oscilloscope ground lead, the +V rail is 8.0V and should of course have minimal ripple. The vibrato input is a global signal that causes all the oscillators to pitch shift, this is quite an intuitive signal to fault find.
 


Oscillator components
 
 

The board above flipped over left-right look like this...

Oscillator track side



So you now have the relevant information to repair all your oscillator boards. Just allocate yourself plenty of time to do the job, the component cost is very low. Hopefully now Farfisa oscillator boards all around the world can now sing again!?