Melosonic 300 Repair

Today we are looking into a melosonic 300 combo-organ. Similar models are Clark/Whippany 300 and the Melosonic 350 (just adds a drum machine). This particular one exhibits the following symptoms:


3 dead keys
horribly out of tune.

lets take a look!

The organ come apart easily enough by removing the woods screws on the bottom side.
The first thing I usually do with these old organs is replace the filter caps in the power supply. This will usually elimate a lot of the noise in the background of the output, and ensure the longevity of the power supply. Remember to Use replacement caps of AT LEAST the same or higher voltage rating. The Capacitance (Farads) doesn't have to match up exactly, but when it comes to the filter caps, its usually better to err on the higher side.


Next to the keys: The main and largest board contains the tone generation and key contacts, and the smaller board contains the tone filtering for the different voices (reed, flute, etc). The way these oscillators work is a bit different then most organs of the day. Instead of having 12 top oscillators (1 for each note) and divide down circuitry for lower octaves, this organ groups its notes in threes. For example, C, B, and Bb will all use the same oscillator . This means only one of those 3 notes can be played at once. The circuitry gives priority to the highest note being played. There are 2 exceptions to this. the first is the bass octave, in which all 12 notes are running from 1 oscillator, and the highest 4 notes, which share one oscillator.

This photo shows the key grouping for oscillators. (the key contacts are the gold wires at the bottom)

Here we see the tone filter board:

The three notes that were not working on this organ all shared an oscillator circuit. After switching out a few components with oscillators that were working, the problem was found to be the germanuim transistor. unfortunately, they were all unmarked, but I found a suitable replacement w/ and NPN silicon transistor I had on hand (C945). I am sure there is a better match, but this one worked fine in a pinch.





Now to tuning:

The notes in each oscillator group are tuned by the large trim pots on the tone generator board. The second to left one tunes the bass notes, and I couldn't figure out the far left one!



The best way to approach tuning this machine is to tune the highest note in each oscillator group. If this brings the other notes in that group out of tune, it is because the tuning resistors have gone out of tolerance, and they will need to be replaced.



The simplest fix here is to just install a trim pot in place of the tuning resistor. I recommend a 2K pot, as most values I found to work were in the 500-1000 Ohm range. Remember to work from Right to left, because tuning the higher notes will affect the lower ones (in a specific oscillator).

Hope this helps!