Wednesday, September 9, 2015

Logan/Hohner String Melody II Repair Project

ELECTRONICS REPAIR HAS THE POTENTIAL TO INJURE, CAUSE FIRE, AND DEATH. ALWAYS TURN OFF AND UNPLUG ANY DEVICE BEFORE ATTEMPTING REPAIRS. Do not attempt electrical repairs unless you are qualified and have adequate skills and experience. ALWAYS exercise safety first. This video was generated for educational learning purposes only. There is no shame in taking your instrument to a qualified professional for peace of mind.

Today I'm repairing a vintage string synthesizer. All the G# notes were dropping out due to some intermittent problem. Intermittent issues, due to their nature of coming and going, can be difficult to track down. Luckily for me the String Melody II is relatively easy to service, even without any schematics, with a little background knowledge in how string synthesizers work.

Normally I would start at the power supply, but I have already serviced that area and know it is functioning properly. Since all of the G# notes are experiencing problems, I begin investigating the Master Oscillator. The Master Oscillator, similar to organs from the 70s, generates all the notes for the top octave. These frequencies are generated all the time regardless if keys are pressed. These top octave frequency voltages are wired to another large PCB located under the keybed, which is responsible for dividing down the notes for all the lower octaves.

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