Korg Lambda ES-50
Korg Lambda ES-50
One of my clients is in a rock combo band. To add a bit of vintage vibe to the sound, they use a Korg Lambda. This model is also known as the ES-50. It is a fully polyphonic string synthesizer, and includes brass, piano, and clav sounds.
They actually have 2 Lambdas. One of them sounds better, so they use it in the studio. The other one is a bit beat up, so they use that one for gigs. This is a smart move, as vintage synths can be temperamental, and they can develop problems at any time.
The Lambda they used on the road got a lot of abuse. My client told me that it had beer and other drinks spilled over it on several occasions. Other than smelling a bit funky, they would just clean it up, and kind of forget about it.
However, after time, weird things started to happen. Some presets were a bit flaky, and it started to sound thin and weak.
Once I got it into my shop, I noticed that every B key sounded thin with the string or orchestral sounds, but the piano and percussive sounds were good. This is a bit of a clue on the architecture of string synthesizers from the late 1970s. Let's get into a little bit of synth theory...
A typical monosynth like a Minimoog or Odyssey has 2 or 3 oscillators. So, obviously a fully polyphonic string synthesizer must have at least one oscillator for every key, right? Sort of.
In reality, most string synthesizers generate all of their waveforms from only one oscillator! The Lambda actually has 3 of these, but we're getting ahead of ourselves.
TOG and Divider Block Diagram
TOG and Divider Block Diagram
The development of the TOG and divider chip caused an explosion in the manufacture of cheap home combo organs. Before that, organ designs needed dozens of separate oscillators, or complex sets of rotating tone wheels and magnetic pickups. All of this complexity was expensive, and a home organ was a luxury.
However, once you could design a keyboard with a single oscillator and a few chips, everybody was making them. Department stores even had entire lines of cheap home organs.
In a string synthesizer, a single master oscillator runs at about 500 kHz, well above the range of human hearing. It is fed into a special purpose chip called a Top Octave Generator (or TOG). The TOG uses the master oscillator to synthesize the 12 notes in the top octave of the keyboard.
Each one of 12 notes in the top octave are then fed into another chip called a divider. The divider simply halves the frequency of the note, over and over, to generate all of the lower octaves. It works just like a 'suboctave generator' guitar pedal.
There are 12 semitones in an octave, so there are 12 divider circuits in total.
For those following along closely, you'll note that the oscillators in the Korg Lambda (and almost all string synthesizers) are completely digital. (Gasp)
Tune Controls
Tune Controls
To enhance the sound, and make it more warm and musical, the Lambda has a trick up its sleeve. It has THREE sets of TOG and divider chips. Two of the sets of waveforms are combined together for the string and orchestral sounds. The third set is passed through a VCA with a short attack-release envelope, for the percussive piano type sounds.
Using a front panel control, it's possible to detune the oscillators, to give a fuller chorus effect. Also, each of the master oscillators can be modulated by an LFO, which creates a phasing effect.
TOG and Divider Circuits
TOG and Divider Circuits
In this photo of the main circuit board, the chip at the top is used to sum the waveforms for the string and orchestral sounds. The chip in the middle (labeled IC-2), is one of the TOGs. The vertical boards to the right contain the divider chips, but they are facing away from the camera.
The TOG chip used in the Lambda is an S50241, manufactured by Mostek Integrated Circuits. This is the same one used in many other organs and synths, including the Korg BX-3 and CX-3, Hammond JM3, and lots of Conn keyboards.
The rows of green capacitors and resistors at the bottom form part of the attack-release envelope circuits and waveshapers for each key.
TOG Output Waveforms
TOG Output Waveforms
Since every B key sounded thin, the first thing I did was scope the output of all three TOG chips. Sure enough, one of them had a very anemic looking output waveform for the B.
The trace on the top is from the B-output of a good TOG, and the trace on the bottom is from the faulty TOG. The faulty waveform was very noisy, with a low amplitude.
The waveform here is a 30/70 pulse, and not a symmetrical square wave. The S50241 comes in two flavors (the S50240, and S50241). The 240 produces square waves, and the 241 produces pulse waves. Each gives a different set of harmonic overtones, and is chosen by the synth designer based on what type of sound they are looking for.
Replacement TOG In A Socket
Replacement TOG In A Socket
Here is a photo of the replacement TOG, in a new socket. The original S50241 is no longer being manufactured, but SGS Thomson makes a modern equivalent chip called the M082B1.
Moldy Switches
Moldy Switches
Other than the B keys, this Lambda had a general flakiness about it. If I wiggled the patch selection switches a bit, things started to work better. This would normally indicate some type of dirt or corrosion in the switch assemblies.
Once I looked at the switches a bit closer, I noticed a weird yellow mold all over the place.
Obviously this is residue from the various drink mishaps. I ended up dismantling all of the switch assemblies, and cleaning them with a small brush and some isopropyl alcohol (ironically enough).
It's always a good idea to inspect the power supply on any studio equipment more than about 25 years old. Capacitors start leaking, voltage regulators and rectifiers run hot, and components start to fail. Some basic preventative maintenance can go a long way in extending the life of your equipment.
Korg Lambda Power Supply Rebuild
Korg Lambda Power Supply Rebuild
The capacitors in this Lambda were due for replacement, and the entire power supply needed a good cleaning. Here is a before and after shot. A buildup of dust and grime on heatsinks and components can affect heat dissipation, and ultimately ruin your power supply (or your gig).
