The Yamaha CS-80

The CS-80 is large and imposing. It weighs 100kg, around 220lb with it’s cover, accessories and wheels. It was produced from 1976 until 1980. It consumes 180 watts of power, a huge amount for a synthesizer – By comparison a Roland Juno-106 uses just 25 watts. It is a 16 oscillator 8 voice synth with two complete synth channels per voice so each patch can play two different tones on each key. It is notable for it’s polyphonic aftertouch and velocity sensitivity, it’s equipped with a wonderful ribbon controller and powerful ring modulator. It is a true live performance synth, it’s expressive capabilities are unmatched. It has a somewhat undeserved reputation for tuning problems – It needs to be approached much like an acoustic piano: located in a room with stable temperature and tuned at regular intervals. Having said that we have used a CS-80 live in recent years and it got through an entire show perfectly with a quick tuning touch up just before show time.

In this post we want to take you on a visual tour inside the CS-80 to show you the normally hidden world within so that you can see how Yamaha approached the construction of this ground breaking and massive instrument. Click on the images for a closer view!

The CS-80 is an extremely heavy instrument. Yamaha built it very solidly and due to it’s power requirements the power supply and transformer are quite massive. Once the top panel is lifted to it’s open position there is a card cage that holds dozens of circuit boards that are all connected together with thick wiring looms. There are very few connectors inside the CS-80, almost every board is hard wired to the rest of the machine. This card cage can be pulled upwards and locked into place to allow access to the cards for repair and calibration and great care must be taken to avoid breaking wires. The rails that the cards are screwed to are live so servicing them requires patience and care to avoid shorts and static electricity damage when working on cards when the machine is turned on.

Servicing them also requires a solid work bench – at 100KG the CS-80 is not an easy machine to move around, once it’s on the service bench it needs to stay there until the job is complete. A full end to end calibration is a time consuming job.

Here is an image of the card cage in it’s raised position. This is a view of the RHS showing the 16 voice cards. Altogether there are 35 cards in the rack containing most of the non-keyboard related circuitry within the instrument. At the bottom of the image you can see the long weighted bars attached to each key to give the CS-80 keyboard it’s semi-weighted touch. Yamaha did not cut any corners in the construction of the CS-80.

One of the reasons that the CS-80 is a challenge to service is that there are almost no connectors inside – Everything is hard wired to everything else. most boards have fine wires soldered all around their edges and so that cannot be removed from the machine. It is very easy to break these wires at the point where they are soldered so during servicing board handling needs to be kept to a minimum.

All of the CS-80’s circuitry that is responsible for the keyboard is located beneath the key bed. To access it requires removing some large bolts, sliding the key bed a few centimetres towards the back of the machine and then swinging it upwards. There is a metal bar clipped to the bottom of the case that can be swing up and guided into a hole on the underside of the key bed to hold the key bed in the raised position, much like the hood of a car. And here especially you can see just why the CS-80 is so heavy – the plywood construction is massive. Once the key bed is raised the circuit boards below are exposed. The card cage needs to be locked into it’s up position before the key bed can be raised. It is typical of Yamaha builds of that period – Quality definitely took precedence, everything solid and carefully thought out.

CS-80 service manual

The ribbon controller and patch preset buttons

The CS-80’s power supply is large and heavy. The mains transformer is the heaviest component, in front of it are the large black heat sinks with the regulator power transistors and in between is the circuit board containing the DC rectifier diodes, the voltage regulator circuits and the large capacitors for smoothing out ripple and noise that would interfere with the synth’s circuits. It’s all mounted onto a steel chassis. As the power consumption is rated at 180 watts it does run quite warm which adds to the tuning issues that occur, particularly if you cover the top grille of the synth. It’s never a good idea to sit another instrument on top!

As with all poly synths of this age it is important to give some attention to the power supply for two main reasons – Firstly it is the part of the machine that endures the most heat and a lot of punishment from power surges. Secondly in order to protect the valuable components inside the machine the power supply must always be in good shape to prevent avoidable damage due to an over-voltage failure caused by an aged component.

Here is another view showing the mains transformer and the heavy physical construction of the synth. You can see the ventilation grilles designed to allow heat from the circuitry to escape rather than building up inside the case which would shift the oscillators out of tune. Air flows from the vents in the bottom of the synth and rises past the voice cards and this is a problem because any change in the ambient air temperature will cause drift, hence the necessity to use the CS-80 in a room with a stable air temperature. The thick black snake to the right of the image is one of the wire bundles connecting the boards in the card cage to the keyboard circuitry below.

When new the CS-80 shipped with a set of removable castors that socketed into the rear of the synth when the lid was in place to allow it to be rolled along the floor. This made it appear to be designed for gigging but really the machine was too fragile to be bounced in and out of trucks and venues day after day. Many brave bands toured with CS-80’s but they were never suited to life on the road.

There are 16 voice cards in the CS-80, referred to as “M” circuit boards, two for each voice. The board contains a custom VCO IC that produces a sawtooth wave with a little pulse at the front edge. This feeds in to a waveshaper IC referred to as a WSC that converts the sawtooth into the other waveforms – Triangle, pulse, reverse saw. The board also contains two VCF IC’s, one for lowpass and one for highpass and two envelope generators, one for filter contour and the other for the volume envelope. There are six VCA’s on the board –  this is one of the interesting features of the CS-80 – The synth contains hundreds of VCA’s. A full calibration to a CS-80 is a big job – there are many trimmers on each voice card and they are on both sided of the card so a good deal of care and time is needed to adjust all 20 trimmers on each of the 16 cards, not to mention all of the other trimmers scattered throughout the machine.

Beneath the key bed are five circuit boards that deal with all key related functions. The large KAS board handles polyphonic key coding and assigning via large mil spec custom ceramic bodied IC’s, D-A conversion and it handles signals from the pitch ribbon circuits and master tune control as well as portamento, glissando and sustain. From there the SH board processes the CV’s that feed the voice boards. The TKC and TSB boards deal with the polyphonic aftertouch and key initial touch. The gold metal box on the left is the storage compartment for the power lead. These boards are the most problematic parts of the CS-80 due to ageing of the CMOS IC’s and capacitors and because of a curious design omission on Yamaha’s part whereby necessary capacitors were left out of the board design.

These IC’s and capacitors are available new and are easily replaced which dramatically increased the reliability of the synth. Replacing the old IC’s and capacitors will not alter the sound or functionality of the synth but they will fix many issues with aftertouch, pitch and voice triggering.

The CS-80 was produced before the age of digital patch storage but it came equiped with four analog memory locations – two each for the upper and lower voice banks which can be selected from the row of front panel preset buttons. The CS-80 presets are composed of cards with sets of resistors that act like fixed controls that create each static preset. The memories are effectively the same but the fixed resistors are replaced by mini sliders that mimic the layout of the main controls on the top panel. These are located beneath a flip-up panel on the top LHS of the synth. The idea was that once you have created a patch using the top panel controls you then copied it using the mini sliders. In practice it is simpler to just create your patch from scratch using the mini sliders. Really it is easier to just think of the CS-80 as having six sets of controls, two full size and four mini sized.

Back when the CS-80 was new some artists were rumoured to have engaged their techs to go into the boards with the fixed resistors for the preset sounds and alter their values to customise the CS-80’s fixed factory preset buttons into new patches more to the artist’s liking.