Old Crow's Synth Shop: Maintaining the Yamaha CS-80

The Remaining Problems

5) Of the LEFT, GENERAL, RIGHT audio output jacks, only the RIGHT and GENERAL jacks are working. The same is true for the PHONES ouput jack.

6) On voice #8, aftertouch LFO modulation was not working.

7) On voice #3, the aftertouch->filter brilliance response was sounding odd; only the high-pass filter seemed to react.

The Remaining Repairs

5) Taking a break from the aftertouch problems, it was time to look at the output jack problem. Using an audio signal tracer (my tracer is simply a little battery-powered amplifier from Radio Shack with a cord plugged in that ends in clip leads), I verified that in fact both synth sections were providing audio to the 'I/II Mix' control. Following the large fold-out overall schematic diagram, I then proceeded to trace the audio signal as it made its way to the output jacks. After the mix knob the two signal source are mixed together into one signal and passed through the expression/wah circuit on the PRA board. The output of wah filter circuit was OK.

The signal then gets buffered and sent to the chorusing/tremolo 'OE2' circuit board where the mono signal is turned into a stereo effect when the chorus or tremolo is turned on. Note that if the effects are off, the output signals from the OE2 board are the same. Tracing the audio signal through the OE2 board showed that things were still fine. These signals then proceed to the OE1 board for blending: 4 VCAs are used to 'move' the audio around the stereo image on the OE2 board, and these 4 VCA outputs are mixed back into 2 signals on the OE1 board. The signals then return to the PRA board for final line-level amplification. The output signals from the line amp circuit section were incorrect: the RIGHT signal was fine, but the LEFT signal was very faint. The problem must be in the amplifier circuit for the LEFT channel on the PRA board, then.

After studying the circuit, it became apparent that aside from a chip failure (unlikely), a wire had to be loose. Using the digital volt-ohmmeter, I measured the jumpers in the local circuit area for continuity, and then moved on to measure the resistor values. On the 100K resistor that attenuates the LEFT signal coming from the OE1 board, a value of 800K was measured. Close inspection of the solder pads for this resistor revealed the resistor lead had broken loose from one pad and was making poor electrical contact. One resoldered connection later, the LEFT output was working fine again.

6) Voice #8, which is only activated in certain cases of the key assigner's operation, was not receiving LFO modulation from the aftertouch pressure on the corresponding keyboard key. Using the overall schematic, I determined the circuit of VCAs responsible for passing the aftertouch modulation to the voice cards for voice#8 were on the TRG5 board. Measuring voltages on the VCAs in this circuit subsection revealed that while the panel setting (DC voltage) was getting passed through to the LFO amount VCA, the LFO amount VCA was not outputting any LFO signals. Inspecting the circuit with the digital volt-ohmmeter, I discovered that a jumper had one end unconnected. I can't be certain, but from the way the solder pad looked I suspect this problem came from the factory itself. There was no evidence of a broken or 'cold' solder connection; it simply looked like the wire never made it all the way into its hole so as to have solder applied to it. This means that for more than twenty years this particular CS-80 had no LFO<>aftertouch operating on voice #8.

Once the jumper was reseated and soldered, it worked fine.

8) At first, I thought the problem was due to a bad envelope generator chip. As it turns out, moving the panel slider for aftertouch brilliance on synth section II caused strange reactions in the filter. Learning this, the big schematic was consulted and it was determined the aftertouch brilliance controls were ultimately used by the TRG2 board. Inspecting the trimmers (I always check to make sure the 'CV reject' trimmers for all these VCAs are set to something that looks normal) for the VCA that passes the aftertouch brilliance setting, I noticed the trimmer for that particular VCA was jammed all the way to one extreme. This produced an offset of several tenths of a volt, enough to foul things up. Adjustment of the trimmer resulted in a properly-operating aftertouch brilliance effect on the section II, #3 synthesizer voice.

At last, the machine was 100% operational. I did replace two cracked keys with two keys from a CS-80 once used by Toto.

Scott Rider -- oldcrow@oldcrows.net