Troubleshooting a strange problem with an Oberheim Matrix Six
I was helping a friend revive a Matrix Six. We were getting very strange sounds from all six voices, and decided to have a look at the DAC output. The waveform at U712 (TL081) pin 6 looked good. Stepping through lots of different voltage levels. But when we checked the output of U710 (TL082) at pin 1, it was sitting at a negative voltage and never changing. Also the voltage at pin 2 of U710 was not 0V, but was at about +3V IIRC. We weren't sure if the problem was U710 or the 4053 analog switch IC that sets different gain values for this op amp. Since 4000 series chips seem to have a high failure rate, we cut out that chip (U711). The voltage at the output of U710 now looked good. So we installed a socket and a new TI CD4053BE at U711, after removing the pins left over from the old chip. When we powered up the synthesizer, the sound was still very strange and we noticed that the new chip we had installed at U711 was getting rather warm. We also noticed that pin 16 of this chip was at about 4V, not the 5.6V shown on the schematic. We checked the 10K resistor and diode (R40 and D4) used to generate the +5.6V. They were fine. (The lower end of D4 connects to +5V, even though my schematic copy seems to show a -5V connection.) With our new 4053B unplugged, we got 5.6V at pin 16 of the socket. So it seemed like that chip was drawing too much current, and pulling down the 5.6V supply. After much head scratching, I think we found the problem. The spare gate of U711 (pins 3,4,5) is tied off. Pin 5 is tied to +5V, not 5.6V. This voltage will come up right away when power is turned on. The 5.6V VDD supply at pin 16 of the same chip will not come up right away, as the circuit that generates it has a 10K resistor and a 10 uF (C37) capacitor. So for a very short time when power is turned on, there will be 5 volts at pin 5, and less voltage at the VDD supply pin of the chip. This is the standard way to cause latch-up in a CMOS IC. Take an input beyond one of the supply rails with a voltage source that can supply a fair amount of current. Some CMOS chips are more prone to latch-up than others, so possibly the original 4053 chips that were used did not show this problem. Anyway, bending out pin 5 of the chip so that it did not go into the IC socket solved the problem. The sound is normal, and U711 is no longer getting warm. Pin 5 could also be tied to ground. Pins 3-5 are a spare switch which was wired up, but the output at pin 4 is not connected to anything else, even though there is a short trace on it (strange). Hope this helps someone else who is seeing a strange problem with U710/U711 in an otherwise very cool Matrix Six analog synthesizer.
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