ARP Omni (Mk1) -1976    Weight= 33Lbs. Number mfg.= ? MRP=$2450

Circuit Overview:
     TOS chip creates HF square waves of octave plus an extra C, and they are divided down to suboctaves with 4520 dual counter chips. All frequencies of square waves are produced on the upper voice board. CD4069 inverter/buffers are used on the 4520 outputs. And on both upper and lower we see a waveform control bus that bleeds a DC reference from the waveform enhancement switch to each signal path which will create an offset for each signal in interacting with the TDA0470 arrays as described below. (ie. it will change the point at which each waveform starts to be diminished by the envelope )
     Both upper and lower voice boards have a string of diodes which isolate the release voltage line from each individual key switch (0V for short to 15V on long releases) so that when a key is depressed, a 470ohm resistor quickly drops the charge on one of the 22uF tantalum capacitors. (500x20micro = 1 millisecond or so of attack time minimum.) When the key is released, the current to recharge the capacitors must flow through a 3.3K resistor and the diode in addition. Thus the mimimum release is set at around 10msec.
     Now, how does this envelope interface? The TDA0470 transistor array is wired with all collectors in common (this is the signal output) and all bases in common (grounded). So as long as a particular emitter pin is at ground or above, no current will flow. So, they create a summing node with resistors, one from a particular key release envelope generator as described above, and the other from the frequency that should sound for that key. In this way the waveforms are mixed as more keys are struck and each one has it's own release envelope. Then op amps sum the related footage mixes as some filtering is performed on 'upper' and 'lower' voice boards to create waveforms for synth, Bass and string timbres. BBD's are used to create the phaser effect.
     Anyway there's a good start. I'll try to finish this up after I get my repair finished up! Just a little problem with the envelope collapsing after reaching peak of attack seg. on the strings. Hmm. It has these circuits labelled "AR supression" and "AR Squelch". maaaybe something isn't quite right in that. These are complex little synths! I hope to frankenstein out my parts machine which appears to have a good filter board and phaser and lower board and power supply. -Bob

Troubleshooting tips:
     Lots of Tanatalum caps that tend to go south on these. I just replace them all. Both the 22uF ones that are abundant on voice boards (envelopes) and power supply ones fail often which includes those used to locally filter voltages on other boards. While expensive tantalums may last longer (though..these didn't...Tellason tells me because on certain settings voltage may exceed 25V slightly.) I use cheap 22uF 35V electrolytic caps and they work just great for the envelopes...as though that's any surprise. Roy Tellason informs me they used Tantalum caps because the tolerences were tighter at the time. These days I don't see a lot of variation in a typical lot of electrolytics.
     Switches are very fail prone. Out of two units I barely got enough for one! They are impossible to clean usually as they use a membrane switch that gets 'bumped' by a metal springy bumper inside. Sometimes they respond some but they seldom come back to good workable condition. The last one I worked on had three failed sections in 4069 inverter chips, a broken lead on a capacitor killing the string signal coming back from the phaser board. ALSO should the problem ever arise, St. Eric notes that the oscillator coil on these is 220uH inductance.

     ANOTHER REASON PHASER MIGHT FAIL.... the circuits can tend to glitch on power up. I noticed this when I was operating on a variac without full 120V but maybe that wasn't the real issue. One board on a unit that came in had only ONE bbd getting clock. The other two multivibrators were latching up. But..if you turn it off and on they come right back! Hmmm. I just replaced the board and all three worked. I sold the unit then..discovered that the problem had returned! IT TOOK SEVERAL minutes of leaking down to get to where they would latch up. All three would latch up IF and ONLY if the -15V supply sank beneath...I kid you not... precisely 4.5mV within a few tenths. I was never able to get just one or two of them to latch up on that board. It was always all three! Fascinating. So anyway the glitch was caused by an overzealous previous repair person who had used 1000UF in place of the 250Uf Capacitor in the power supply. It caused the -15V to rise just a little too slow and at that particular voltage and below caused the multivibrator to latch up.