ARP 4-voice/16-voice - 1978    Weight= 53 Lbs.(4 voice) Number mfg.= ? MRP=$

User Manual: Manual Manor
Reset Proceedures: N/A
Operating System code: N/A
MIDI or other control protocol: N/A
Software related Links: N/A
Patches or knob settings: N/A
Circuit Overview: below
Scematics/Service Manual: If 'music parts' guy comes back. keep calling him.
Common Service Issues/Tips: below
Parts Sources: Keys knobs semiconductors misc
Uncommon chips/modules used: CA3086 transistor arrays (Osc), M083B TOS, AMI S10430 divider/keyer
Modifications: Use your imagination! Turn it into a real synth!
General Info Links: Synthesizer Museum

Circuit Overview:
      4 and 16 preset instruments with extensive analog signal conditioning over a tone generation similar to korg Lambda. Except there are 2 instead of 3 clocks. There are two identical gen. boards, each with 2 TOS chips and 4 divider/keyers offering a wide range of tones to work with and two independently clocked 'ranks' that can be detuned from one another. The upper board's left TOS's signal comes from the 4069 chip by the interconnect that goes to the PS/osc board and Z11 (another 4069)Pin 10. pin 11 the input for that inverter is tied to pin 12 and pin 13 it's in is tied to the clock source but also the clock in for Z8 which is a 4520 counter whose Q3 outputs are the clock source for lower generators. The right side TOS's clock follows that same kind of path but uses Z12 on the PS board with same pins on it. Hopefully that will save time for someone else. IN SUMMARY the HF clock signals can be tuned together w/main tune and trimmer on right side of PS board. And detuned only useing Detune on panel. And the counter drops that frequency 3 octaves for this 73 key keyboard to be split in two halves; allowing more specific tonal control for lower and higher notes. The panel board also has a bass / treble eq. and vibrato speed/depth circuitry. I didn't have to pull that board but did get a schematic copied for that and the board with the mixing amplifiers, 4016 analog switches and all the test points. If anyone needs help please contribute some if you can. I spent a lot of time on this one.

Service Tips:
      OUr 4-voice had bad half-wave distortion/noise at the final op amp in front right of mixing board. I thought maybe 4007 chip or the op amp but... tracing back i found control for mosfets in 4007 was just pos. rectifier out (17.5V). While I had board out I esr tested electrolytics and some bad 1uF, 10uf and a 47uf. Whatever the case that fixed the issue. Signal looked ok to that point so...I'm baffled. It then had a problem tuning. Someone had left it tuned down several notes and about 50 cents below being in tune the far right TOS would go into convulsive jitter! In probing with it tuned down where it sounds ok, the probe even on x10 would cause nasty turbulence also..but only on that TOS. Before replacing TOS I tried probing back and found that it would only distort back to pin 11 on Z12 (see circuit overview above) I replaced that 4069 which tested ok on my chip tester...and that fixed the tuning issue! And detune also didn't respond before and now it does! Yay it doesn't need a tos i guess. Now I'm wondering why there seems too much grand cipher noise...but it disappears when sustain switch is opened. (OH and i had to retension that switch contact on was open thus causing constant sustain). I found 30 ohms ish between the ground in on iec connector and front panel/headphone jack. It was oxidization between panel and the grounded standoff on upper left side. It helped some but still a bit noisy. hmm. See VIDEO Size is 46"x24"x6" incl. rails on bottom roughly.

I Have the switches w/ black push button knobs for 4-voice. Missing two knobs on my own here so no exact ones for those. Nothing for 16-voice.

Parts: Lots of discrete parts in these. Always feel free to ask or check my Parts
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