Integrated Circuits
I've got thousands of integrated circuits, I'll only list the more sought after. But if you're looking for a synth IC, please mail. I only list new ICs but if you're desperate for a unobtainable unit, maybe I can provide you a used (tested) IC.
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uA726HC

Roland Jupiter 4, SH-2, SH-5, System 700

uPD8253C-5 - Programmable Interval Timer

Kawai SX-210

4051 8ch Multiplexer/Demultiplexer

Sequential Prophet 600,

AN6551 Dual Operational Amplifier

Used in Korg Delta

AY-3-0214

Korg Delta, Roland RS-505, VP-330

AY-3-0214 replacement circuit

This Top Octave Generator from General Instruments was used in Korg Delta, Roland RS 505, Roland VP-330 and in some Wurlitzer Organs.
UPDATE - NO NEED FOR THIS - I HAVE DOZENS OF AY-3-0214

These chips are next to impossible to find, so I had to do something. My replacement solution takes very little more space than the original chip. The input frequency must be a little down though or else you'll end up with a half step sharp. For most people it will not be a problem as the tuning potentiometer compensates this, it simply won't be in the center. But I'll provide you instructions to correct this in the Korg Delta (adjusting VR7) , the Roland RS 505 (adjusting VR-1) and VP-330 (adjusting VR1A).

BA618 - LED driver

Korg Poly 800, EX800, Poly 800 II

CEM3328 - Four Pole Low Pass VCF

Crumar Bit 01, Bit 99
Ensoniq Mirage, SDP-1
Doepfer A-122

CEM3340 - Voltage Controlled Oscillator

Crumar Spirit
Doepfer A-109, A111-2, A-111-3, A-111-4
Oberheim OB-8, OB-Xa, OB-SX
Octave Plateau Voyetra-8
Paia Proteus
Roland SH-101, MC-202, Jupiter 6, MKS-80 early models
Sequential Circuits Prophet 5 rev 3, Prophet 10, Prophet 600, Pro-One, Prophet T-8
Simmons SDS7
Steiner Parker EVI
Synton Syrinx

Curtis Electromusic Specialties

CEM3374 - double VCO

Oberheim Matrix-12, Xpander, Fender Polaris

Curtis Electromusic Specialties

CEM3379 - Voltage Controlled Signal Processor

Ensoniq ESQ1, SQ-80, Sequential Circuits Prophet 2000, Prophet VS, Doepfer A107, A122, A106-6 and A109
Curtis Electromusic Specialties

CEM3394 - µP Controllable Synthesizer Voice

Sequential Circuits Six-Trak, Multi-Trak, Max, Split8, Akai AX60, AX73, VX90, Doepfer A111-2, Dark Energy (original model), Simmons Drums SDS9/SDS1000/SDS800

Curtis Electromusic Specialties

CEM3396 - µP Controllable Dual Waveform Converter / Processor

Elka EK 22, Cheetah Ms6, Oberheim Matrix-6 & Matrix-6R
Narrow version: Oberheim Matrix 1000, Cheetah MS6
For the Oberheim Matrix-1000, check also my DIL wide to narrow adapter.

CEM3350 - 30 ch Multiplexed Sample & Hold

Used in Sequential Circuits Prophet VS, Studio 440 and Keytek (Siel) CTS-2000
Cirrus Logic

Cirrus Logic/Crystal 4391KZ DAC

Novation Supernova II keyboard and rack.

D63200, uPD63200 - DAC from NEC

Roland JP-8000, JP8080

DAC71-CSB-I D/A converter

used in Sequential Prophet 5, 10, 600, T8

IR3109 Roland Quadruple Operational Transconductance Amplifier VCF Envelope Generator

Made by International Rectifier to Roland's specifications

Used in Roland Jupiter 6 and 8, Juno 6 and 60, SH-101, MC202, JX-3P, Jupiter 4 with sn higher than 800800, MKS 80 revision 4 and other Roland gear. The SMD version is inside the Roland module 80017A.

IR3R05 Roland Quadruple Operational Transconductance Amplifier

Improved version of the IR3109, designed by International Rectifier to Roland's specifications
Used in JX-8P, JX-10, Alpha Juno 1 and 2 models, MKS-50, MKS-70 and the second version of MKS-80

LM3216 - Sanyo Six Stage Divider

Used in Roland RS101, RS202, SH3, SH3a, SH7, SH1000, SH2000, EP20, EP30, VK9, Hitachi HMS30 and so on...

M110 B1 - Monophonic Synthesizer - SGS

Used in Jen SX-1000, Crumar Composer, Synthaphone, Siel Mono, Cruise, Sequential Circuits Fugue, Farfisa Soundmaker, Synthorchestra 4, Gem PK 4900

M112 B1 - Polyphonic Sound Generator - SGS

Used in Siel DK 70, DK 80, Solton Programmer 24, 24S, Arranger Plus

M114S - Digital Sound Generator - SGS

Solton TS4K, TS4, TS3K, TS3, K160, Keytek (Siel) CTS-2000, CTS-5000

M114A - Digital Sound Generator - SGS

M5451B7 LED display driver

Used in Siel DK 70, DK 80