A digitally controlled analog subtracive synth, a trimmed-down version of classic AX60 lacking all cool real-time controls with a more "S" like sampler design S900 (the previous ax60 was style- matched with S612). The Ax73 (rackmount version is VX90) is a 6 voices polysynth based on "synth chip" Curtis CEM3394. It's basically a 73 keys velocity keyboard, with LCD 16x2 display, classic 4 poles VCF with resonance and a MN3009/MN3101 chorus board.
Not mine, this has been brought by a customer friend of mine. He bought two of them , really in love with that machine! I still have my XV90 sometimes, thought I never been totally impressed by those machine (not the juno 60 killer as I read somewhere). Anyway I always loved the sampler combo with S612 - filter.
Back to the synths, one is working and needs only a cleaning, capacitor check and ROM (27c128) upgrade, It also feature some nice custom wood sides. The other seems to have trouble in voice assignment and makes strange noises when turning ON.
Well time to grab the screwdrivers!
UPPER: The AX73 is very easy to open with just 4 of screws on the front panel. The lower side is composed by 3 boards: voice, cpu and PSU.
UPPER: There is no official AX73 schematics, but the AX60 is quite similar on the lower boards with same VOICE BOARD and Voltages. Checking ba40 bridge rectifier, then: -5 +5 +15 and -15 7815 7915 + M5230L on CN. All seems to be there, capacitors have good ESR. Of course be safe and discharge electrolytics.
Reading across the net, it's highly advised to check first if 20 pins CEMs are still ok. First measuring the voltages across and then high failure rale CMOS 4051 ANALOG MULTI/DEMULTIPLEXER and 4066 Quad analog switch.
UPPER cems are on sockets, a good news.. Voice assignment will start from VOICE1, so just remove them all (with number label) and tried them on first socket starting from left. Of course pay attention to notch, the 3394 are not so easy to find out.
The cems are good, the voltage too. Missing values on A6082S op (082 opamp). Probing the 4051 reveals some problems with no output on 16 15 and more . Another CMOS seems pretty warm.
Time to replace them with nice sockets.
After replacing some CMOS the AX73 began to express some sounds. However higher octaves on its keyboard just played random or not at all. Connecting an expander to AX73 midi out, also the same troubles. Finally, the AX73 plays normally using the MIDI IN.
I just disassembled the keyboard and found out a lot of oxidized parts with missing tracks. A good cleaning, tuning and MUSIC!
AX73 USER MANUAL
AX73 SERVICE MANUAL (vx60)
CEM3394 datasheet
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