MATRIXSYNTH: Restored Vintage ARP Quadra Synthesizer Keyboard with MIDI

Thursday, April 29, 2021

Restored Vintage ARP Quadra Synthesizer Keyboard with MIDI

video by Synth Chaser

via this auction

"This ARP Quadra has just been professionally overhauled by Synthchaser, the foremost expert on ARP Quadras! Work done included new orange LED sliders (illumination can be disabled before shipping at buyer's request), recap of all tantalum and electrolytic capacitors, new touch sensor, keyboard refurbishment, new IC chips, reliability enhancements, and perhaps most unique of all... this may be the world's first ARP Quadra with MIDI!

While many synths are listed as "serviced" or "fully serviced", they are usually devoid of any details of what was actually done. Those that do provide details or service invoices often reveal that the synth was merely superficially serviced, and not genuinely "future-proofed". I have not only serviced this Quadra, I've taken it down to the chassis and overhauled it. Work done included:

Synthchaser ARP Quadra MIDI adapter - Provides MIDI capability to all 4 sections of the synth (bass, strings, polysynth, and lead). Via MIDI in, aftertouch is processed over the full 5 octaves of the keyboard, not just the top 3 as in the original design! MIDI IN and OUT jacks have been fitted. More info on the MIDI is in the functional demo video linked above.

Synthchaser Precision LED slider upgrade. All 35 of the old original sliders have been replaced with new, smooth, high precision LED illuminated sliders. The difference in the way these sliders feel and perform compared to the originals is a huge improvement.

Illuminated slider caps - Slider caps with a translucent plastic stripe that allows the LED illumination to shine through, while giving you something substantial to grip onto. They look and feel awesome!

Power supply rebuilt - All tantalum and electrolytic capacitors in the power supply have been replaced, and high quality, high temperature 105 deg C capacitors have been used for the power supply filter capacitors.

All polarized capacitors replaced - The original tantalum capacitors in ARP synths are ticking time bombs, and 40 year old electrolytic capacitors are past their prime. All of the 150+ tantalum and electrolytic capacitors in this synth have been replaced with new electrolytics to help provide stable operation for years to come.

4075 Filter Enhancement - The ARP Quadra uses the same 4075 filter submodules found in the Odyssey, Omni, Avatar, and Pro DGX which was designed with a calculation error that limited the cutoff frequency. Both of the 4075 filter submodules (for the lead synth and the poly synth) have been recapped, and corrected to provide superior frequency response, and tested for proper operation in the Synthchaser custom test fixture.

New aftertouch sensor - The original aftertouch sensors are a common failure point-- the aftertouch sensor has been replaced with a new long-lasting one to ensure the aftertouch will work for years to come

Keyboard rebuild - Keys removed and ultrasonically cleaned, buss bar and key contacts cleaned and adjusted, new rubber key bushings installed

Oscilloscope calibration - Calibrated to factory spec using oscilloscope and frequency counter.

PCB Interconnect reliability upgrade

Many connectors replaced

Batteries relocated - ARP Quadra presets are preserved by alkaline batteries mounted to the microprocessor circuit board. In the majority of Quadras, these batteries have leaked and damaged both the microprocessor board and also the keyboard electronics circuit board below. This was an ARP Quadra that actually didn't suffer from battery leakage, so the boards are clean! To keep them clean, I've relocated the batteries to a battery holder in the bottom of the chassis, a standard upgrade I make to all Quadras that I refurbish.

This synthesizer looks in good shape for a 40+ year old synth. There is a sticker from an old music shop on the right side panel. Overall it looks pretty sweet, particularly with the new upgrades! See the pictures and judge for yourself--or better yet, come check it out in person and take it for a test drive."

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