The synths in this store were built between 1970 and the late 1980s, and nearly every one that still works has been opened by a tech at some point. That's normal. Service on an old synth follows a familiar list, and once you know it, you can read a listing for what's been done and what's still waiting. Here's what usually needs attention, what you can check in the photos, and what a seller should be able to show you when they write "serviced".
Power supply capacitors
Most of the big capacitors in a vintage power supply are aluminum electrolytics with a liquid electrolyte inside. That electrolyte slowly dries out. As it does, capacitance drops and internal resistance rises, and heat speeds the process up. Electrolytics built in the 1970s or earlier can call for gentle reconditioning, a step called preconditioning, before they go back to full use. That's one reason to be careful with a synth that's been sitting unused for years.
Replacing the power supply's electrolytics as a set is routine work on a synth this age. You can't judge a capacitor's health from a photo, but an inside shot can show the obvious ones, like a can with a domed top or brown crust around its base. If a seller says it was recapped, ask which boards were done. Some jobs cover the power supply only, and some cover every board.
Memory backup batteries
Any vintage synth that remembers its sounds after you switch it off has a battery keeping that memory alive, and old batteries leak or die.
- Korg Polysix: the battery sits on the KLM-367 programmer board. When it leaks, the alkaline fluid eats the board's traces and can short the circuit. Clone KLM-367 boards have been sold since 2009 for this repair, and their maker says thousands have been installed. Ask for a photo of that board before you buy a Polysix.
- Roland Juno-60 and Juno-106: both owner's manuals say to replace the backup battery every five years, and the Juno-106 service notes say to replace it once it reads under 2.8 volts in place. Back up the patches before the swap.
- Sequential Prophet-5: the owner's manual promised a battery with a 10-year life. Every original is long past that, so ask when it was last changed.
- Yamaha DX7: its 32 internal patches live in battery-backed memory. Ask the seller whether they survive switching it off and on.
- Roland TR-808 and TR-909: these keep their patterns on ordinary dry cells in a compartment underneath, three in the 808 and two AA cells in the 909. Roland told owners to change them once a year, and the 909 manual says to take them out for long storage because they can leak. Sound On Sound's buying advice for the 808 is to look in that compartment for corrosion.
In the photos, look for the battery compartment or the board the battery sits on. On arrival, pull any loose cells out of a drum machine you won't use for a while.
Keys, pedals and contacts
Each key on an old synth closes a small electrical contact, and contacts collect dirt and oxidize. On the Minimoog, Sound On Sound found that worn keyboard bushings leave many vintage units with uneven, clattery keys and dirty contacts that don't always play. The Moog Taurus bass pedals have their contacts exposed on the underside, where they pick up grime, and Sound On Sound's advice is to check for notes that don't trigger or that hang on too long.
Cleaning fixes some of this. Worn parts take longer. Ask the seller for a video that plays every key, one at a time, from the bottom of the keyboard to the top. Every note should sound, and none should stick.
Knobs, sliders and switches
Scratchy pots and sliders are a common fault in instruments this old. On the TR-808, Sound On Sound warned that noisy pots can make sounds thin, weak or unpredictable, and that the Tap button and the step keys take the most wear. Cleaning sometimes helps. Replacement is harder. Sound On Sound noted that Polysix pots are often sealed and peculiar to the maker, and the Allen-Bradley Type J pots used in original Minimoogs are no longer available.
Ask for a video that sweeps every knob and slider slowly while a note plays. Listen for crackle, dropouts or sudden jumps in level.
Tuning and calibration
Analog oscillators drift. Roland gave the Juno-60 and the Juno-106 digitally controlled oscillators for stable pitch, but a synth with voltage-controlled oscillators needs calibrating from time to time. Sound On Sound said the Polysix's oscillator trim can drift and need recalibration by a service engineer. Vintage Synth Explorer lists unstable tuning among the Prophet-5's problems. The CS-80 is the extreme case. Sound On Sound's view was that a full service is the only way to keep one in tune for long.
Calibration is bench work with the service manual and the right test gear. When you judge tuning yourself, let the synth warm up first, then play octaves up and down the keyboard. A little drift after warm-up is a calibration job. A voice that won't hold pitch at all is a repair.
Voice cards and voice chips
A polysynth makes each note on its own voice circuit, so when one voice goes bad, one note in a chord sounds wrong. On the Juno-106, Roland's service notes show each voice running through its own 80017A chip, which holds that voice's filter and amplifier. Our Juno-106 voice chip guide covers what to listen for.
Parts supply matters as much as the fault. Roland stopped building the TR-808 once its semiconductors couldn't be restocked. Sound On Sound wrote in 1999 that few service centers would repair a Prophet-5 Rev 2 and that many of its parts were unobtainable. Before you buy anything with voice cards, ask for a video that plays single notes up the keyboard and then holds a chord, so you hear each voice.
Finding a tech
Look for someone who has worked on your model before and has its service manual. Owner forums and user groups for the model are good places to ask for names. MIDI retrofit makers are another lead. Kenton, for one, sells MIDI kits for the Polysix and the Memorymoog, and Vintage Synth Explorer notes that Kenton and Lintronics will install Memorymoog upgrades.
If the synth will need work, call before you buy. Ask whether they take that model, how long their queue is, and how they want it shipped. A heavy synth that has to travel to a tech in another state means two freight trips, so count that in. Our voltage and shipping guide covers packing.
What "serviced" should mean in a listing
A listing that says "serviced" or "recapped" with nothing behind it tells you very little. A good one gives you details you could check.
- Who and when: the tech or shop, and the date. A recent invoice or work order in the photos is the strongest proof.
- What was done: which boards were recapped, whether the backup battery was replaced, whether keys and pots were cleaned, and whether it was calibrated afterward.
- What wasn't: any known weak spot left alone, like an original Polysix battery or the original voice chips in a Juno-106.
- Parts: which original parts were swapped for new ones, and whether the old parts come with it.
- Mods: MIDI retrofits and other changes, with the maker's name.
A receipt that says "cleaned and checked" describes a lighter job than a recap and calibration. If the seller can't say what was done, assume the work is still ahead of you and compare the listing with the next one on that basis. Our eBay red flags guide covers the rest of the listing.
