Synthi 100 Full Log

Really Nice Audio · Repair case records

Case record 003: EMS Synthi 100: power supply restoration

A retrospective record of one repair, reconstructed after the work.

Retrospective record

Retrospective: reconstructed from video and the restorer’s recollection. The record was compiled after the work was finished. Video captions were automatic and untimed; every number taken from them is marked (caption, unverified).

This page is a structured record of one repair: the fault, how it was traced, every decision taken, and what was kept, repaired or replaced, so that the work can be checked, repeated and learned from.

Bench time
4 hours
Parts
£60
Videos
3
Outcome
Fault resolved

Record prepared 5 November 2026 · Version 1.0 · About this record · Download as JSON

The videos

Video series “EMS Synthi 100 Repair” by Really Nice Audio, 3 videos: https://youtube.com/playlist?list=PLs8nMG67pcogcWpqT7wEBgAULZrfUnDK8

Video inventory
#TitlePublishedLengthSummary
1Not recorded (Part 1)Not recordedNot recordedThe mains path is traced, the faulty −12 V supply is found by its ripple, and substituting a bench supply cleans up the oscillators
2EMS Synthi 100 Repair Part 2 Belt and Braces Approach to bring back to life a beautiful linear PSUOn or before 16 January 2026 (inferred)Not recordedThe −12 V supply is restored on the bench: capacitors and bridge rectifier replaced, a damaged trace repaired, tested under load
3EMS Synthi 100 Repair Part 3On or before 20 January 2026 (inferred)Not recordedThe restored supply goes back in; hum gone and oscillators correct Restorer

Chapters: Not recorded.

The instrument

Identity and provenance

Instrument
EMS Synthi 100
Serial number
Not recorded
Owner
Pete Townshend Restorer
Location
on loan to the University of West London, housed in the Townshend Studio, Ealing Restorer. The studio was established in 2024 and opened on 10 October 2024. The university’s web page describes the collection as donated; the restorer confirms this instrument is on loan.
Restorer
Really Nice Audio (RNA)

What was wrong

Condition on arrival

Intermittent hum, harmonics on the output, and oscillators that appeared to frequency-modulate themselves VIDEO 1. The fault was demonstrated on video by Theo.

The diagnostic path

Diagnostic sequence

  1. 1
    The mains path was traced: mains input, the key switch (on/off), a 13 A five-way distribution socket, then two identical linear power supplies for +12 V and −12 V. A separate transformer was thought to supply the lighting, but this was not confirmed VIDEO 1.
  2. 2
    The +12 V rail measured clean. The −12 V rail showed heavy ripple VIDEO 1.
  3. 3
    A bench supply was substituted for the −12 V supply, and oscillator 4 (caption, unverified) immediately sounded cleaner. This confirmed the −12 V supply as the cause VIDEO 1.
  4. 4
    The −12 V supply was tested on the bench through a light-bulb current limiter, into an 8 Ω load (caption, unverified) with the sense wires taken to the load. It reproduced the fault: about −10 V instead of −12 V (caption, unverified), with ripple at 2.2 kHz (caption, unverified) VIDEO 2.
  5. 5
    From the ripple frequency, the restorer judged a rectifier fault unlikely. Rectifier ripple on UK mains would be at 100 Hz, so a ripple in the kHz range points towards oscillation in the regulator (inferred) VIDEO 2.
  6. 6
    The main filter capacitor was found to be leaky VIDEO 2.

Open question: all capacitors and the rectifier were replaced at the same time, so the specific part causing the oscillation was not isolated (inferred).

Faults and causes

  • Ripple and low output on the −12 V supply, putting hum and modulation onto the oscillators.
  • Leaky main filter capacitor.
  • The specific cause of the oscillation was not isolated (inferred).
  • Damage caused during the work: a PCB trace lifted when the old capacitors were removed, creating a short. The captions mention a couple of places with disturbed traces; the restorer describes one repair.

The decision record

Decision record
ActionOriginal partReplacementReasonAlternativesReversible?Original kept and where
Replace main filter capacitor10,000 µF 50 V electrolytic (caption, unverified)Smaller Nichicon capacitor rated 5 A ripple current (caption, unverified); part number Not recordedLeaky; quality chosen over physical size, since the load is about 2 A (caption, unverified)Not recordedYes (inferred)Yes: labelled bag, RNA workshop Restorer
Replace bridge rectifierNot recordedNew bridge rectifier; part number Not recordedPrecaution, after rectifier failures seen in an earlier EMS Vocoder repairNot recordedYes (inferred)Yes: labelled bag, RNA workshop Restorer
Keep rectifier support parts1 kΩ resistor across the DC output; 100 nF capacitor across the AC input (caption, unverified)ReusedBoth measured fine; judged low-stress——Reinstalled
Replace regulation-board capacitors, including a small capacitor glued to the bracingNot recordedNot recordedPart of the full recapNot recordedYes (inferred)Yes: labelled bag, RNA workshop Restorer
Repair lifted traceOriginal PCB traceWire link soldered across the break RestorerRestore the circuit and clear the short—Partly: the wire can be removed, but the original copper can’t be restored (inferred)—
Re-sleeve nicked heat-shrinkOriginal sleevingNot recordedNicks in the insulation—Yes—
Secure new capacitorOriginal mountingCable tie through a hole in the capacitor housingThe new part is smaller—Yes—
Keep the original linear supply——It could be restoredSwitch-mode supplies were considered and rejected, because the restorer understood EMS had tried them with poor results (stated but unsourced)——
+12 V supply—Checked only, not serviced RestorerMeasured clean———
Transistors—Checked, found fine Restorer; test method Not recorded————

Testing

  • Bench test through the light-bulb limiter into the 8 Ω load, with oscilloscope and meter: −12 V with no visible ripple, then 11.96 V (caption, unverified) once the capacitor was secured. Nothing ran hot under load VIDEO 2.
  • Reinstalled in the instrument and the voltage set to specification in situ; exact value Not recorded Restorer.
  • After reinstallation, no hum was heard and the oscillators sounded correct VIDEO 3Restorer.

Outcome

Outcome

Fault resolved. The original linear power supply remains in service Restorer.

Time and cost

Time
4 hours of bench time (inferred to exclude filming and editing) Restorer
Parts
£60 Restorer; itemised breakdown Not recorded
Labour cost
Not recorded
Dates
arrival Not recorded; work completed on or before about 20 January 2026 (inferred from when Part 3 was first posted online)

Tools, sources and the role of AI

  • Tools seen in the videos: light-bulb current limiter, bench power supply, 8 Ω load resistor, oscilloscope, multimeter, continuity tester, magnifier.
  • Schematics or service documents used: Not recorded.
  • AI in the repair: Not recorded.
  • AI in this record: the record was compiled with the help of Claude (Anthropic) from the video transcripts and the restorer’s answers. Every fact was supplied by the restorer, the videos or the cited sources.

Sources

Design history

Design history
ClaimSourceStatus
EMS (Electronic Music Studios) was founded in Putney, London, in 1969 by Peter Zinovieff, Tristram Cary and David CockerellWikipedia, “Electronic Music Studios”Confirmed by several secondary sources; no primary document seen
David Cockerell designed the Synthi 100EMS product history, emssynthesisers.co.uk/emsprods.html; Engineers Australia heritage listingConfirmed by several sources
The Synthi 100 dates from 1971Wikipedia, “EMS Synthi 100”; muzines.co.uk magazine archiveThe year is confirmed; the month (January) is stated but unsourced
It was built as a commission from Radio BelgradeWikipedia, “EMS Synthi 100”, citing Radio Belgrade documents and an interview with CockerellStated, citing documents not seen
Earlier names “Digitana” and “the Delaware”EMS product history; Sonic State report, 2003Uncertain: one source treats Digitana as a former product name, the other as the name of the studio one unit was built for
EMS worked from Peter Zinovieff’s Putney home, with an office at 277 Putney Bridge RoadNational Music Centre collection, nmc.emuseum.comThat EMS was in Putney is confirmed; which building the Synthi 100 drawings were made in is uncertain (inferred to be Putney)
When the schematics were drawnNone yetUncertain; the dates on the drawings themselves would confirm this DOC
David Cockerell left EMS in 1972Sound On Sound, “All About EMS, Part 2”Confirmed by one secondary source
The schematics were being drawn up a couple of miles from the restorer’s family home, around the time the restorer was bornRestorerStated by the restorer; the place and date of the drawings could be confirmed from their title blocks

What could confirm the details: the title blocks on the schematics (date, drawn-by initials, address); the Electronic Studio of Radio Belgrade’s archive; the current EMS company in Cornwall; Graham Hinton’s EMS history; and David Cockerell’s published interview.

Knowledge that is hard to write down

  1. Use the ripple frequency to rule causes in or out: a ripple in the kHz range points away from the rectifier (inferred from the restorer’s reasoning).
  2. Reproduce the fault on the bench before opening anything, using a load close to the instrument’s real draw, with the sense wires taken to the load.
  3. Old traces on this board are fragile and lift very easily, so take extra care when desoldering.
  4. Carry lessons between jobs: the rectifier was replaced because of an earlier EMS Vocoder repair.
  5. Prove it works before tidying up: test with the new capacitor loose, and secure it only once it works.
  6. Listen as well as measure: the oscillator cleaned up audibly as soon as the bench supply went in.

What was captured

The videos captured

the diagnosis, the bench repair and the bench test results.

The restorer's recollection adds

the Video 3 outcome, ownership and loan status, time and cost, the trace repair method, the checks on the +12 V supply and the transistors, and where the original parts are stored.

Neither captured

the rectifier swap (deliberately not filmed), the part numbers of the new components, the final in-situ voltage, the dates of arrival and completion, and the schematics used.

About this record

How the record was made

Retrospective: reconstructed from video and the restorer’s recollection. The record was compiled after the work was finished. Video captions were automatic and untimed; every number taken from them is marked (caption, unverified).

Sources

Source badges

VIDEO n
Video n of the series “EMS Synthi 100 Repair” by Really Nice Audio.
Restorer
The restorer’s recollection.
DOC
The documents themselves (here: the schematics’ title blocks), not yet seen.

Honesty conventions

Not recorded
The sources don’t say. Gaps are shown rather than filled.
inferred
A conclusion drawn from the sources rather than read directly.
caption, unverified
A value taken from the automatic, untimed video captions, not yet checked.

Publication and consent

Owner
Pete Townshend has approved documentation of the repairs carried out for him Restorer.
Custodian
the University of West London houses the instrument on loan.
Third parties
Theo, who demonstrated the fault, consents to being named; first name only Restorer.
Documents
No schematics, manuals or other copyrighted documents are reproduced; they are cited as sources only.

Record details

Record
Case record 003
Date prepared
5 November 2026
Version
1.0
Permanent identifier (DOI)
To be assigned
Machine-readable copy
Download the record as JSON

Gaps and unverified items

Not recorded (12)
  • Serial number
  • titles, dates and lengths of Video 1 (and lengths of Videos 2 and 3)
  • chapters
  • arrival date
  • labour cost
  • itemised parts cost
  • part numbers of all new components
  • the original rectifier's part number
  • the final in-situ voltage
  • the transistor test method
  • schematics used
  • the specific cause of the oscillation
Inferred (6)
  • Video publication dates
  • the completion date
  • the reasoning from ripple frequency to regulator oscillation
  • that the four hours exclude filming
  • the reversibility of each change
  • that the drawings were made in Putney
Captions not yet checked (7)
Captions not yet checked
VideoCaption value
1Oscillator 4
28 Ω load, about 1.5 A; instrument draws about 2 A
2−10 V before repair; −12 V and 11.96 V after
22.2 kHz ripple
210,000 µF 50 V original capacitor
2Nichicon replacement, 5 A ripple rating
21 kΩ resistor and 100 nF capacitor
Other open items (2)
  • Number of trace repairs: one according to the restorer; the captions mention more than one place.
  • Caption references are untimed; timestamps can be added from YouTube Studio caption files.