Repairing Vintage Synthesisers – Ekosynth P15

This is the last of the Italian Synthesiser’s made by Eko. Made in 1979 while the company had already sold the electronic segment to another Italian company Farisa.

I found this Synthesiser on a secondhand website and decided I would purchase it. It was located in Vienna and shipping would be difficult to Graz. It’s quite a heavy bulky beast. I was delighted that Philip, the guy selling the EKO would be able to deliver it in person, as he was visiting his brother in Graz this weekend. How lucky was that!

We had arranged to meet outside the Bahnhof Graz, the main train station. He presented the EKOSynth to me wrapped in a blue Ikea bag. It was heavy when he passed it over. He was certainly reluctant and sad to let it go. He had asked a couple of people to repair it in Vienna. Unfortunately no one was willing to take on the project. They had expressed concerns over vintage Italian synthesisers, as they’re supposedly too delicate and cumbersome to repair. They had advised him to sell it.

In the online advert and by messaging with Philip he had mentioned that, ‘Unfortunately the bottom Octave until the low Eb doesn’t give any sound at all. After that every C key plays the same octave C, every c# key the same octave c# etc. Only the top C plays an octave higher than the other Cs.
Also the Vibrato doesn’t seem to do anything.’

Asked if the presets worked and he replied that they do. So I agreed to take the P15 off his hands.


In this video, I repair, demo’ed and talk about the Ekosynth in depth. I’ll show you the insides and the some of the important rare synthesiser chips in Eko P15. I’ll even present a demo song of my own making!

Circuit Diagram

Once I got it home and on a suitable table, it was time to peek inside the black box. The Ekosynth P15 casing was made by the furniture department of EKO. Their main business was guitars and guitar amps. Certainly their woodworking department knew what they were doing when they made this machine back in 1980. Believe me, the proof is worth seeing, with the removal of a couple of screws the front panel lifts up on a hinge.  Such style and ease to open. The circuit diagram was still stapled to the side of the case as you can see below in the pictures. The IC’s through are dated between 1979 and 1980 using CMOS/MOS and TTL technology.

EKOSynth P15 Insides

The Ekosynth is pretty straightforward stuff for the time. The logic and switching, the “digital part” is done mainly with simple CMOS 4xxx series logic and counter chips. These were the chips I played about with in my teenager years, by the way of paperback books from R.A Penfold. Many hours were futtered away building CMOS circuit, I tell you!

The thing about CMOS chips, they are inexpensive and don’t require a stabilised 5V line. These little IC’s in fact run over a good range of voltages, so perfect for 9V battery projects. Also operating on very little current indeed. CMOS/MOS chips are used through this synth and all are inserted into DIP sockets. The overall wattage of the syncth is only 10 watts.

EKOSynth P15 Insides keys

DIP Sockets

A note about DIP sockets (also know as IC sockets), all the chips in EKO synth sit in these. While this is very practical for repairs however over time oxidation happens and sockets may have bad connections. You can of course replace or re-sit the chips into their sockets. I would recommended using a nylon or glass fibre stiff brush on the chip pins and the socket. You can also use contact cleaner on both the chips pins and sockets. (Do wait until the sockets and chips are dry before reinserting them.)

Chips with DIP sockets

In the worse case replace the socket by desoldering it out and soldering in a new one – gold plated DIP sockets are the best solution. This though is a lot work, so consider before taking this on.

Recaping Capacitors

Recap old capacitor means replace them with new ones. Specifically electrolytic ones. However this is a bit tricky one to answer because certain manufacturers had real problems in the 90s. In the case of EKO Synth I’m less inclined to recap just yet.

Cleaning the keys

It maybe necessary to contact clean the key in the synth. If certain notes are not making contact. This is pretty easy enough to do. See this link to an idea what to do.

My Repair Job

The idea around my repair job is to replace the logic and other simple components first. Each repair job is always an educated guess. There are main ways to approach repairing a synth.  More complex synth always start with the power supply. In my case the synth is in good condition, the presets are all working and just some keys are not working. Even the faders work well and don’t make any noise from dust or wear, which is pretty impressive for the age of this machine.

The reasoning behind this is, two fold.

One, some keys work and others don’t and most of the key controller is done by the CMOS logic chips. Bare in mind they are over 40 years old!  The second reason the rarer chips should be left alone until all the logic is working. These chips are impossible to find. I’ll talk about that later. Also if there is sound coming out of the machine, it’s very possible it’s just a logic chip.

Many of chip replaced in the EKO synth
Many of chip replaced in the EKO synth

The following logic chips are found in the Ekosynth P15 and no doubt in many other Italian synthesisers too.  They are inexpensive to replace, so order the lot and slowly replace each one, remember to clean the DIP sockets too! (see my note on DIP sockets above)

  • 4011  NAND logic chip
  • 4012  NAND logic chip
  • 4069 Logic Hex Inverter
  • 4042 Quad Latch
  • 4046 PPL
  • 4512 Counter – this one of the main culprits in the repair btw!

Other components

  • 74LS221 – TTL Logic Schottky
  • LM 324 – Quad OP Amps
  • various BC transisters

Rare Components

Whatever synth you work on, often there are components which will be hard to source or replace. Avoid exchanging or touching these parts until it’s absolutely necessary!

In my case the S50242 and SAA1005P chips are the rarest chips on the EKOSynth P15. The S50242 is known as TOGs, top octave generators. I’m going to be really unhappy if either the TOG or SAA1005 chips are faulty. They are next to impossible to source  as far as I know.

It’s important to remember many organs and synth were often built down to a price, so the designers used all sorts of cheap tricks to get them to make a sound. And in many cases it is those very tricks that give those organs/synths of the past their characteristic sound. Imperfections are easy in hardware, but actually quite difficult to do in software.

Reading the Hackaday article on DIYing one of these chips is mind boggling to say the least. There is even a solution!

Below are the specifications and datasheets of both chips S50242 and SAA1005P in the EKOSynth P15.

The ITT SAA1005P is a seven stage frequency divider specifically for use for electronic organs according to the datasheet.

SAA 1005 P ITT Datasheet

The S50242 can be replaced by FPGA chip from Flatkeys in the UK but quite expensive.

S50242 chip
S50242 chip


How do string synths and organs work?

In general, they used the following scheme. A high frequency clock (typically 1 or 2MHz, sometimes 4MHz) drives a top octave generator chip (“TOG”, also known as a “Top octave synthesiser”, “TOS”, or “top octave divider”) which produces all the note frequencies for the top octave as basic square waves. These square waves can then be fed to flip-flop dividers to produce a note an octave lower. The output of that flip-flip can then be fed to another flip-flop to get an octave below than that, and so on. Luckily, we don’t need a ton of individual flip-flop chips, because that arrangement is basically a binary counter, and there are many counter chips available which will produce all the required octaves when fed the highest note frequency.

This all uses quite a few chips (one oscillator of some type, the top octave generator, and twelve dividers) but it produces all the tones you need for an entire organ – around 100 pitches. One hundred pitches from fourteen chips is pretty good, especially in the Seventies!



  1. Replace cheap logic chips first!
  2. Fix dry joints and bad connections
  3. Avoid the rare chips
  4. Avoid dismantling the keyboard as they are delicate and  replacement pieces are impossible to find. Instead clean contacts and slowly identify faults.


All in all it was a lot of fun but a daunting task to fix one of these old vintage synth. My advise before entering into this domain of insanity (and it is).

  1. Ask as much questions before purchasing a broken synth.
  2. Ensure a circuit diagram of machine is available.
  3. If the keyboard is completely broken consider moving on, as parts are often too rare to find.
  4. Search Internet forums and vintage synthesiser groups concerning common faults regarding the model you are attempting to purchase and repair.
  5. Take a calculated risk and be prepare for not everything working 100% with the possibility of other issues happening over time! They are old and need TLC.

In the end I had success, however the first 10 keys are still not working 100%, I can live with this as I can make music with the Eko and that’s what means I guess in the end!



Leave a Reply