Category Archives: Organs / Synthesizers

EKO TIGER

This wee little Italian Organ actually got brilliant sound. It came predominantly working with few little issues here and there.

Main problem was the bass section, it is bound to develop issues by design. All the secondary contacts for bass section C1 to G2 are connected in series and as they tarnish over the years you get regular random malfunctions affecting not just single key which might not have such a severe impact but the entire section stops working. Contacts are made of some alloy with high silver content and blacken if unused for long periods of time. The only effective way was to use Nitric acid with Thiourea and rinse it properly and neutralize.

There was one issue which really took me a while to discover. Selector for BASS chorus behaved in a very unusual fashion. With Bass Chorus disabled only bass section C1 to G2 would play but it would play both the Bass as well as the selected voices when it was  Enabled it would play both bass and selected voice on the lower part and selected voice on the top. I couldn’t figure out what have happened to it. At the end I found out somebody was there before me and swapped by mistake two wires from keyboard bus bars.

All there was left was to tune it and 3D print new slider handles.

 

Musical Blackboard

This is one of the weirdest instruments I had on my table for repair to date. 

It isn’t a synthesizer, or at least not the ordinary type, it is a analog touchscreen of old. Educational prop made in Czechoslovakia sometime during 1980’s. This blackboard is made of the large PCB painted over with  black paint. There is a separate thick PCB track for each note and you play touch. However since it’s a old school analog you need to hold the wire (or connect yourself using bracelet) for this to work.

The heart of this “thing” is a “organ in one package” Tesla MHB208. This is a Czechoslovakian clone of Thompson M208. It’s rather basic and foul sounding chip but it’s definitely got it’s use. It’s one of those “so bad it’s good”

Whats different is how keyboard is simulated by the touch mechanism. There is a set of 3 state switches Flat, Regular, Sharp for each note. You select the key you want to play in by selecting respective #’s and b’s  for each note and go for it.

 

There are high impedance flip flops triggered by the small current carried over your body triggering individual notes. It is fully polyphonic and great fun to play once you get used to the individual notes boundaries.

This thing came without external PSU and I had to figure out the power and voltage requirements. It’s dead on straight forward AC input of anything between 15 and 40 V AC.

There is rectifier bridge and MA7812 which is a TO3 package for of usual 78xx.

I discussed with lucky owner and added separate internal transformer to make it easier to use on stage on mains.

clumsy demo below, enjoy the creepy sound

 

 

Vermona ET 6-2

Reviving old records and doing a retro tours became huge trend lately. I was asked by local band  Tatabojs to resurrect their dual manual organ Vermona for their upcoming retro tour Futuretro

Upon inspection I discovered several functions weren’t working.  No Vibrato or Tremolo, no Reverb, whole percussive section was dead. bongos were rather crunchy, synth section (bass pedal) controls weren’t working  and there were about 3 or 4 keys not playing the 16′ register. Not to mention all the switches and pots were properly noisy.

 

Actual oscillators themselves were in good shape and only troublesome devices were the electrolytic decoupling caps. Following the replacement with new ones signal was much stronger and according to the specs in the manual.

Design is rather simple, it consist of precision foil caps and adjustable coil master oscillator for each note followed by transistor dividers. These circuits are incredibly stable and don’t drift noticeably at all.

I started slowly one by one following the smaller daughter boards and tackle issues one by one.

Tremolo and Vibrato functions are based on low frequency oscillator and there were no oscillations at all. Problem was with 3 foil capacitors in the oscillator circuit. They deteriorated over time and following the replacement with modern WIMA foil caps both Tremolo and Vibrato function sprung to life.

The further I was troubleshooting individual functions it became apparent capacitors (predominantly the electrolytic ones) are the main cause of all the issues. I have therefore decided to do full recap of the instrument. Pretty much all of the old caps with few exceptions I removed measured really bad ESR or some were actually partly conductive as if they had a resistor in parallel.

Following the full recap most of the organ functionality was restored, WAH WAH, Vibrato, Tremolo, Bongos, Percussive section to some extend as well and overall signal to noise ratio changed dramatically. Especially top manual was giving really strong signal but the lower manual remained weak and the manual ratio pot had to be pushed all the way towards the lower manual to have any decent balance between the two. Issues with missing 16′ notes remained and percussive section while now working was rather short and the slider made no difference to its length.

One of the missing 16′ notes was due to bad germanium transistor in the divider circuit while the others were due to physical wires not being soldered on properly. There were cold solder joints on two and one wire fallen off completely.

Percussive section just needed trim pot cleanup and re-calibration and all was good.

Broken reverb was purely due to broken off wires from the spring reverb, all was good when i re-soldered them in place.

Now with entire organ working and all functions doing what they should I remained puzzled by the sheer volume difference between top and lower manual. It took me several hours of head scratching just to realize after all that somebody who repaired the instrument before me wasn’t thinking quite right.

Slider pots used by Vermona are of amazing construction. Virtually indestructible and easy to access to clean them. They were clearly designed to be maintained and the tracks themselves are made out of some sort of conductive plastic which doesn’t wear at all. After just a brief wipe with IPA and re-lubrication with fader lube, they are like new. The only weak spot is the track itself is secured to the rest of the mechanism by plastic lugs which ends are melted to secure it in place. These get fragile over time and break which results in the track coming loose. Somebody before me didn’t think quite right and replaced this plastic lug with screw and didn’t think about the need to insulate the nut and bolt. It cause this dual pot to be partly short connected between each other and therefore it was impossible to get the manual balance right. I have used some fiber washers and all was good since. Use of plastic M3 nuts and bolts would be even better, but I didn’t have any at hand.

Once this last issue was resolved only left to do was new hinges, locks and handles on the case and this monstrous organ was ready to go back to the customer.

 

 

Antares 2 – Kolafon

Antares 2 is a rare breed. This Synth was developed in Czechoslovakia in 1979 by VURT (Research Institute of Broadcast and Television) and only few prototypes were ever made. There was one small batch manufactured (<10 pieces) before the project was finally retired.

There were few issues with this particular piece. Mechanical problem with one of the keys and associated damage around the key J wires.

Major problem were the capacitors. I had to replace both the dried up electrolytic caps as well as metalised paper caps and ceramic caps which absorb moisture over time. Following the  cleanup of all the pots and switches as well as full recap Antares sprung back to life.

I’m giving special credit to the mechanical construction of this piece, all the electronics apart of keyboard itself are in one metal enclosure. This can be tilted upwards to allow for better access to the controls.

More pictures are little history in Czech language can be found here.

http://czechkeys.blog.cz/0912/syntetizer-antares-ii

All the electronics are in 5 modules and use gold plated high quality connectors. The entire PCB section can be also tilted and secured in place using thumb screws allowing full access to all the components.

Real joy to work on.

Vermona E-Piano

E-Piano was a bit more complicated repair compared to the Vermona string synth. PSU had dried up filter electrolytics and one of the earth wires fell off.

Following a pot cleanup all was working well except tremolo.

See the unusual but very reliable slider pot

I noticed some of the electrolytic caps were getting warm due to ESR. After the replacement piano was quieter with less strain on the PSU however the tremolo generator still wasn’t oscillating.

Both of the transistors were OK, all the surrounding resistors as well, electrolytes were replaced so the only culprit left were the foil capacitors and indeed once replaced by small electrolytic caps (the only 470nF I had in my spares drawer) tremolo sprung to life. I managed to later replace electrolytics with foil polypropylene condensers and tremolo was much nicer and more stable sounding afterwards.

Korg Poly 61

This Synth came to me following a damage suffered during transport. Joystick was loose /broken and it wouldn’t turn on or even when it turned on it would just display nonsense and wouldn’t react to any of the presets.

The core of the issue was a semi shorted battery and the spring clip from the joystick wedged under one of the PCB’s causing short of some sort.

With battery and the offending spring clip removed all sprung to life and worked just fine when presets were loaded.

I managed to glue the joystick together, but I sourced replacement module in the meantime.

 

Vermona Piano Strings

 

Vintage synth from Eastern Germany, you can smell the communist era inside of it. Very typical earthy smell of brown PCB boards. All it needed to bring back to life was thorough cleanup of all the keyboard contacts and slider pots.  After an hour and a half with compressed air and IPA it’s back in business.

Slider pots are actually very good design, fully open accessible and designed to be cleaned, sadly I forgot to take picture.

The way this one sound is just pure filth, it’s harsh…….