Hammond X2

This Hammond X2 came in a weird shape, it was basically working apart of last octave, upon opening the organ I could see there was somebody there before me. It was modified to add pedal octave mod. It was done sometimes during 70s or 80s in Czechoslovakia and it was predominantly made using integrated circuits from eastern Germany. I was for a while contemplating fixing the mod but after discussion with owner it was decided to chop the mod out and revert back to original design.

This extra board (impossible to read any labels but ended up being dividers) creates the pedal octave and the smaller board with double VCA and two transistors mixed the pedal octave and lowest octave together. Rest of the issues were down to thorough cleaning of switches and contacts.

It took me a while to find the service manual but at the end I managed to get hold of it.

Here is a link for those who might be in the same situation



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.



Dynacord Echocord S62 and S65

It must be Echo season,  I have now 5 tape echos in for repair. Dynacord Echocord Super 65 was not working at all, however it was in incredibly good shape otherwise I don’t think this particular piece done many active hours because there wasn’t a lot of wear on the original heads and everything was in perfect order except the motor which clearly had some moisture induced damage. Shaft was rusted in the bearing but I managed to free this up, clean and lubricate.

I had the pinch roller refurbished, cleaned the heads and made a new loop out of master BASF tape, but I struggled to get this rolling, it wouldn’t spin on adagio and even on presto setting it would stop eventually. I was puzzled for quite a bit and learned several lessons.

1) master tape is no good for echo loops, it is too thick, abrasive and sticks to the brass heads.

2) Brasso polish is great for refurbishing worn tape head surface, it made it roll with much less resistance

3) tension settings and pinch roller pressure settings are extremely important, even a slightest tweak makes whole lot of difference to the overall ability to spin freely, especially at low speeds

With mechanical problems out of the way it was time to check electrolytic caps and they were generally OK with exception of the main PSU filtration. Non functional Magic Eye indicator was caused by faulty diode as well as faulty EM84 itself.

Main issue was the combined pot / pull switch which had to be taken apart cleaned and reassembled Echo is working flawlessly since.

I have also replaced burnt light bulbs illuminating the  selector switches. It’s a beauty to behold


I expected the Super 62 to be easier to repair because Nachhall function was working and the only missing bit was the Echo functionality…. I couldn’t be more wrong. This piece gave me proper headache to repair and countless hours of head scratching.

Capacitors were generally in really poor state, all the Electrolytics were measuring very high ESR and even several of the ceramic caps had problems. I rarely ever see ceramics to be the problem but these wax sealed units probably somehow soaked up moisture and became partly conductive which resulted in valve self oscillations.

I have eventually decided to do complete recap.  This was especially surprising because just 3 years younger unit S65 was nearly perfect in this regard.

After the recap this unit was still only partly functional but substantially quieter. There was remaining issue with insufficient voltage on the erase head. Problem was eliminated by replacement of ECC82 with new known good example. After proper cleanup of the combined Pot/Switch I could hear faint echo coming through on Echo 2 settings but not so on 1 or 3. I measured the selector switch and had to de-solder entire board and take the selector switch cluster apart and clean thoroughly.

Now I was at the point where I really didn’t know what to do, it appeared as if there was a bad valve socket for the Ecc83 acting as a playback head amplifier I could hear clearly when I tapped one of the heads with screwdriver but only faintly when I did the same with the other.

The root cause of the problem ended up being bad playback head, after desoldering it one of them measured 1kOhm but the other was open circuit so when I was tapping the bad head what I was hearing was just a cross talk from the other head!

At the moment I’m facing challenge with sourcing replacement playback head and may have to have it custom manufactured which would sadly result in already largely uneconomical repair to become entirely uneconomical. Nearly 40 hours of time in total and it’s still not running. I hope there is some spare head lying about.





Fender M12

I never knew Fender did mixing desks but then out of the blue this one had to be saved from going to the scrapyard.

There were several issues

  1. PSU had a toast rectifier diode and burnt out stabilizer. I could tell there was somebody in there before me and added various modifications. One of them was extra huge electrolytic cap to get rid of the PSU induced hum. Thing is, PSU hum was there due to dried out electrolytic capacitors with massive dielectric losses so while this modification removed the unneeded hum the ever increasing load on the transformer and rectifier diodes eventually led to the diode failure and associated chain events.
  2. one of the meter  module opamps were toast and presented beautiful smelly smoke following the power up
  3.  few of the channels had a bad solder joints
  4. one of the channels had a bad OPAmp

When tackling vintage console the main question is that of economical feasibility of the repair

If things are to be done properly, every switch and pot taken apart cleaned and lubricated, every dried out cap replaced it takes enormous amount of time which will be worth several times the price of the console itself

With everybody moving to digital nowadays , there are high end analog consoles available for unbelievably low prices

However it was decided to rescue this vintage piece and 2 weeks of cleaning have begun

I stripped every channel, carefully opened every single pot and cleaned using three stage process First I remove the coarse debris with compressed air, clean all the contacts and pot track with Kontakt 60 contact cleaner, wash it down with Kontakt WL and finally lubricate with Kontakt 61 fader lubricant

Nylon scratch brushes were particularly handy in this effort

This before / after picture of the slider pot shows the difference thorough cleaning can make, all of the sulfides dust dried out grease are removed and surface freshly lubricated

Difference it made to the feel of the slider pots was quite eye opening

The end result is reliable vintage mono mixer with fairly high noise bed but it’s nice analog noise, nothing particularly irritating


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.


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.

Trace Elliot GP 11

This preamp was humming and some of the graphical sliders worked as a noise generators rather then equalizer.  Also the two momentary switches were bad. I couldn’t find the exact momentary switch so i had to modify the switch cap slighty to fit modern replacements.

Re-cap and upgrade of main filter cap from 2200uF to 15000uF resolved all the PSU induced hum and one of the transistors in the graphical equalizer actually had a small crater where the silicon chip used to be inside of the plastic casing. It was interesting that the transistor was still working-ish but it was acting as a noise generator.