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Technics PCM E33 Organ Repair

This post is a work in progress

A guide and full schematic documentation of the Technics PCM SX E33 electric organ. I wrote this in hope of helping anyone interested in these, how they work and break down its component and modular system. It was a really interesting project and I learnt a lot about synthesis, Serial bus, Filters and PCB’s

Ive been after a digital organ for a little while as they go really cheap, sometimes easily replaceable and simple in electronics enough for my skillbase.

I found this Technics PCM E33 on Facebook Marketplace for £30, it worked perfectly for a full week until mysteriously a control board failed on me. All the LED’s died and none of the buttons worked. The board that failed controls the built in rhythm machine as well as the tab selection keys for the voicing options of strings and flute lengths, chorus effects and upper harmonic couplers.

I decided to start with checking the service manual for power and ground pinouts, I found that all the control boards daisy chain using a serial bus network.

They run +5v positive rail and common negative across the PCB’s. The control boards all go back to the main motherboard [MB] which acts as a network interface letting them all talk to eachother.

The motherboard supplies rail voltage and a common ground for the entire system from the power supply [AS] I couldnt find any issues coming to the conclusion that something on the board had failed and locked it out, my money was on one of the IC’s which ended up being confirmed later. In the end I ended up finding another E33 on eBay and bid just under £30 for it. I swapped the boards over and got it back up and running and a huge box of spare parts!

I scrapped the carcass in the end taking all the real wood sections for future projects,most of this is laminated chipboard which isn’t much good. I also poached all the hardware fixings, mouldings and sliding cover.

I asked for help on Facebook and got some great answers from Tim Pullen on the technics organ group who told me a common problem with these organs where the Main Technics MN15621GSA IC’s. As he explained the following:

The ICS are all the same. They have 3 or 4 pins where there is a binary code set by the jumper wires which are circled in your picture. So the upper and lower circuit boards are the same and the jumper tells the IC which serial data channel it must respond to. So somewhere in the data packet is a number which wakes the ICs up when they are being spoken to.

Basically the same MN15621GSA IC is used in several places around the entire system (LIST PCBs)

There are 20 modules in total (PCB) inside the Techincs PCM E33 Organ its a fairly straight forward system once you understand how the modules work with eachother, Something that was hugely complex and advanced at the time so I’ve been told!

List of Modules

  1. AS
  2. MB
  3. CPSL
  4. LKB
  5. UKB
  6. CPL
  7. CPR
  8. PKB
  9. NW
  10. EXP
  11. TGM
  12. TG
  13. ORG
  14. OP
  15. ACC
  16. BD
  17. SPS
  18. CSM
  19. SM
  20. JACK
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Motherboard and Power Supply

Mother Board [MB]

Amp & Power Supply [AS]

The transformer is a ?????.

3 pole live and a common ground. Two 12volt and two 24v. The rails are rectified and regulated down to three rails +5v and +/-12v respectfully. These three rails supply the motherboard [MB] and amplifier [AS]

Amplifier is driven with +/-15v

Additional Power Supply Unit

Selector Mixing and Systems Assigner

Systems Assigner [SAS]

Selector & Mixing Circuit [SM]

Tone Generator [TG]

The Control Panels & Voice Selection

Control Panel Switch Left [CPSL]

Control Panel Switch Right [CPSR]

Control Panel Left & Right [CPR & CPL]

Keyboards and Pedals

Upper, Lower & Pedal Keyboard [PKB]

The Synthesiser & Presets

Organ Circuit Board [ORG]

Orchestral & Percussive Presets Circuit Board [OP]

Accompaniment Circuit Board [OP]

Bass & Drum Percussion Circuit Board [BD]

Solo Synth Preset Circuit Board [SPS]

Celeste & String Ensamble Circuit Board [CSM]

A look at the audio system

The audio system consists of a 6ohm 12″ bass driver directly connected to the on-board amplifier (code)

The tweeter is a 6.5″ 6ohm driver connected to a crossover mounted half way between the cabinet.

Crossover Network

The crossover is a simple high pass network consisting of a ??? Capacitor and ??? Coil (check name)

The pedal board and ???? Switch have a  potentiometer (check this) and are wired back to the ??? Control board. There is a pitch bend switch on the pedal

The spring reverb

The spring reverb has 3 attenuation settings controlled by (code) controller board its a dual spring which resonates quite alot via the cabinet from the vibrations from the 12″ driver. There may be a fault here but high frequencies tend not to pass through it so I will be swapping it out at some point. I’ll be using external reverbs so it’s not a function I need really

The organ is now back up and running and fully functional! Sure to make an appearance on something in the future!

It’s been a really fun project to work on and learning how these things were build really helps in understanding synthesis using electronics, something I hope to expand on later on!

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