Upgrading a Faulty Hammond XB5 Organ

Fig.1 - Hammond XB5 organ

Over the past 25 years I have had several attempts at learning to play Hammond organ. I recently decided I would have another go, but found that my ageing XB5 digital organ has developed an annoying fault. That is, it has developed a crackling sound on the upper manual when the 5 ⅓', 2 ⅔ ', 1 ⅓ ' or 1' drawbars are pulled out. I traced the fault to the output of an LC7881 digital-to-analogue convertor (DAC), IC69 on the main board. I obtained a replacement LC7881 and replaced the chip, using an IC socket to make it easier if I needed to do this again.
Fig.2 - XB5 main board

Unfortunately it made no difference, so I now suspect IC66 to be at fault, one of the custom “MUSE2” chips. A common problem with old XB5 organs (as well as the more common, single manual version, the XB2) is faulty MUSE chips and/or faulty DRB chip. I replaced the DRB chip a few years ago, but now both of these parts are obsolete and unobtainable. The XB5 is very rare therefore it is highly unlikely that I would ever be able to find a replacement main board from another organ, at least with functioning DRB and MUSE chips! This left me with three basic options: 
  1. Scrap the organ. 
  2. Replace the internal electronics with the HX3 board made by Keyboard Partner in Germany. 
  3. Use the faulty XB5 purely as a MIDI controller for an outboard MIDI organ sound module (expander).

Option 1 

I quickly eliminated option 1 and seriously considered option 2. 

Option 2 

The HX3 is highly regarded by ‘real’ musicians for its sound and there is a fair bit of information out there on how to install it. It can be connected to existing keyboard scanners and has ample analogue inputs for drawbars and other controls. It would be a big project to take on, however, what finally put me off what the cost. The HX3.5 (the current version of the HX3) is priced at 600 euros, but when you add on shipping to the UK plus VAT and import duty it becomes a lot more expensive. Too expensive for a ‘wannabe’ (aspiring to be amateur) musician! 

Option 3 

I decided on option 3, using the XB5 as a MIDI controller for an organ sound module. After doing some research, I purchased the Ferrofish B4000+ as this seemed to give the best ‘bang per buck’. Overall I am very pleased with it. The sounds are better than the original XB5 and it includes a decent (although not the best) Leslie speaker simulation. Sadly I had to part with my vintage Leslie 760 as part of a ‘downsizing’ exercise. Probably the biggest flaw with the B4000+ is the fiddly controls. My intention was, as far as possible, to operate many of the controls from the XB5 organ using MIDI CC (control change) messages. 

Operating the Sound Module Using the XB5 Organ’s Controls 

It was very easy to get the B4000+ working with both manuals (keyboards) and bass pedals. By changing some settings I was even able to get the XB5 expression pedal controlling the “swell” level via MIDI. The good news is that the XB5 outputs most of its controls (including drawbars) via MIDI. The other good news is that the important B4000+ controls can be operated via MIDI, including the drawbars, vibrato/chorus and Leslie speed. The bad news is that they do not use the same CC numbers. In the case of the drawbars the format is also different. The solution would be a MIDI box that remaps the messages, but a suitable device does not seem to exist! 

 I have set myself the task of designing an interface to remap the MIDI messages, hence allowing the B4000+ to be played using the XB5 organs controls. I intend to eventually build the electronics into the organ. I have some experience programming Arduino microcontrollers, so decided to use that platform. A MIDI library is available for Arduino that features ‘callbacks’, which should simplify the coding. For convenience I will use the Sparkfun MIDI ‘shield’ instead of building all the hardware. 


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  1. More Option? PC Mainoard, soundcard, NI B4 software by autorun in XB5 case.


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