Fixing my Nord Rack 2
2024-11-08
Fixing synths vs making music
Creative work is difficult and it triggers avoidance strategies for me. One avoidance strategy is fixing synths rather than making music with them. Still it has to happen sometimes and I enjoy doing it.
Working alone and getting help
I do these fixing projects on my own but I do get help. What I usually do is that I look for a forum or mailing list where it seems that people who know the gear hang out, and then I start a thread that is half asking questions and half keeping a log of what I've tried. It's a bit like what I used to do working with an issue tracker at my last job. Not all reactions are equally useful but I always end up getting some good hints and learning something, and it is also very helpful to think out loud to stop yourself going in circles. I often end up starting a thread in the DIY section of the Dutch synthesizer forum and that is also what I did this time.
Nord Rack 2
My Nord Rack 2 (the rack version of the Nord Lead 2 keyboard) stopped working. This synth was made from 1997-2003 and the quality control sticker inside mine suggests it's from 1997.
It started with a blown mains fuse. Like many appliances, the Nord Rack 2 has a fuse next to the mains lead that you can pop out and replace without having to open up the synth itself. I naively ordered some new ones and replaced it. Second fuse blew too. Then on the third it didn't. I probably made things worse by doing this.
The next step was to look at the power supply. There are 3 circuit boards inside the Nord Rack 2: panel, main and power. The panel board has the knobs and switches of the user interface on it. The main board makes the sounds and does all the IO. The power board takes mains voltage and converts it into DC for the main board. I unplugged the power board from the main board and looked at the DC voltages. They were close to zero which told me that the power board was broken.
Now I went into a detour of thinking "I need to fix the power board" and not thinking about why the power board broke. The power board has a custom transformer on it which turned out to be broken. Only after spending quite some time trying to find a new transformer (I couldn't find one) did I bother to take a closer look at the main board where I spotted some things that did not look good.
On the left you see transistor Q7 with black gunk on it. Also the upper right-most electrolytic capacitor (metal can shaped component) has gunk around it.
I have read many times that a bench power supply is useful for electronics work but I never got myself one. This seemed like a case where it would really help. Before spending effort fixing the power board, I need to find out if the main board still works. But for that I need power. A bench power supply can let me temporarily hook up the main board to power to see if it still works.
So I go off to buy a bench power supply. I found a good deal for a second-hand professional-quality one. It cost about as much as what the Nord Rack 2 is worth but that is OK because I don't do this stuff to save money.
Besides providing power to a device that lacks a power supply, a bench power supply has two other very useful functions. First of all it can measure the current going into the device under test: it is a measurement tool. Second of all you can configure a current limit. This is great when you have a short circuit because it can protect the device under test from catastrophic failure.
By using the bench power supply I found out I had a short in the transistors that make up the class B amplifier for the right headphone channel: transistors Q7 and Q6. That is why Q7 has all that gunk, it was overheating because too much current went through it. The Nord power board does not have good protection against overcurrent so this short ended up blowing the mains fuse and frying the power board.
Circuit diagram for the headphone output of the Nord Lead 2X. I could not find Nord Lead 2 schematics but these are close enough. The components are numbered differently but it's the same circuit. Q5 and Q6 in this picture are Q6 and Q7 on my main board. They make up a class B power amplifier for the right channel of the headphone output.
This failure also leads to a theory of what originally went wrong. Headphone sockets use TRS jack connectors: Tip Ring Sleeve. Right next to the headphone jack are multiple mono TS jacks: Tip Sleeve. When you insert a TS plug into a TRS jack then you short the Ring contact to Sleeve. And for a stereo jack, the right channel is connected to the Ring contact. I probably plugged a mono plug into the headphone out and left it in there for long enough for the amplifier of the right headphone channel to fry itself, sending current to ground. The funny thing about plugging a mono plug into a stereo jack is that as a user you can't tell that you did it wrong: you are getting sound, namely that of the left channel.
I cleaned up the circuit board with craft beer and replaced the blown capacitor and the transistors. Because of the short circuit some traces on the circuit board burned through leading to broken connections. I restored those connections with wire wrap wire.
You can see that the new transistors Q7 and Q6 are a little crooked because I held them with tweezers in my left hand while holding the hot air wand in my right hand. (I am right-handed.) The little blue "bodge wires" are there to fix burned-through traces on the circuit board.
Now the awesome thing about the bench supply was that I could test the synth and see that my repair on the main board did the job. I could also see exactly how much power the Nord uses which allowed me to design a replacement power supply. I used some relatively expensive DC-DC converters on the new power board but I think that is OK because it saves me time. The synth now has a standard DC barrel jack on the back instead of a mains plug. It takes 9-12V DC and needs about 9W.
I think this was much easier than trying to build a new linear power supply because space is limited and a linear supply has a heavy transformer in it which you need to bolt to the case. The new DC-DC supply weighs almost nothing in comparison. I think it's also easier to build an asymmetric supply this way. The +5V rail used about 900mA so I used a 5V 1A converter for that. The +12V rail used about 40mA and the -12V and -5V rail together (the -5V is derived from -12V) used about 90mA. I used a 3W bipolar converter which can deliver 125mA into +12V and -12V.
I hand-sawed a protoboard to size and hand-drilled some holes in it to be able to mount it on the stand-offs of the original power supply. It doesn't look pretty but it works and it's hidden from view. The new power supply uses a Traco TSR 1-2450 to provide +5V for the digital circuits, a Traco TVN 3-0922 to create +/-12V for the analog circuits and an LM7905 to create -5V for the audio DAC's. The TVN is expensive overkill but I had it lying around.
Conclusion
My Nord Rack 2 works again and it's lighter and quieter now that the old transformer-based power board is gone. (The old transformer made an annoying humming sound.) I learned how to use a bench power supply and I did my first project with an isolated DC-DC converter to create a bi-polar power supply for analog stuff. I am happy that the synth is working again and I feel like I learned something, yay!