Well, I’ve managed to sort out the cause of the noise. I swapped over the Op Amp chips and the noise practically disappeared. So I imagine one of them must have been damaged in some way. The residual noise that’s remaining is only audible when the gain is whacked right up, and Izotope RX can easily sort that out.
I haven’t made any progress with getting the left channel up and running, and it’s proving difficult to find a replacement long-pole dual gang potentiometer, so have decided to cut my losses and stick with a mono output only which is fine.
I also noticed the LCD had a bit of a flicker to it, so thought I’d look again at the power output on the motherboard, now that I’ve established there was a problem with the old PSU. And I’m pleased to report that it’s outputting power, so I’ve redirected the LCD backlight back to where it should go and the flicker has totally gone.
So all in all, a success and I’m already quite a way through multi-sampling it. Will put together an audio demo once I have a full palette of sounds to choose from.
Finally, some progress! I replaced the relay, but this didn’t seem to make any difference. Next, I replaced the nearby capacitors (quite a tricky job on a 30 year old module) but this didn’t help either. So I resorted to my trusty multimeter and determined that there was no power getting to the negative voltage regulator.
Now, I’ll fully admit at this point, I was way beyond the knowledge gained in my Electronics GCSE. But my theory was that the display, navigation, MIDI etc. all used positive voltages but the sound making areas must need negative voltages. So logically, it can’t have been receiving an alternating current (AC).
It was at this point that I found a forum post along the lines of “Do make sure you are using an AC adapter, not a DC adapter, otherwise you’ll get no sound”. So I thought I’d better order a new one to be on the safe side.
It arrived this morning and low and behold, we now have sound! Sort of.
More issues
The first issue is that there is only sound in the right channel. Also, the volume knob seems knackered and only works when turned fully clockwise. So I suspect that could be the cause of the left channel problems too.
There is also a lot of background hum, particularly around 50hz. Fortunately, the Izotope RX De-hum seems to get rid of most of and I’m sure with a bit of tweaking should give me an acceptable sound.
But what does it sound like?
As expected, the pianos sound really nice, if a little one dimensional (i.e. Don’t appear to have many velocity layers). In fact, most of the sounds seem to be piano based with varying degrees of brightness, attack and detuning. For want of a better word, they sound ‘Expensive’, but are perhaps a bit too full sounding. Fine for solo piano, but I would guess they might need to be thinned out a bit to fit in a mix.
There are also a selection of growly synth sounds, soft pads, digital bells, harpsichords and a quite usable Acoustic Bass too.
The Electric Piano is an interesting one. It appears to have two velocity layers, but they sound nothing like each other. The lower velocity has an almost FM like quality, but the higher velocity is a lovely Rhodes sample. It does sound a bit unnatural moving from one to the other, so I’ll probably just multisample them into two separate instruments.
What’s next?
Well, I probably will have a go at replacing the volume potentiometer at some point and tracking down the cause of the hum. If the volume pot isn’t the cause of the left channel being silent, I’ll probably try swapping over the Op-Amps as they are mirrored for left and right channels. Otherwise, I don’t think I’m losing too much in Mono and can always widen the sound in my DAW.
But before I do any of that, I think it’s best to multisample the sounds into Kontakt just in case I manage to fry anything.
Well, it was a bit of a battle, but I finally have a working display for the Kurzweil!
The removal and replacement of the LCD itself wasn’t too difficult, but the backlight didn’t appear to be working on the new LCD. So I checked the two connection pins on the circuitboard, but couldn’t detect any power coming out of them.
I then checked the specs for the LCD and which revealed it accepts a maximum of 13V, so I hooked up a 9v battery to it and the backlight burst into life.
I guess there’s a component somewhere in the circuit which has failed or a dry solder point perhaps, but a quick visual inspection didn’t reveal anything obvious, so I decided to look for an alternative 9v source.
Fortunately, the 900 MX has an external 9v wall wart PSU, so I ran wires from the input socket and power switch to the backlight. The reason for running it after the power switch of course is because otherwise the backlight would be permanently on!
After reassembly, I did some testing and found that there was good and bad news.
The good news
I could navigate through the menus fine, select patches and change MIDI settings. All buttons work too.
The MIDI indicator responds to incoming data from my controller. In fact, the MX has a handy built in MIDI monitor function so I could see the note on / off information as well as pitch bend.
The bad news
It doesn’t make any sound. Pretty bad news I think you’ll agree.
So I whipped the top off again to have a good stare at the circuit board. I noticed two IC’s with Left and Right screen-printed below them on the circuit board, so guessed that might be a good place to start. The IC’s were marked PCM56P, which I think are Digital to Analog converters (DAC’s).
However, the likelihood of both DAC’s failing at the same time seemed unlikely, so I carried on looking for any shared components in the same general area. That’s when I chanced upon a relay by American Relays Inc, marked AD2A05D. I must profess, I didn’t really know what it did, but it was positioned on the circuit board between the DAC’s and the output phono jacks so I guessed the sound must be somehow routed through it.
I then chanced upon a webpage dedicated to problem solving for the Kurzweil K1000 http://k1000.net/problems.htm which revealed I could be onto something. Under the heading ‘No Sound or Distorted Sound’ it read..
“This is most often caused by a faulty audio output relay – a relay that connects the audio to the two output jacks on the back of the expander or keyboard. If the relay gets stuck or fails, no sound will be heard at the output jacks.”
So I’m hoping that’s also what’s wrong with my MX. Being in the UK, the chances of finding the original relay are nil, but I managed to find what I think is a compatible one on ebay. It’s the Hamlin HE722A0510 which is a 5v DPST reed relay with diode.
Picked up what I hope will be a bit of a bargain on eBay, the Kurzweil 900 MX MicroExpander.
There’s not a lot of information out there, but I believe it was launched in 1989 as part of their Home Product range and features 63 sounds taken from their flagship K250 including pianos, bass and strings. Being a home product, there are some compromises such as using phono plugs rather than 6.5mm jacks and having a wall wart power supply rather than a built in PSU, but it does retain the 12 note polyphony of its older sibling.
This particular unit looked a little worse for wear on eBay, with some scratches on the top and no power supply. It didn’t look too bad when it arrived today though, but I had no idea if it would actually work.
Connecting a spare 9V AC PSU, the MIDI light flashed on for a second, but it appeared the backlight on the LCD was dead. I could just about read the text though and navigate through the sounds – so far so good.
The 900 MX uses a 16 character, single-line LCD so I set about sourcing a replacement, finally settling on a Winstar WH1601 which you can find on eBay. Unlike my Akai S2000, the LCD isn’t wired directly to the circuit board, so this should make it much easier to swap out. I’ll follow up with more details once I get the LCD replaced, but for now, check out this video from Espen Kraft where he demos the similar 1000 PX
Update 8/1/2021
Still waiting for the replacement LCD, but it has at least shipped now so should arrive next week. Meanwhile, I’ve started reviewing the pinouts on the old LCD as it’s in 2 rows of 7 pins (plus separate pins for the backlight) rather than a single row of 16 as is on the replacement.
This website has been quite illuminating (pardon the pun) and revealed that the pins are numbered in pairs.
