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YAMAHA
STRINGS
SS-30
RACK-MOUNTED WITH MIDI
MIDI STRINGS

Friday, September 15, 2017

2017 - Wrapping up for another year

Time's up! The summer here is over. Yep. Three months and we're done.

I could have done more, but the rest of life gets in the way - which is only right. This is a hobby project and one amongst many other interests. I've also had to carry out a few unforeseen repairs which slowed things down a bit.

There is a bit more warm weather left and I could get a heater for the garage but something else has come up which I need to focus my energy on. I'm breaking the habit of a life-time and playing music live. It's less than a month away now and I really have to get my act together. By the time that's done it will be mid-October and, well, we'll see.

What I have done this year
  • Fixed noise and other issues
  • Built interface and played the SS-30 via MIDI
  • Started a front-panel design.
What I also wanted to have done this year:
  • Sort out the power supply for the interface
  • Fix up a solution for the power supply, MIDI and audio connectors on the back panel.
  • Fit everything in the enclosure and fix temporary front-panel to it as well
If I'd managed that I could have taken the SS-30M inside for the winter and, y'know, actually played it. Well, some of the front-panel controls aren't working very well. The Orchestra section switches in particular are failing. This is my fault for not looking after them properly but it's another hurdle.


What I did that you can't see yet (aaaah!)
  • Bought components for the front-panel 
I'll save that for another few posts on the subject of knobs, sliders, switches and, err, cannibalising old Yamaha organs (ahem). Oh yes!







Build-a-board


Let's build this thing!

First board with Lite-On opto-couplers placed ready for soldering.


The first problem I encountered was that my lovely connectors were too big for the wires on the K-boards. I had to go and order another batch of wires and then add new wires from the K boards to the connectors. This took a lot longer than I was hoping but I had no real choice.

Lotsa, lotsa wires.
K1 and K2 boards wired to connectors on the first interface board.


Much soldering and wiring later I had the first board (err) wired up and - somewhat to my surprise - all working perfectly.


The first interface board wired up to the MIDI decoder

MIDI IN


I ploughed on to the second interface and suddenly I had a MIDI controlled SS-30!



MIDI Interface almost complete


 So, I'm almost there. I have to sort out the power to the decoder now though. So far it's been run of a bench PSU and I hope to get it running off the SS-30 supply. Thats' another post though.

I meant to get a recording of the whole thing being played, but it's not done yet so I'll end this post on that tantalising prospect.


'Opto-mum' Currents?

This post is a bit behind events now (more on that to come) but here it is anyway.

'Opto-mum' Currents?

Before I launch into building the coupling boards that will interface the MIDI decoder to the key switches I need to actually do a bit a electronics design with equations and everything! The first task is to ensure that opto-couplers have enough current on the input to make sure there is enough current on the output. The input is an LED, so if the current is too low the light emitted by the diode will not be sufficient to turn on the photo transistor fully. In that case the voltage drop across the phototransistor will be too low and the switch will only be partially on. It might be enough to hear something, but I don't want a quiet SS30.
The second task is to limit the current to each input so that the cumulative load of the switches is not drawing hundreds of milliamperes. With 49 keys the potential is there to simultaneously draw more current than the power-supply can handle and blow the fuse. The power rails on the SS30 are rated up to 500mA and if possible I want to use the +15V supply for the MTP8 and the couplers.
Therefore I need to find the optimum current for each optocoupler input - not too low and not too high.

MIDI Decoder 

The MTP8 draws around 15mA for it's own logic etc.
The maximum current per output in the MTP8 is 500mA. But, each group of 8 outputs is limited to 2A which translates to 250mA per output with a possible maximum for 49 keys of 12.25A(!). All that means is it can comformatbly handle large currents. But I want as low a current as possible.

K Boards

The K boards key-switch 'input' is held at -7V through a 3.3K resistor. That means we should getting around 2mA through each switch.


KS-M-C3 is what goes to the key to be switched to ground

The measured value I got from one I picked at random was over that so my calculation must be mistaken somewhere.





And, hey, look! That's my new multimeter.

2.37mA means that the voltage is actually higher. I usually measure around 7.7V which results in a current of 2.33mA, which is much closer. In any case I measured approximately 2.4mA so that is my target output current.


Opto-Coupler Characteristics

Transistor outout optocouplers have many charascteristics but the main one I'm interested in the CTR. The Current Transfer Ratio - expressed as a percentage - simply decribes the relationship between the input current to the LED and the output current. This is essentially the gain of the coupler. All I have to do is make sure that the gain is 100% and they will be no loss through the output, thus mimicing the elecro-mechanical switch I'm replacing. But, what if the gain is a lot greater than unity? Does that mean that somehow the current through the going to be higher? Err, no. The current cannot be any higher than the 2.4mA I measured, but you can have a CTR above 100% because the input current can be lower than the output. This is the kind of coupler I need because if I had to have 2.4mA available for every input I would need 118mA to cover the entire 49 keys. I want to target 50mA or 1mA per switch. 


When I set up the single octave test I had just grabbed some opto's from Maplins - Vishay IL74s - without looking too closely at this CTR characteristic.
The IL74 - data sheet here - has a quoted typical CTR of 35% for 16mA. But the datasheet also shows that it ranges above 100% when the current is greater than 20mA. To be honest I worked through the datasheet and it was a very long and boring process which required this guide on how the CTR graphs should be interpreted. Vishay take the long road and I'm not interersted in that here, so instead here's the shortcut.
I was using 1K current limiting resisistors with a 15Vsupply in that test. The input current (If) was then measured as 13.34mA (the voltage drop across the resistor being 13.34V). A lot higher than I was looking for but that comfortably switched the keys on. The effective CTR in that case was then (Ic/If) 2.34/13.34 x100 =  17.54. Experimenting with other resistors and supply voltages I managed to get good results with 9V through 1K2. This was 2.59mA If and 2.08mA Ic giving 80% CTR. However I couldn't do any better and I want to get the input current down to more like 1mA. This would require a CTR of  234%.




The LiteOn LTV-847 has minimum CTR from 50% (for 5V/5mA) but a maximum of 600%, which seems much better than the Vishay part, and they are quite cheap, so I ordered these.
Avoiding the datasheet again I worked through a series of resistsors from 1K to 10K and measured the input current and output voltage drop. I was looking for the point where the input current is low but not so low that the output current is throttled and the voltage drop across the output collector emitter is rising.


Since taking these measure I seem to have lost the info what the supply voltage was. Was I used 9V or 12V or what? Ah well. When I go the 6K8 resistors I decided they were too high and although the voltgae drop shoudl have been relatively little i t was much higher. May be I changed the input voltage? In any caseI decided to change to 5K6 and as I had enough 'in stock' I used those instead. It was time to start building!



















Thursday, August 31, 2017

Billy Currie talks String Synths and Ultravox


Big, exciting updates on the SS-30M progress coming soon!

But in the meantime, here's a very interesting video from GForce Software. Billy Currie (of Ultravox fame) talks about stringers with reference to the Elka Rhapsody and SS-30 in particular. He describes how both were used on the smash hit Vienna for different parts, their relative strengths emulating real strings and the feel of each.The video is to promote the Virtual String Machine/Re-Strings plug-in emulation of strings machines,




In the video Billy is at the GForce studio and is playing the mother of all string synths - the Freeman String Symphoniser.I might have to write something about that one day (and correct a few of my mistakes).



I've written some commentary on what he's saying as it's just a general chat really and some of the points fly past pretty quickly. I then took the opportunity to write an analysis of how the SS-30 and Rhapsody were used on Vienna "of course you've got to talk about Vienna".


So Much For Solina

Ultravox were doing experimental things so they decided to get a strings machine. I'm not sure quite what that means, but I think this would be part of a general move towards electronics. It was the Solina they got first though. Billy doesn't like it because it was 'so weak'. He was not impressed! "Very middle of the road" and a "Mantovani" type of sound.

So then they got the Elka Rhapsody. The Elka had an "emotional" character. This matched his interest in German classical music gained at music college - Schoenberg, Bartok etc. And by extension then Kraftwerk and the "emotional and lost" feel. In Particular on The Ascent - from Rage In Eden


This string's something to me. 

Violinistic


For Vienna the SS-30 was used on the chorus because it was violinistic (that this a word, btw). What Billy wanted was the right envelope because as a violin player he wanted to play it like a violin.  It was obviously to Billy's liking when compared to other stringers for this reason. He sings and mimes the way a violinist would play to better describe the effect.
As he says: "You could do a very slow fade in" and "You were able to create the feeling of a bow". The SS-30 attack is actually quite short, even in the 'Slow' setting, but we're dealing with the articulation of a bowed string, and not a slow fade in.This was in contrast to sustaining the notes as suggested to him later by the guys from Mellotron (Streetly?).

I think it's quite hard to hear what he's talking about on the first two choruses though. He's specifically referring to the melody, which is picked out on piano. The strings are following the piano part but the piano dominates your attention. The strings are more washy. You need to pay a bit more attention to notice that they are following the piano. I think that's because they first sustain into chorus and then out to the "Oh, Vienna" line. You are somehow distracted from the way they articulate in the melodic phrase. The final chorus is more definitive though and this time you are left in no doubt.

In a way it's similar to how ABBA used the SS30 on Gimme Gimme. It's right there, in the hook, but as an undertone. 
In this video you can see Billy playing the part on the SS30, which gives you a better idea of what he was doing - the sound is quite poor on this though.


He doesn't really mention the bridge into the chorus, which is arguably a slower attack. This is more of a swell though and I wonder if the volume pedal was used here.


Middle String Section

 The middle section is preceded with an instrumental version of the verse. The piano takes the lead here with a single high pitched sting over the top. It's not clear which stringer but the higher parts tend to be Elka.
Then the middle section proper starts with a cello from the SS30 - "Great sound". This is the simple descending figure underpinning Billy's violin part. The attack is slow and it tucks in behind the beat to counter the synth bass line. 


I have to agree that it does sound very authentic. Very wooden.
 Then the orchestral part which comes in next is the Elka. I assume this was chosen here because of the feel. The sound is somehow 'shinier'. And then you can contrast it with the SS30 which comes marching in for the climactic final chorus. The SS30 sees the track out with one last sustained chord.

There you have it a pop masterpiece helped along by the SS-30.

Tuesday, August 01, 2017

Connecting you to the Switch Board

All-a-board

I have found the solution to my key switching PCB worries. After the quick lash-up I did on a breadboard I started to fret about what kind of permanent, solderable board to use. A standard Veroboard or matrix perfboard was the obvious answer, but only with a lot of soldering. I didn't want to have to cut dozens of wires and tracks and link everything together by hand, or rely on solder links. I also wasn't keen to design custom PCB. Apart from the costs the learning curve for creating a PCB is not one I wanted to commit myself too. If only there was a PCB just like a bread board, I thought. Well, apparently, there are! There's several on the market in fact, but I found one that matches my needs the closest.


This is the SparkFun Solder-able Breadboard - Large PRT-12699

"a real FR4 fibreglass board, with soldermask, plated through-holes, and the layout duplicates the connectivity of a solderless breadboard"

You can lay this board on top of a breadboard and insert the components through the PCB to test everything as normal. Then you can lift the PCB up with all the components already in the right place and you just have to solder them in.


Features/Specifications

  • 63 split rows
    • Like the breadboard there's "twin rows of 5 holes each, spaced 0.3" apart, to accommodate DIP ICs."
    • And there's an additional line in between the rows, but I might not need that.  
  • The holes are 0.1"/2.54 mm pitch (apart)
  • The hole size is 0.040"/1.016mm
  • The rails can be hooked up at either end 



Board Layout


I will need two of these large sized boards which will accommodate twenty-four and twenty-five key switches. With 16-pin, four-channel opto-isolators there will be three per octave (with one extra on K1 for the extra ). That's six 16-pin ICs per board needing (6 x 8) forty eight rows. K1 will need an additional single-channel 4-pin device. This will leave room to spare on both boards.
I might use some of this extra space for power rail circuit to the MTP8. A regulator is all that's needed if I can use one of the rails from the SS-30 supply. But that's another post...

The design then is simple. Every other pin on the opto-couplers will be an input or output wire and the alternates will be a resistor to one of the rails. What could be easier?

I have created a diagram of the layout in Excel  - here's the first board. Note that as this has 25 keys I'm going to reuse a spare connect on the connector and then have to cut a track (column h, row 9) on the board and link it to the correct one.







Getting A Good Connection


The only construction concern I have is how to get the wires to the board. The 8 strand wires from the K boards are around 0.7 mm with insulation but the conductors are approximately 0.5mm. That's probably 24 AWG or maybe less.

The options are:
  • Solder wires directly through the PCB holes 
  • Use some sort of pin or tag soldered to the board to solder them to 
  • Terminal blocks to hold the wires (screw or spring clips)
  • Use some sort of IDC connector
  • Solder the wires to another smaller board which is connected to the main board with some sort of  male to female headers.


Wire through the PCB 

The simplest and easiest option is to push the end of the wire through the hole on the PCB and solder like a component leg. The main drawbacks of this approach are that if I need to de-solder it there is more stress on the holes. Also the wires will probably be subject to stresses during construction and maintenance which could break off and needed re-soldering - as I have seen time and time again on the original boards. I could probably mitigate the stresses quite a lot by threading the wire through the holes. Including the rails there will be 5 available holes to go through and the stress on the join between wire and solder should be eliminated altogether.

In one's cups

On the K-boards and G boards the very fine wires are soldered to pins with cups (or 'solder wells') in them. The parts list is a bit vague here and I don't think there are even listed.

Here's a satisfyingly detailed piece of vintage instruction on  soldering such terminals.




Digikey (for example) list a part like this as PC PIN receptacles of type solder cup . The solder termination is like those you find inside connectors (DIN, Centronics, etc. see video above.

I can also use the threading technique to remove tension so there is double the advantage over simply soldering through the board.

One downside of this approach is cost. The ones at Digikey come out at over £40 before tax for the number I need. I can also see the potential for a lot of work there too. 

Terminal blocks


Avoiding soldering the wires altogether is another option. Screw and spring clamp terminal blocks could be fitted to the boards and are relatively cheap. Strips of 12 can be had for as little as £1.70 before tax.

This makes disconnection a very easy job compared to soldering and again there is strain relief possible.
 On the whole though these are not my favoured choice. The thin wires from the K boards may not hold in the clips very well and I'm not a fan of these sorts of terminals. That said, the MTP-8 uses these and I'm not currently planning to replace them.

IDC Connectors

IDC connectors are a possibility because you can get the wires into the connector without soldering or paying for fancy crimp tools*. For the kinds of wires in use this might be a good option. As the Molex notes, err, note:
"overcoat seven strand cable is the easiest and often performs as well as solid wire."
Now, because each connection is going direct to the optocoupler ICs's pins via the board, and I don't want to cut any tracks or add bunch of link wires, it will need to be double spaced (2 x 2.54mm) 5.08mm. Most IDC is single spaced at 2.54mm though. If you want 5.08mm spacing the pins are bigger because it's designed for higher current purposes. This is a drawback of using a prototype board. I'm looking for a connector that will match the board not designing the board around the ideal connector.


The photo above is 5.08 pitch but is for 18-22 AWG, so it's probably a bit to large for the wires.

 I can solve this by getting one that is double length and then using half the contacts though. That then creates an issue of sourcing the right part as the longer ones with more contacts have more limited options. For example you might find a 24-way single row IDC female header, but it's probably for the wrong wire size, or no-one stocks it in quantities under 100. Also some of the more fancy solutions will be less cost effective if they are 50% redundant. For example there are AVX parts in the 8284 series which are almost ideal but the total cost would be over £100!


* Why are crimp tools so eye-wateringly expensive anyway?

Other options

There are other options including replacing the existing wires with ribbon cable somehow so that it's a snap to plug then in and out again. I really don't think there's a better option than those above though. 

Conclusion


The MTP8 has terminal bloc connections so I don't necessarily need more at the other end for those wires. Also, I will use fatter, stronger wires for those connections so that will probably be fine to solder through the board holes directly.

The thinner wires from the K boards are the bigger concern.
They are already soldered at the K-board end, so I would prefer not to do it at the other too. Once they are soldered in there's no quick way to disconnect if needed.  However, because they are so small, even after tinning the ends with solder then may not hold in clamps very well. I would probably have to add a pin to the end of each which will be tedious to do. 
Therefore IDC is the best way to go (short of crimping) and I eventually decided to get these connectors - Autocom from Stelvio Kontek - available from RS Components

Autocom HE14 90° IDC Connectors

  • Single row IDC connectors with right angle exit for flat or single wire cables
  • No special preparation of cables required
  • Can be assembled by hand
  • Double blade contacts
  • Rated at 3A/1000V
  • Flammability rating UL94 V-0


12-way Autocom Connector


These come in the correct pitch of 2.54mm and in single rows. Individual wires can be inserted in each hole and then the two halves are simply squeezed together.








Any 2.54mm pitch header would probably work but they come with their own range of headers which key in to the connectors.





Concept Art

Here's a mock-up of what I want the panel to be like.


I've assumed I can replace all switches with potentiometers. I've opted for slide potentiometers instead of the tablet switches except for the Attack setting which I've made rotary in common with the Sustain.

I've grouped the Cello and Violin settings together, which differs from the original where the tuning and vibrato setting are followed by Sustain and then keyboard split and then the switches.


This is just a first pass at the design concept and there is a list of problems. The logo is probably too large, there is no power switch, power indicator, or MIDI activity LED. There may be other things I decide to add during development too. The whole layout might change again to get a better balance of elements, or some other whim of mine.

Nonetheless this is roughly what it will look like.



 

Thursday, July 27, 2017

Viola Woes

Chords

video

So, here's a short bit of actual chords on the SS30-M.

No Alla Viola - Silenzio

(These heading puns are getting more convoluted by the week. Top marks if you can unpick that one)

Last time I fixed the missing clock to G3. Finally I was done with the SS-30 problems! Or was I? No.

After having missed the fact that I had an issue with G3 I went more carefully through each octave and found another problem. On the bottom octave (K1/G1) there was no output from the Viola.

Tracing this problem took me to a board I've not had any issues with so far - LF.

On LF there are a pair of analogue switch ICs.

4016 Analogue Switches on LF Board


The Old Switcharoo

These 4016 devices control the signals which get selected in and out by the Keyboard Switch control. Either the Cello is selected or the Viola/Violin voices. This is called the KYB. Split Gate.


KBD. Split Gate

In this case 16U from G1 was arriving at the switch gate but not being routed through. I could see that the voltage from the switch was working fine so the problem was really with the IC.

I've ordered the replacement so it should be another easy IC swap when it arrives. The main concern is to make sure no wires break loose whilst the boards are being moved around.

And then I can start on the full MIDI interface. I have a couple of posts in the works about that.

LF Board


The LF board hosts the vibrato, pitch control, sustain control and the oscillators for the Orchestra effect.

I guess the LF board is named Low Frequency because the Vibrato and Orchestra oscillators are essentially LFOs. 

F Board


Wednesday, July 26, 2017

Flippin' Flopped Flip-Flop

There's something about blogging and cafe's isn't there? Well, I am writing this from a cafe in Paris, so this blog now has a certian je ne sais quoi it was lacking before I think. But really, je ne sais quoi. Whilst I'm away from home it's a good time to catch up on progress, of which there has been plenty since the last post.

Last time I was basking in the glow from having got one single note to play. Well I wasn't going to stop there.

Striking a Chord.


I decided to buy a few 4-channel opto-couplers from Maplin's and just see what would happen if I got a whole octave working at once.


Well, this was pretty exciting!


The thrill of playing actual chords from the SS-30on a keyboard for the first time in what must be 20 years was quite something for me. I went to bed late but very happy that night, but there was something bothering me at the back of my mind...

 Feeling Jitttery

In the post before last I mentioned that although the master clock on G3 was back in business I had some concerning waveforms. When I looked at the test points I saw that the waves we jittery. Jitter is a term from digital electronics to describe variation in timing between the signal and a clock, so I'm misusing it here but the effect was that the output from the octave divider IC was not stable. My ears told me everything was fine so I didn't immediately know what to do.

Just before connected the K2 board up for my coupler board test above I ran through each octave to decide which to use. It was then that my ears detected a jittery sounding problem with the top octave from K4. No all the keys but about half had a noise that sounded digital and related to the clocks.  After the triumph of playing a chord I came back to investigate this. Initially I was confused because it was coming and going. Some keys which had the problem seemed okay. Then I noticed that the vibrato was effecting the noise so I turned that off and played with the pitch and detune. Now I could make the problem come and go with the detune. Each tone was effected differently so I surmised that this must be related to the interaction between the two oscillators clocks for each tone and that it was specifically effecting the top octave.


Getting busy with the oscilloscope I soon saw that in fact there was no clock at all on one of the G3 board octave dividers. 
Oh no! could this be a dead YM25400? If it was I was in for some serious heartache. Thankfully tracing the input to that IC showed that the problem was further back. So, the jittery signal I'd seen and heard was because one half of G3 wasn't getting the second oscillator.

Divide and Concur


The problem actually stemmed from the flip-flop divider chip on G3 which divides the 500KHz master clock oscillators from G3 and G4 in half, to 250KHz. The octave dividers provide a 1/4 of the input clock output, as well as the octave division for each note. The dividers on G4 provide a 125KHz clock to G2 and the G3 divider 62.5KHz to G1. Hence the input for G3, which must be half of G4, comes from the flip-flop dividers.

Flip-flop - TC4027BP


The TC4027P dual JK flip-flop IC is thus fed inputs directly from the master clock oscillators of G3 and G4.

Master Clock Oscillators from G3 (top) and G4 (bottom)



I could clearly see that whilst the inputs to both of the two flip-flops was okay, the only output was the one with the G4 clock input.

G4 Master Clock Oscillator input (top) and flip-flop output (bottom)


Even after disconnecting the wire running from the output on one side of G3 to the other there was still no signal to speak of. Reducing the scale showed something was there - all but nothing compared to the other output.



Flip-flop input 2 (top) and output 2 (bottom)


The 4027 was half dead.

The fact that this chip is adjacent to the blown transistor I had just replaced and uses the same -15V supply cannot be a coincidence. 

Flip-Flop Swap-Shop


After being slightly disconcerted that none of the main electronics shops sell these devices anymore (I assumed these parts would be around for ever) I found some at a reasonble price on eBay and they were posted the next day.

A few nights later I setteled into my comfort zone (I must have replaced dozens of DIPs back in my test and service tech days) and replaced the part with alacrity.

If you're not familar with changing DIP ICs the trick is to snip off all the legs before you go near the soldering iron. That is unless you have a pot of solder on the heat nearby, and then you can (after protecting the neigbouring parts and being liberal with your flux) place it pin-side down in the bath and pluck the IC out as soon as the solder has melted.

Faulty TC4027BP with legs snipped off.
 Then it's time to deloder each leg in turn. Doing it this way avoids putting additional strain on the pads as you try and get each one free. Ideally you have through-hole plating so that you can desolder from the top, where you pull the legs through. In the case of the SS30 there is no pad at all on the top side for the ICs so I had to do it in two stages: first on the bottom side I heated the solder on each pad and desoldered with wick (suction is liable to pull the pads of); then I used the iron to push each leg through to the top-side. As there is no solder through the hole they usually fall through without too much trouble.

Top-side of G3 board with IC removed


For most I have to do some more desoldering after pushing the leg through and then turn the board over and heat the leg from the top using tweezers to remove the last obstinate couple. Finally I went back to the bottom-side and used the wick again to remove the remaining solder. The main concern with that is to make sure that the legs of the new chip would slide cleanly through. With plated through-holes this can be a real pain to do cleanly.
 Even though I was careful I still lost a part of one of the pads though. This is typical for a pad with no track, as in this case. Because the pin is linked to the adjacent pin it still held enough to solder though.

Bottom-side of G3 with IC pads cleaned.
The new chip was a very close match to the original Toshiba part.

Old part (left) new part (right)


New part soldered in place


Soon everything was back to normal and the jitter was gone as all clocks were present and correct.


Both outputs from the flip-flops






Friday, July 14, 2017

MIDI Interface - Test #1



I am conscious that this project is taking a long time. As you can see I'm going through another rush of activity whilst the weather is warm and I can sit comfortably in my drafty garage but how long it will last, I don't know. In the last post I said I would pack the boards carefully into the rack enclosure before making a start on the MIDI interface at long last. Well, last night I thought again. Now I have my new bench power-supply I decided I would just check if the j-Omega MTP8 I bought back in (checks blog) 2009 was powering up okay.

I'm pleased to report that when I set-up a 15V supply the MTP8's power LED came on. At this point I couldn' t resist trying a simple test with an LED. And it worked.

video


The MIDI keyboard I'm using is my trusty old Yamaha (of course) PSS-580. A lovely example of the Portasound breed with a programmable 2-operator FM synth and useful MIDI spec. It says it's a Workstation and it really is. The fact that it's nearer in age to the SS-30 than to now is slightly amazing to me though. It is possible to record sequences on this keyboard too so for testing it will be possible to set up a simple note on, note off for each key which I can play at will. I could do this from my phone or iPad with a MIDI interface too. I might re-think and do that but for now I like having a keyboard and I can easily use it for a tuning reference.

Here's a fun demo I found on YouTube of some of the more extreme possibilities of the synth section.


Handily, the keyboard is exactly the same size as the SS-30 - 49 keys from C1->C5 - so it's ideal for this project. And although I still have a soft spot for this keyboard  - it was my first ever synth - it's not in use in my studio these days. I have a Volca FM performing FM duties, so it's not really required.
Something else I will need to do is check the range of the keys used by the MTP8. I connected output number 32 (of 64) and that mapped to G3. If output 1 was C1. C2 would be on 13 and C3 on 25, C4 on 37 and C5 on 49. G3 would then be on 32 so it is mapped exactly as I would have wanted it to. Great!


With the MTP8 working and the SS-30 ready and working I still had time to take the obvious next step -  try to control the SS-30 from MIDI! At long last.

First though I had to find something to handle the switching. As you may remember from this post, on how to switch the -7V keys to ground, there are a few options but the obvious choice is some sort of opto-coupler/opto-isolator. I thought I must have one somewhere and remembered that I have a CNY17 chip. Originally this was used for a failed attempt at a DIY MIDI CV converter. And then it was stolen for a Gameboy MIDI interface. For MIDI it's always recommended to use an opto-isolator and on the MTP8 there's a similar chip for the same purpose. The CNY17 is a passive component of the type I expect to use so I de-soldered it from the GB interface and with a bit of wiring and a couple of resistors on the bread board I could switch the LED on/off through the opto-coupler.

It was the moment of truth. I knew it should work, but could I play one note of the SS-30 via MIDI? Yes, of course  :-)

video


So there you have it. All I need to do now is make another 48 similar circuits and it's job done!




Thursday, July 13, 2017

Jitter Bug

Fix the jitter bug...!

Well, that's a relief! Last night I fitted a new transistor on G3 and the clock came back to life with a far more healthy looking waveform.

Tr1 Replacement on G3 - A bit wonky as I'm trying to be careful handling these boards to avoid breaking more wires.

 To be frank I'm still not totally happy with it though. I need to spend a bit more time measuring and checking because it still seems a bit, jittery... Not as bad as before but on the scope it's not as stable as the clock on G4.

I've Got The Bench Power

Before trying the transistor I thought I would have another go with a bench power-supply. But this time it was a linear supply, not the switch-mode one I borrowed. Ah, yes, this one is mine! I decided I needed a new toy for when I start work on the  MIDI interface so I bought cheapest one Farnell offered - a Tenma - and it's rather good for fifty notes. It'll appear in a later post no doubt.

Transistor Saviour


Initially I was a bit worried that I couldn't find a replacement for the 2AS509 transistor. I could find expensive ones or cheap ones in the states that would be expensive to ship over but nothing in the UK. Eventually I noticed that BC636 were close match and were cheaply available at Farnell. I bought a few just in case, as there we so cheap.


Back in the S.S.3.0

In any case, it works with the new part fitted and I plugged the output back into my mixer and had a listen - More noise! Oh, but this time I know what to look for. Sure enough a ground wire had snapped off G3. A quick fix and I was finally listening to the SS-30, as it should sound. Hurray!



To celebrate I decided to create a short video of me fiddling with the temporary front panel. Not the most exciting thing you'll watch today I would imagine but it's nice for me to have a record of these things.

video

You will notice that some of the pots are a bit noisy and sometimes when I switch things in it gets quieter, but overall things are working.

After a few more checks I should now be able to put everything back together in the rack case and after verifying it still works turn to the MIDI interface. The idea is to keep all the SS-30 boards inside the case, have the front-panel easily accessible and then just have the switch PCBs available to connect up with the interface.