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Old 17th Nov 2017, 5:10 pm   #1
ITAM805
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Default MIDI keyboard fault

Is there a MIDI doctor in the house? I've been given a 90's Studiologic SL-880, an 88 note weighted keyboard with an odd fault. Utilising 'Midi Ox' software the k/b occasionally and momentarily switches MIDI channel by itself. If for instance I'm playing F# on ch 4, F# ch 1 appears in the Ox list?

Is this likely to be the microcontroller (ST9036C1) on the blink or something more insidious?

I should say I have reflowed the solder joints on the board and laboriously cleaned all the keyboard membrane switches

Many thanks
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Old 17th Nov 2017, 6:56 pm   #2
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Default Re: MIDI keyboard fault

How do you -intentionally- change the MIDI channel on those? By going into a menu of some sort, or just by pressing a front panel key and a note key together?

Depending on how sophisticated it is, it may be possible to 'split' the keyboard so that different sections send on different MIDI channels so that playing at the bottom end sounds one instrument and playing higher up sounds another.

I realise this is not what you're describing (from what you say, you could hit the same note thirty times consecutively and mostly it will send on the channel you have set, but occasionally sends the same note on another channel). Is that right?
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Old 17th Nov 2017, 8:09 pm   #3
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Default Re: MIDI keyboard fault

Is it possibly a data transmission error? I understand the kbd uses old-style DIN MIDI connectors. What is at the computer end? Is it a MIDI-USB dongle? Some of these are not known for their reliability, I believe. If you strike the same key repeatedly, do you see any other anomalies, for example pitch errors? The channel ID is included in each note on and note off channel voice message, so the kbd is not actually switching channel in the conventional sense, but message appears to be getting corrupted.
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Old 17th Nov 2017, 8:33 pm   #4
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Default Re: MIDI keyboard fault

Hi AG, yes I'm using a MIDI/USB cable but that said, the k/b has had problems according to the the last owner, spurious notes playing and so on. I have a tower with Cubase installed which I can fire up that uses MIDI cabling in/out. I haven't noted any other anomalies so far?

Hi SH, the MIDI changes are made with buttons on the fascia and data entered through key presses, quite rudimentary as you can see. I did a factory reset which assigns channel 1-4 to zones 1-4 respectively and this works according to Ox. You have a point about k/b split, I never considered that but that wouldn't account for the spurious channel changes when hitting one key?
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Old 17th Nov 2017, 8:55 pm   #5
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Default Re: MIDI keyboard fault

If you have a 'scope, I'd be inclined to take a look at the data line, both with and without the other end connected. Also stick the 'scope on the power supply lines.

Edit: a quick foray into Google suggests that you're luck to get away with the occasional wrong channel with this keyboard. They have, by all accounts, somewhat flaky electronics. The user manual isn't exactly informative, is it?

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Old 18th Nov 2017, 1:33 pm   #6
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Default Re: MIDI keyboard fault

AG, I have a scope but the k/b is too big (and heavy) for my bench, aka kitchen worktop. I could remove the board but I'm not sure how I could trigger notes. But what would I be looking for anyway, one data stream looks much like another - to me anyway?

I had the external PSU apart and the capacitor tested good as did the voltage. Today I tried to get a screenshot of the MIDI strangeness in 'OX' but Sod's Law prevailed, it worked without issue
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Old 18th Nov 2017, 2:04 pm   #7
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Default Re: MIDI keyboard fault

I'd probably not be looking at the actual bits in the bit stream, but whether or not the signal looks clean and free of jitter. Similarly, I'd take a look for noise on the power supply rails. If, however, the fault only affects the channel ID in an occasional channel voice command, it seems unlikely to me that either PSU noise or data line noise would be so specific.
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Old 19th Nov 2017, 12:20 pm   #8
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Default Re: MIDI keyboard fault

Thanks AG. I was wondering if I replaced the microcontroller and EEPROM (93C46) if I might cure this problem, as I want to move this k/b on? I found a processor on Aliexpress for under a fiver but the 93C46 would have to be programmed, not something I can do. I contacted Fatar but they never bothered to reply, so I'm rather stuck
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Old 19th Nov 2017, 12:26 pm   #9
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Default Re: MIDI keyboard fault

Itam805, If I remember correctly you do a little bit with microprocessors, do you have a PicKit 2 PIC programmer by any chance?

The latest / final version (V2.61?) of the standalone PC support software for those includes a three-channel 'logic capture' tool, perfect for capturing radio-control, infra-red, serial data and in this case, MIDI data packets.

You could use that (or any other single channel logic capture tool you may have) to see if the keyboard really is sending different packets every now and again, or if it is just sending the same packet over and over again badly formed so that it is difficult for the receiver to read it consistently.

MIDI outputs are usually open-collector so you would need to connect a load (a 470R resistor, perhaps) between pins 4 and 5 on the MIDI OUT of the keyboard in order to see anything coming out.
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Old 19th Nov 2017, 12:43 pm   #10
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Default Re: MIDI keyboard fault

Hi SH, well not really - read: not at all! My brain fogs over when such thing as programmers are mentioned I did knock up a little PIC short circuit finder but that only worked with the help and guidance of the good folk hereabouts.

Thanks for tip about the MIDI termination
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Old 19th Nov 2017, 1:37 pm   #11
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Default Re: MIDI keyboard fault

I presume you've tried a hard reset which, I gather is "Hold Up , Down and Enter buttons while Powering on , hold for 5 seconds, then release."
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Old 19th Nov 2017, 2:01 pm   #12
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Default Re: MIDI keyboard fault

AG, yes, see post #4
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Old 19th Nov 2017, 2:06 pm   #13
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Default Re: MIDI keyboard fault

Sorry, missed that.
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Old 19th Nov 2017, 11:42 pm   #14
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Default Re: MIDI keyboard fault

OK Itam805, I think you have a few bits of synth gear, but possibly only one computer and / or MIDI interface?

If you have any MIDI sound source which is multitimbral (ie, you can set different sounds on differerent MIDI channels so it can be a 'band in a box') then set each channel on it to a wildly different sound and connect the keyboard directly to the sound source, missing out the computer and its MIDI interface altogether.

If the multitimbral source always sounds the right note but sometimes the wrong instrument, then your suspected problem with the keyboard is probably true.

I agree with AG that, anecdotally, the commonly available cheap 5-DIN -USB MIDI interfaces can be a bit flaky.

If the PC is old enough to have a 15-pin 'D' "Game" port either as part of its motherboard or as part of a plug-in soundcard, the 'Game' port usually has the digital half of a MIDI interface built in, needing only the addition of an open-collector buffer (outgoing) and an optocoupler (incoming). These used to be available in the form of a "Game port MIDI cable".
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Old 20th Nov 2017, 12:04 am   #15
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Default Re: MIDI keyboard fault

Thanks SH, I have a purpose built music pc with an M-Audio 24/96 soundcard with MIDI in/out, and dozens of multi-timbral plug-ins, I'm only using the USB cable for convenience as my laptop is easier to get to at present...

I also have a Korg k/b that I can plug the 'thing' into which is multi-timbral
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Old 20th Nov 2017, 12:45 am   #16
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Default Re: MIDI keyboard fault

Ah, then you obviously don't need anyone's help finding a top class MIDI interface (!) but since you have one I think it is only fair to ask you to repeat the experiment using MidiOX running on the dedicated PC with its high end MIDI interface.

Your earlier experiments may only be revealing that the USB MIDI interface is not working as well as you might hope, as AG suggested. I would certainly try that before going to the extreme length of trying to find a replacement microprocessor (you would need not only the microprocessor but the code which goes in it, unless it has a separate firmware eprom).

'Always try the simple stuff first'. If there are things you can try using just whatever you already have to hand without changing or buying additional parts, do that first.
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Old 20th Nov 2017, 12:58 am   #17
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Default Re: MIDI keyboard fault

SH, thanks, I shall try your suggestion and report back.

I have ordered the microprocessor from Ali Ex (post #8) for 3 odd, so worth a punt even if it's a duffer, but the EEprom, that's a dead end I'm afraid?
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Old 20th Nov 2017, 9:35 am   #18
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Default Re: MIDI keyboard fault

I think it's most likely that the EEPROM is used only to store configuration data. All the code will be in the microcontroller which, by the way, has a mask ROM. A quick question for you:

The microcontroller is a PLCC package. Is it soldered in, or in a socket? If the latter, some cleaning might be in order.
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Old 20th Nov 2017, 12:38 pm   #19
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Default Re: MIDI keyboard fault

Hi Geek, it's socketed, I removed, cleaned it as well as reflowed the socket pins to the board, I also reflowed the Eeprom
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Old 20th Nov 2017, 3:47 pm   #20
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Default Re: MIDI keyboard fault

Thought you would have. However, I still say you need to check the power supply with a scope while operating. From what I can see, there is an on-board regulator with a filter cap next to it, which you should also check. Also, I gather that not only is the unregulated supply routed through the power switch, but so is the regulated supply, and this has been known to cause problems. The key scanner will be driving fairly long lines and I reckon that good supply and decoupling will be crucial.
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