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Synth Fix
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LektroiD
Joined: 26 Jun 2009
Posts: 11
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 Posted: Mon May 17, 2010 7:09 am Post subject: Little phatty pitch wheel issues |
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Dumping this info here until I can get round to sorting the problem. This will be of no use to anyone without the necessary files that accompany it. Please PM me for the rest of the data if you are having similar issues
| Quote: | If you have done all calibrations including the "pw amount" calibration, then it's possible that something is awry with the pitch bend DAC circuit. If you haven't also done the PW amount calibration, of course do this... but it sounds like you've probably done them all now, yes?
It's a great thing that you have the equipment and enthusiasm to look into this more closely yourself. Here are a bunch of pics which should help. Start by hooking up your scope to TP13 - pitch wheel (and TP19- analogue ground) and look at the voltage as you move the pitch wheel. I expect it's probably smooth, should be centred around 1.25 volts and should change by +/- a volt or so as you move the wheel through its range.
If this looks good, then we know the pitch wheel input is OK. Next, put your scope probe on U40 pin 1. This is the pitch-bend output after it's been through the processor. I expect it will not look so good, based on your problem description. You will see a change here in response to MIDI pitch bend also. If this does indeed look bad, then check also U40 pin 3. This is the direct output of the DAC, so it will look a bit funny as it's unfiltered... you should see periodic voltage spikes whose amplitude corresponds to the amount of pitch bend voltage. On a working unit, the change in amplitude of these spikes will be smooth as you move the pitch wheel. If you get a jerky or discontinuous change in amplitude of these spikes, then there is some sort of digital problem. Possibly it's the de-multiplex chip that takes the multiplexed output from the DAC and gives you a single voltage for pitch bend. This chip is U39. The pitch bend CV output is pin 4, but this is the same signal you see at U40 pin 3. The multiplexed CV input appears on U39 pin 8. If you can interpret the multiplexed signal, you should see one little segment of the stepped waveform moving smoothly up and down as you move the pitch wheel. If you see this and it looks smooth, but the demuxed signal on U39 pin 4 looks bad, then U49 is bad and needs to be replaced.
If on the other hand you are seeing jerky/garbled motion of the one segment of the multiplexed signal when viewed at U49 pin 8, then there is digital problem that might be firmware-related somehow. At any rate it's a digital problem that is independent of all the analogue circuitry in the synth.
This should help to clarify a thing or two... have a look around and let me know what you find! |
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