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midiDAC pitch bend way too subtle
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author midiDAC pitch bend way too subtle
lasselakken
I have very little response when pitch bending via the midiDAC. Even with the pitch bend pot all turned up, I bend less than a semitone. I checked the pot and that's the correct value (I didn't desolder it to check if its faulty, though). Any tips on where to check?
Kind regards
Lasse
Synthbuilder
lasselakken wrote:
I have very little response when pitch bending via the midiDAC. Even with the pitch bend pot all turned up, I bend less than a semitone.


The first thing to check is the output from the bend socket. If that goes from -5V to +5V, then check R5 is 47K and not something like 470K.

If it isn't going from +5V to -5V then check that R28 and R29 are both 10K.

Tony
lasselakken
Thank you for the reply, Tony.
The output from the bend socket only goes from +0,14V to -0,14V when I turn the pitch bend wheel and the bend pot is fully clockwise. I tried several MIDI-keyboards with the same result.
R5, R28 and R29 all seem to be ok, though. Anything else, I should check out?
Synthbuilder
lasselakken wrote:
R5, R28 and R29 all seem to be ok, though. Anything else, I should check out?


R23, 24 and 25 could also be the problem. All of these should be 10K. R23 and R25 being 0.1% tolerance 10K types.

The raw bend CV signal comes from pin 7 U12. Check that this moves from 0V to 10V when you wiggle the PB wheel or lever from one to the other.

Tony
lasselakken
Synthbuilder wrote:
R23, 24 and 25 could also be the problem. All of these should be 10K. R23 and R25 being 0.1% tolerance 10K types.

The raw bend CV signal comes from pin 7 U12. Check that this moves from 0V to 10V when you wiggle the PB wheel or lever from one to the other.

All that turns out to be fine too. I measure between 0,01V and 9,97V on pin 7 of U12. hmmm.....
Synthbuilder
lasselakken wrote:
I measure between 0,01V and 9,97V on pin 7 of U12.


That's good. So the problem will probably be due to the gain in the op-amp stages based around U2b and U4b.

What's the voltage of pin 7 U2 doing? This should vary between +5V and -5V. If not it's probably resistors, R23, R25 & R24. Don't trust their colour codes - remember that 23 and 25 will have a different code because they should be 0.1% types. Take them out and measure them. If pin 7 does vary from +5V to -5V then check for shorts on the board around C8 and pins 6 and 7 on U4. If OK check again the values of R27, R28 and R29.

Tony
lasselakken
Thanks again Tony for your impressively competent help here.
I checked all of the above, and all seems fine. I get a steady +5V to -5V on pin 7 of U2. I desoldered one end of R5, R23, R24, R25, R27, R28 and R29 and all measure as expected. There are no visible shorts on the board around C6 and U4 - I scraped a bit around the solder tags to make sure (but of course there could still be a short beneath the ic-socket).
I measured the voltage on the bend socket again and it seems a little weird. It goes from about +1V to -1V, but the bend pot doesn't seem to change that value at all. That is: I measure between +1V and -1V on the bend socket with any setting of the bend pot.
As I mentioned, I haven't checked the pot yet since it is such a pain to desolder - but maybe that should be my next step?
Synthbuilder
lasselakken wrote:
It goes from about +1V to -1V, but the bend pot doesn't seem to change that value at all.


That's correct. The bend pot only affects the amount of bend applied to the main pitch CV output.

It could be the pot that is dragging the voltage down. But it'd have to be a really low value of pot - something like 100R or something to cause a problem.

With the module removed from the power supply, measure the resistance between the output lug on the bend socket and the ground lug on the bend socket. It should be quite high - not sure exactly but anything under 100R would indicate a problem.

To check for shorts around U4 remove U4 and measure the resistance between pins 6 and 7 of the IC socket. It should be quite high - again anything under 1K would indicate a problem. C6 perhaps?

Tony
lasselakken
Synthbuilder wrote:
With the module removed from the power supply, measure the resistance between the output lug on the bend socket and the ground lug on the bend socket. It should be quite high - not sure exactly but anything under 100R would indicate a problem.

To check for shorts around U4 remove U4 and measure the resistance between pins 6 and 7 of the IC socket. It should be quite high - again anything under 1K would indicate a problem. C6 perhaps?

Hi Tony
I've been away from DIY for a while, but now I'm back :-) I measured the resistance between output and ground on the bend socket, and there is only 31R! Is that an indication of a wrong or faulty bend pot? Between pins 6 and 7 on the socket of U4, I measured 9K ohms, so that seems to be alright.
Kind regards
Lasse
Synthbuilder
lasselakken wrote:
I measured the resistance between output and ground on the bend socket, and there is only 31R! Is that an indication of a wrong or faulty bend pot?


It's either a broken pot or there's a problem with the wiring to the bend CV socket. It could be the socket too but I'd expect that to be a complete short (ie. near zero ohms) if that had failed.

Tony
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