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Discontinuity calibration issue
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author Discontinuity calibration issue
terjewinther
Finished building some more modules, and the Discontinuity is up next.
The first two calibration trimmers worked fine, but I am unable to get OFF1 to zero volts. Also, the offset pot does not seem to work properly - very tiny change in sound (compared to the other pots). All other pots, the LED, CV inputs and volume seems to work as they should. Sound is getting through all right, and gets modulated.
The OFF1 trimmer changes the voltage of pin 7 of U13 from over 6 volts down to approx. 2,4 volts. Double-checked that R72 is 390K, and that both R69 and R70 are 330R. I have not calibrated the unity trimmer yet, but turned it a bit to check that it is working, and it seems to be.
The calibration is for U11 I guess, which is LM13700. I am running low on them, but have some LM13600. Not sure if that is an OK substitute in this application, though.
Any hints appreciated.
Synthbuilder
It'll be worth checking the voltages on pin 1 and pin 7 of U4. Both should be around 0V with the threshold pot in its central position which it should be to calibrate OFF1.

If so, try replacing the LM13700 with a 13600 - they are compatible in the Discon - and see what happens. If that doesn't fix it, check R73 is a 15K resistor and not something like 150R.

Tony
terjewinther
Thanks. Some interesting discoveries:
Voltages on pin 1 and pin 7 of U4 are 1,6 volts (positive and negative of course). With the threshold pot in the middle there are 0 volts coming out of the pot, and there is no CV coming from the threshold CV input, so the pot and CV input is OK.
It seems to me that the 1,6 volts are coming through the "Thresh_L" line.
If I turn the Threshold pot, the voltage at the outputs of U4 change, but it is almost impossible to get 0 volts at the outputs of U4.

Tried with new 13700 and new 13600. No change.
R73 is indeed 15k.
Checked some other parts too, and all seems to be good.
Swapped some ICs just in case any was faulty, but no change.

I can desolder R23 to disconnect the Thresh_L line and see what happens.
Should possibly desolder R25 too?

Synthbuilder wrote:
It'll be worth checking the voltages on pin 1 and pin 7 of U4. Both should be around 0V with the threshold pot in its central position which it should be to calibrate OFF1.

If so, try replacing the LM13700 with a 13600 - they are compatible in the Discon - and see what happens. If that doesn't fix it, check R73 is a 15K resistor and not something like 150R.

Tony
Synthbuilder
The output of U4 (pins 1 and 7) should go from around -6.5V to +6.5V (pin 1 going the opposite direction to pin 7) when the threshold pot is moved from one end to the other. What's the voltage at pin 6 on U4? It should be zero. If not replace U4.

I don't think any fault is coming through R23. It's too big a value to be able to force the output of U4 to do anything other what it should be.

Tony
terjewinther
The output of U4 does indeed change from -6.5V to +6.5V. However, it jumps from about -2V to +3V, even though I try very tiny movements with the threshold pot, so it is not possible to get 0V on the output.

Replaced U4. No change.

Thanks for the comment on R23.

The OFF1 trimmer never gets the pin7 of U13 lower than about +3V.

I tried a full, new calibration, and I also tried following the descriptions in the builder and user manuals. Listening to the sound and watching it on a scoop shows that all pots does indeed change the sound. So all seems to be working somehow. However; I am not able to get all the scoop images as in the users manual. I get some, and for some I can get close, but others are not able to do at all. So something is not all correct, I think.

Two additional remarks:
1. The offset pot seems to be doing very little in the TRK mode, but quite a lot on the OPP position.
2. The polarity switch change the sound considerably when changing position.
If this is as it should be, fine.

Interesting module, though. It can actually be used as it is now, but it would be nice to have it up to full working order.


Synthbuilder wrote:
The output of U4 (pins 1 and 7) should go from around -6.5V to +6.5V (pin 1 going the opposite direction to pin 7) when the threshold pot is moved from one end to the other. What's the voltage at pin 6 on U4? It should be zero. If not replace U4.

I don't think any fault is coming through R23. It's too big a value to be able to force the output of U4 to do anything other what it should be.

Tony
Synthbuilder
terjewinther wrote:
The output of U4 does indeed change from -6.5V to +6.5V. However, it jumps from about -2V to +3V, even though I try very tiny movements with the threshold pot, so it is not possible to get 0V on the output.

That sounds like the pot has gone bad. If you monitor the voltage with your scope on the wiper (middle pin) of the pot and see what happens when you rotate the pot. It should go smoothly from -5V to +5V. If there are any sudden jumps in the scope trace the pot is probably damaged. You should be able to get the wiper voltage to close to 0V when the pot is in the middle.

Quote:
The OFF1 trimmer never gets the pin7 of U13 lower than about +3V.

I think with the threshold pot misbehaving it will be difficult to get this to work. Just to check that OFF1 is doing its job, remove U4 completely and try adjusting OFF1 again. If you can get it to 0.00V then the pot is probably bad.

Without the Threshold pot, or the circuitry around U4, doing what it should, the module will not behave correctly.

Tony
terjewinther
Synthbuilder wrote:
That sounds like the pot has gone bad.


Yes, spot on!
It was a broken Alpha pot. A good part of the revolving was of no use. A bit annoying that, since it was a new pot, and it takes some work to get everything in the module apart again. Also, I didn't have any extra Alpha pots with the right resistance, so I had to use a 249 type pot, and that took some modification to fit, and the first pot I put in had lots of scratching, so I had to replace that too. Not a good pot day, for sure! But in the end it turned out fine, and I can now get correct trimming results, as in the manual. Great sounds too!

Thanks a lot!

Terje
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