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 9720 VCO testing 
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Joined: Sat Jul 30, 2011 5:46 am
Posts: 63
Post 9720 VCO testing
another module with a challenge. Sorry for the multiple posts, but I'm on an assembly binge of late.

The module powers up fine and the AR modulator tests out as good.

When testing the VCO's ramp waveform I get the following resutls:

OSC A sunds significanlty lower in PITCH than OSC B
the output VOLUME of both oscillators is the same

when testing the DC voltage at the ramp outputs of the oscillators I get the following:

OSC A is at 58mV
OSC B is at 128mV

OSC B sounds like I would expect it to.

when turning the scale trimmers for both oscillators, they both function, but the pitch of OSC A is still significatly lower than OSC B.

this leads me to think that maybe I have a potentiometer or a trimmer in the wrong position?

how can I test the potentiometers and trimmers with a volt meter to see if I need to swap or move or replace them? Would prefer to test 1st, before removing parts, etc.


Mon Oct 03, 2016 10:34 am
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Joined: Sun Nov 09, 2008 2:49 pm
Posts: 733
Post Re: 9720 VCO testing
It is normal for VCO A to be at a lower frequency than VCO B for similar knob pointer settings -- roughly, one is usually about 10-11 o'clock while the other is about 1-2 o'clock for unison between the two.

The LFO/Normal switch is in the Normal postion, and has an effect when actuated, right?

The mV tests are really just listed to help confirm the waves are in the proper DC range and the readings will vary somewhat according to the frequency. If they were +5V or -5V, etc. (way off), then it would be time to check soldering of resistors and that the specified values are the ones installed installed on the board.

Check out the tips for setting the trims on the MIDI2CV8 and VCO in the P9700S overview:

It is unfortunate the trims have the value marked on the bottom and not visible once installed, but a resistance test can be made to confirm that both scale and symmetry trims have similar readings. Switch off power to the VCO before making the resistance tests. The readings will not be 10K and 1K due to extra paths for the meter current flow when the parts are on the board, but when they are all set to mid-rotation and the probes are put to the two legs at the flat end of the part, the 1K will have a low reading and the 10K, higher.

Confirm the panel connections for the A and B panel Pitch controls. Their wipers, wired to points I and R, should have DC voltage which varies from 0 to 12VDC.

Scott Lee

Mon Oct 03, 2016 11:19 am

Joined: Sat Jul 30, 2011 5:46 am
Posts: 63
Post Re: 9720 VCO testing
the VCO pitch difference between the 2 is very very large. When setting both scale trimmers to center and then listening to each oscillator, they are very very different. So much different that even when I turn OSC A pitch knob all the way up and OSC B pitch knob all the way down, they are still not even close to each other. Which makes me think maybe I have a potentiometer wrongly placed?

yes, the Normal / LFO toggle switch does effect the OSC as expected.

I haven't gone thru the full calibration of the OSC's yet, because the ramp outputs don't seem to be following what the testing instructions say they should be doing, before moving on to calibration.

Maybe I should go thru the calibration procedure anyways and see what happens?

trouble is, I need my midi2cv8 working 1st..:) which I'm working on now.


Mon Oct 03, 2016 12:31 pm
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Joined: Sat Jul 30, 2011 5:46 am
Posts: 63
Post Re: 9720 VCO testing
VCO appears to be normal after all. I was able to calibrate it just fine after getting the Midi2cv8 up and running. Great sounding VCO for sure!

On to more modules...


Mon Oct 10, 2016 9:22 pm
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