9720 testing
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Author:  voxtrot [ Fri Dec 27, 2013 11:41 am ]
Post subject:  9720 testing
I have completed the assembly of the 9720 VCO. I have begun the testing scenarios from the assembly manual. I am on page 18 at #4 of the testing portion. It says "Each should be a low pitch and of equal amplitude".

Osc A and Osc B have somewhat similar amplitude, but the pitches are not at all within each other's range. Furthermore, the voltmeter does not match the suggested ranges.

I have checked IC4, Q2, and Q3 as well as IC5, Q8, and Q9. All of the connections look solid and placement appears to be correct.

Also, when connected to the MIDI to CV converter with a midi controller, the VCO does not track as expected.

To be honest, I am fairly new to all of this, so I just wanted to touch base and see if there is something wrong, or if this is normal.


Author:  voxtrot [ Fri Dec 27, 2013 2:15 pm ]
Post subject:  Re: 9720 testing

I removed R5 trimpot to ensure that I had the correct trimpots (10k / 100k) in their place. They seem to be in the right place. I think I toasted the trim pot in removing it. So much for that!

Author:  voxtrot [ Sun Dec 29, 2013 3:23 pm ]
Post subject:  Re: 9720 testing

Having destroyed the 10k trimpot by removing it, I made quite a mess of the contacts on the PCB. After some trial and error (I'm still pretty amateur) I managed to use some spare wire to reconnect a new radioshack 10k trimpot ($2 is ridiculous). A few connection mistakes yielded no results...

I was about ready to call it a day (and possibly walk away from the VCO for quite a while) but I found that I had missed one connection. I plugged it in and was somehow able to tune OSC A to OSC B.

The only problem now is, OSC A is an octave below OSC B, rendering the LFO setting useless. When switched to LFO, it is so low, it barely sounds because the frequency is so low.

I probably won't mess with it anymore, I'm just happy to get it back semi-functioning. Maybe I'll build another VCO if I can't get this one to full functionality.

Author:  voxtrot [ Sun Dec 29, 2013 5:22 pm ]
Post subject:  Re: 9720 testing

The testing for OSC A and B seem to work fine.

As far as tracking goes, it does not track at all. I assume this is not normal but I did not read anything about it in the manual. The MIDI to CV converter tested fine for each output too, so I'm not sure what to look at. ...

Author:  ModularMe [ Sun Dec 29, 2013 6:13 pm ]
Post subject:  Re: 9720 testing

Have you checked Scott's advice on tuning at ?

Author:  voxtrot [ Sun Dec 29, 2013 6:17 pm ]
Post subject:  Re: 9720 testing

I think the tuning is fine. I don't really know what is wrong with it.

Author:  voxtrot [ Sun Jan 05, 2014 8:51 pm ]
Post subject:  Re: 9720 testing

Scott gave me some encouragement via email and pointed out a few spots to re solder. I've got it tracking properly now.

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