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Europanaut
Joined: Apr 04, 2014 Posts: 46 Location: Nanaimo, BC, Canada
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Posted: Sun Mar 29, 2015 12:42 am Post subject:
Delayed Modulation, with no release. |
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Hello!
After many successful MFOS builds in a row (thanks to Ray's great documentation, not due to my barely-adequate skill level), I've come across an anomaly.
I've just finished the Delayed Modulation Module. The modulation part works great, and the gate/trig attack seems fine so far, but the release doesn't work at all. I'm triggering from a J-wire keyboard, using Ray's single-buss keyboard circuit (which works well with my other envelope gens).
I've tried both gate and trigger, but once the modulation starts, it won't stop.
I'm running 15v, and have implemented the appropriate changes to the circuit, as described on Ray's site.
Any suggestions on where to start my troubleshooting process, or what the likely culprit might be?
Thanks! |
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Skrog Productions
Joined: Jan 07, 2009 Posts: 1196 Location: Scottish Borders
Audio files: 155
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Posted: Sun Mar 29, 2015 6:34 am Post subject:
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Hi
Do the usual visual checks at the wiring around the pots area & pcb circuit soldering points in that area too , also check the zener diodes are keeping the A/R levels around + & - 5V with your multimeter.
Although I have this pcb ... I've not built it yet (hoping to build soon on my 12V system)
Dave. |
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Europanaut
Joined: Apr 04, 2014 Posts: 46 Location: Nanaimo, BC, Canada
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Posted: Sun Mar 29, 2015 3:37 pm Post subject:
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Thanks Dave,
I've measured Z1 and Z2.
At the striped end (cathode?) Z1 is showing -.78v. The other end is showing -5.74v.
At the striped end of Z2, I'm getting -.78v. The other end is showing 0v.
What should I be expecting from these test points?
Thanks again! |
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Europanaut
Joined: Apr 04, 2014 Posts: 46 Location: Nanaimo, BC, Canada
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Posted: Sun Mar 29, 2015 8:54 pm Post subject:
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Ok, the plot thickens.
The module DOES appear to be working correctly. Sort of.
I measured the gate voltage, at the Gate In of the module, when receiving gate voltage from the keyboard (which works fine on everything else), and I've made some interesting observations.
Gate in from keyboard - no key pressed: -1.32v
Press key: +12.75v, modulation starts.
Release key: -1.27v, modulation does not stop.
Remove cable from module: -13.25, modulation stops.
When modulation starts and stops, all attack and release functionality works 100%.
So the module CAN work properly, but is not "releasing" unless I physically unplug the cable.
The same results occur if I run it from the gate out on the sequencer. It's like the module needs to be pulled down at zero, rather than requiring -12v.
Again, any suggestions? |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
Audio files: 16
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Posted: Sun Mar 29, 2015 9:29 pm Post subject:
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So first of all, those voltages sound totally normal. When you have a keyboard plugged in, generating no gate, it's holding that point at ground. So you have ground then 10k then 200k, then -15V. 1/20 * -15V = -.75V if everything is perfect. You're pretty close to that, within tolerance of your resistors and your supply voltage, I'm guessing.
This is the second time recently I have been reading about Ray's EGs not cutting off when an actual signal source is plugged into them, and it's the same basic circuit in this case. The other one was a SL Mk II -- more inputs, but the same structure with the FET and NPN.
Honestly, my bet is the FET. A 2N5457 is supposed to cut off entirely at -0.5V, and you're getting lower than that (at least I think so; you should measure at the junction between the 10K and 200K resistors, which is also the gate of the FET). |
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Europanaut
Joined: Apr 04, 2014 Posts: 46 Location: Nanaimo, BC, Canada
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Posted: Sun Mar 29, 2015 9:53 pm Post subject:
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Thanks elmegil.
I get 15v at the G of the 5457, with no gate signal applied.
Being new to this, what should I expect at that test point? |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
Audio files: 16
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Posted: Mon Mar 30, 2015 6:56 am Post subject:
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If you have a ground signal at the jack, you should have -.75V or so at the gate of the FET, I believe. I don't have one to validate that. |
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Europanaut
Joined: Apr 04, 2014 Posts: 46 Location: Nanaimo, BC, Canada
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Posted: Tue Mar 31, 2015 7:57 pm Post subject:
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elmegil wrote: |
Honestly, my bet is the FET. |
Just tried a quick 5457 replacement. No change. |
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Europanaut
Joined: Apr 04, 2014 Posts: 46 Location: Nanaimo, BC, Canada
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Posted: Wed Apr 01, 2015 12:16 pm Post subject:
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Thanks to help from Ray, I swapped R2 for a 120k, and it's working. As I work through the calibration, I'll report back if there are any further changes to the circuit. |
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Europanaut
Joined: Apr 04, 2014 Posts: 46 Location: Nanaimo, BC, Canada
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Posted: Wed Apr 01, 2015 1:24 pm Post subject:
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Here's the output of Mod Out, set to sine, low range.
Does it seem a bit messy? I have nothing to compare to, so maybe this is normal?
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