neutron : no sweep

[mitia]

Hello

Two days I'm on this Neutron Filter    http://www.geofex.com/PCB_layouts/Layouts/neutronpub.pdf

I use two batteries, no power supply, no 7660,  a pair of VTL5C3 for the opto-electronic
I put a DPDT switch for true bypass, Input and output jack are grounded but the input stereo lug is free, bat (-9V) is connected on the board as shown on the GEO layout.

I can hear the filter (some distortion too...) but there's no sweep at all ! I've tested several resistances for Rx, from 0 to 100k, I couldn't hear a difference, of course, the sweep direction switch has no effect.

Maybe the wiring is wrong...Pin 8 of all Ics shows 9V, pin 4 shows 0V (should be -9V I guess)
How to check optoisolators ? Maybe do they not work correctly.


If somebody can help me, thank you
I've just finished the GGG's Phase 100, worked the first time 
Beautiful forum
mat

[mdh]

Sounds like you should definitely check the wiring of the batteries.  The battery snaps should be wired as follows:

[ red    black] [ red    black]
+9V    GND    GND    -9V

And of course if you're actually going to use the thing, you should put in a DPST switch to break the +9V and -9V lines when off.

[mitia]

Thank you MDH, you were right, I've checked the batteries connection and I've got now the +9V/-9V OK (found this post a bit to late sorry http://www.diystompboxes.com/smfforum/index.php?topic=54534.0 ...but still don't have any sweep effect. Only both range and mode switches act on the sound.
So I've got for all IC's pin 8: +9V  (well my batteries beguin to go down)
                                pin 4: -9V
For U1/U2 others pins read a few mV or 0V
For U3 sweep switch down
1   -2.15         8    8.68
2   -2.50         7   -7.65
3    3.44         6   -7.32
4   -8.66         5    0

For U3 sweep switch up
1   -7.54         8    8.68
2   -2.50         7   -7.65
3   -5.02         6   -7.32
4   -8.66         5    0


I'm going to check again all connections and values, I don't know what else to to.
I saw your post about your Neutron, did you finally get something ?

[mdh]

Glad you got the power worked out.  The thread of mine that you saw was probably the one about the version I tried to build on a breadboard.  That one never worked, but I built one to RG's layout, and it works.  It has some issues with volume change on bypass (I true bypassed it, so changes in sensitivity lead to changes in volume), as well as a little distortion and ripple that I never worked out.  But it definitely works.  I'd be willing to open it up and take some voltage readings, but I won't have time for that today, maybe tomorrow.

In the meantime, a little bit about the optocouplers.  In my original (unsuccessful) breadboard build I had used some surplus (single) LED/LDR optocouplers that varied quite a bit in spec.  In the successful build, I used the Hamamatsu dual optocoupler that was used in the original unit (Steve at Small Bear has them: http://www.smallbearelec.com/Detail.bok?no=360).  I have a page from the spec sheet for the Hamamatsu unit (http://two.ucdavis.edu/~holland/electronics/P873_13.gif), and it indicates that the "on" resistance for the LDRs ranges from about 250k @ 100uA to 2k @ 20mA.  So if you can get a similar resistance range from your optocouplers under the conditions in the circuit, you should be able to get it to sweep.

If the optocouplers are soldered in, I'm not sure how I would go about testing them.  I guess one thing you could do is check the resistance across each of the LDRs with power removed from the circuit.  Since they're in parallel with 220k resistors, the resistance should be not less than about 100k if the LDRs have an off resistance of around 250k or greater.  If that resistance is less than 100k, that could be your problem.  Another possibility that comes to mind is that you fried the LEDs in the optocouplers by experimenting with Rx < 560.  I don't know how likely that is, though, because I don't know how much current the op-amp feeding the LEDs can deliver under normal conditions in this circuit.  Finally, you could have oriented one or both of the optocoupler LEDs incorrectly.  I guess to check for each of those conditions you could try to check the voltage across each LED with the circuit on, and possibly with input (bang on your guitar).  Seems to me you should see a forward voltage of about 1.6V across each LED if they're oriented correctly and not damaged.

If the optocouplers are socketed or if you have more than the two, you can test them outside of the circuit, by wiring up a current limiting resistor (at least 390 ohms) in series with a pot wired as a variable resistor, in series with the LED of the optocoupler.  Then with your DMM, measure the resistance across the LDR leads as you change the resistance of the pot.  If you can get a range of resistances similar to what I cited above, then in theory it seems that the optocouplers should work.

Like I said, I may be able to give you some more concrete info by making measurements on my Neutron, hopefully tomorrow, but in the meantime, this should give you something to chew on.  I think checking all the values and connections is a good place to start before you get to mucking about with the optocouplers, especially if they're soldered in.  Good luck!

[mitia]

After checking and testing everything i decided without conviction to change U3 for another TL072 I had, and at my big surprise, my Neutron is now sweeping !
Thank you mdh for interesting you in my problem.

[mdh]

Excellent!  Glad to hear you got it working.