VCS wiring

[macumbista]

[EDIT: less confusing now that I removed some extra circuitry, including some DC Mixers and one Analog Logic board. Second "wobbly" VCS works fine, first still acts like an ass....]

OK, the other VCS is even more confusing than the first. Again, input, pots and CV all seem to work, trigger and end out definitely do not. This time, the LED works, but the whole thing seems more like a Source of Uncertainty module! The voltage wobbles all over the place, and seems to be affected by my body resistance/capacitance!

For the latter problem, I'll check the wiring again. But since (besides the LED), both PCBs have trouble with the trigger/end out/cycle feature, I wonder what the similarity is? I replaced one of the LM3900 ICs, just to see if that was the problem (it wasn't).

ICs seem to be getting power on the correct pins. I don't have a scope, unfortunately.

HELP! Suggestions more than welcome at this point.

[macumbista]

Got the first VCS almost working. Everything including the LED works fine, except End Out/Trigger in. So I can't get it to cycle.

I get a faint flicker on the LED from an external trigger, but no noticeable output voltage from the Output. Using a patch cable to connect End Out to Trigger in gives no response at all.

All ICs replaced at least once. Also most of the transistors, espc the one in
the End/Trigger section. Wiring triple-checked. Most of the solder joints reflowed as well.

Anything else I can try?

[macumbista]

OK, the last problem turned out to be a cracked trace on one end of the 820K resistor between pins 9 and 11 of the LM3900. Apparently, the force of cutting the lead alone cracked this trace, so I've asked Ken if this might be a result of his new PCB manufacturer, since I didn't have anything remotely like this problem with other/earlier CGS PCBs.

Also, for the archive... if you can't get the VCS to cycle then setting it in cycle mode and then adjusting the trimpot can help. The effect might not take place immediately, but on the next power-up or trigger it should. My procedure was:

1. Flip the module around, with the Out connected to a VCO so to hear when it starts cycling.
2. Adjust trimpot a little bit.
3. Power down/power up. Listen for cycling in the VCO.
4. When you get the VCS cycling, ease back on the trimpot until just the point where it stops, then bring it up again so it starts. I'm guessing this gets it as close to a 5V trigger out as possible without a scope.

[macumbista]

Build complete!!!! More info at http://macumbista.net/?p=421

[numbernone]

I also wired my second VCS last night. They are crazy.

[robotfunk]

got the same problem as Macumbista.
I can slew rise and fall fine, it works as a filter fine, just can't get the damn thing to cycle! Trig and End Out don't seem to be doing much..

tried the trimpot all the way to no avail, double checked joints on trig/end out and around LM3900 .. .all seems fine ..

[The Bad Producer]

you could try changing the 10N cap to 1N as per Kens suggestion on the site?

[robotfunk]

I figured if it locked up at certain frequencies, it would at least cycle at others? I don't get any sound in any position of the pots or the trimpot..

[robotfunk]

also read this in the Yahoo group:

> >>> the 10n/1n only has any affect when it is set to run fast. If you can't
> >>> get
> >>> it to oscillate at any setting, you have a different problem.
> >>>
> >>> Ken

[J3RK]

Didn't think I should start a new thread for this. I'm also having a speck of trouble with my VCSes. The first one is probably fine short of me frying the 074 chip. Replacing that today, so hopefully that one will work.

The other is a bit more puzzling. I've checked all the components and was extremely careful installing and orienting parts (I've done tons of builds in the past.) Basically, I plug it in, and the LED blinks once. I can't get it to cycle (though for a little while the LED was stuck on. Now it seems to be stuck off. When I probe around the board, I don't see much in the way of positive voltages over .5V. There are some points (one of the opamps, AC out, etc.) that have -25V. So it looks like I'm having some power rail issues, but I'm not sure what's causing it.

I've read about the LM3906/LM3904 pinout differences on Ken's site, so I checked them, and they seem to be correct. BTW: these are Bridechamber kits, not my own part sourcing. I haven't had any trouble with other BC kits parts-wise, so I assume that since there aren't any notes on the site about orientation of the transistors, that they should go in the "normal" way.

Any ideas? Please let me know if I can provide any additional information that may be helpful. I've got someone coming over to help me figure this out, but any information I can gather in the meantime may prove helpful.

Thanks!

[andrewF]

with the power turned off and the board disconnected from your power supply, measure the resistance between the +V rail and Gnd on the circuit board.
Is it very low, zero?

[J3RK]

This seems odd to me, but I'm not a total expert on these things.

I probed the negative side first, and saw 0. Then the positive side, which read around 5M, then slowly started dropping. Then hit the negative side again, and it started around 3M, and started slowly rising.

This is with no power to the device, probing the power pins. I didn't hit the v+ and ground connections in the panel wiring area. Now that I re-read your post, you may have wanted me to check there. Can try that in a bit.

[J3RK]

Ok, checked the V+ and GND on the actual PCB (rather than power pins,) and I get 4.2K

[andrewF]

Well at least it isn't a short.

Sounds like you have to get methodical.

check the pinouts of your transistors, not on a datasheet but with a multimeter, or make a simple circuit to light a led (see CGS LED driver).

Check the power pins of the chips in the circuit to make sure they are getting +V and -V as required. Also, with the power off, use a make sure pins connected to ground are indeed connected using the continuity buzzer.

Getting 25V reading on this circuit means something is wrong....or you measured between +V and -V......

enjoy

[J3RK]

Ok, I feel very stupid now, but I totally missed the jumper wires on the board.

So, it isn't cycling right now, but I have proper voltages around every part of the board I've checked. I only seem to have +15 on the LM3900 though. Is it a + only part, or should there be - as well?

Thanks for the help by the way!

[J3RK]

Just got it working. It was cycling, but something happened to my LED connection. Don't know if it was a bad joint, or what, but I resoldered the connections, and it works correctly now. Thanks!

[andrewF]

[J3RK]

Thanks! Totally enjoying them now! (I've had the VCS in Frac before, had a Serge Creature (right half of TGO is basically a VCS,) and some Wiard Envelators.) These ones though are quite feature-packed.

[Luka]

great minds think alike derek
I also have the 2vcs + analog logic combo



100% functional and making insane slopes

[macumbista]