Chorral Chorus Debug (with voltages)

[mills]

So, I think I'm getting pretty close to geting my CE-2clone debugged, but I can't quite get an effect.  Just an uneffected signal, that seems a little low on volume.  Its being done as a vibrato right now, so the direct signal isn't the one being passed, R21 isn't in right now.

I measured my voltages, and compared them to andre's that were posted in some other threads.
Here's the link to the PDF with schematic/layout.  http://www.tonepad.com/getFileInfo.asp?id=101

I think that most of them are close enough except Q4.  I have

Q4
C    8.78
B    3.835
E    3.317

And andre's working voltages were

C    9.0
B    4.4
E    4.5


I assumed that this meant there was somthing off with R36 or R37, but I checked them, resoldered a joint and no change.   

Thanks for anyhelpor thoughts

In case it helps (my emitter and base voltages might be too close?  I dunno, theres maybe one or two other voltages that could be a bit different), here's the rest of my voltages, with the working version in brackets...

Q1
C   8.87 (9.0)
B   5.26 (5.4)
E   4.88 (4.

Q2
C   8.86 (9.0)
B   5.74 (5.9)
E   5.25 (5.1)

Q3
C   8.86 (9.0)
B   5.43 (5.6)
E   5.00 (5.1)

Q4
C   8.78 (9.0)
B   3.84 (4.4)
E   3.32 (4.5)

Q5
C   7.77 (7.9)
B   0.32 (0.4)
E   0   (0)

IC1
1: 6.00 (6.1)
2: 6.01 (6.1)
3: 5.99 (6.2)
4: 0      (0)
5: 5.87 (6.1)
6: 5.87 (6.1)
7: 5.87 (6.1)
8: 8.86 (9.0)

IC2
1: 4.55 (var)
2: 4.39 (4.5)
3: 3.98 (var)
4: 0      (0)
5: 4.39 (4.5)
6: 4.47 (4.5)
7: 3.71 (var)
8: 8.87 (9.0)

IC3
1: 8.78 (8.9)
2: 4.49 (4.5)
3: 5.24 (5.3)
4: 0.59 (0.6)
5: 0      (0)
6: 4.46 (4.5)
7: 4.92 (5.4)
8: 4.91 (5.4)

IC4
1: 8.78 (8.9)
2: 4.46 (4.5)
3: 0      (0)
4: 4.48 (4.5)
5: 0.23 (0.2)
6: 7.89 (8.0)
7: 2.91 (2.9)
8: 0.59 (0.6)

[mills]

Also, Just in case anyone is thinking about this, I can sort of get vibe-ish sounds by touching one leg of the time setting capacitor (c19).  The other doesn't do anything...  I don't know if that'll make a difference, as I can't see anyting wrong with the capacitor either.

I dunno, I assume something isn't right in the LFO section, or its connection to the 3101.  Can't figure out what yet though.  Any help is definately appreciated!

[PerroGrande]

Unfortunately, this is one of those times when an oscilloscope would come in really handy.  However, I think you're looking in a reasonable spot when you're considering the LFO.

Q4 is nothing but an emitter follower, from the look of it. Sure, it has the voltage divider formed by R36 & R37, but the transistor just "sees" the series resistance.  The emitter will follow what it has appearing on its base (minus a diode drop).  The divider looks like it just adjusts the level for the next stage (along with current gain, of course).  This buffers the LFO output from the BBD driver input.

Truth be told, your Q4 looks a little more like what I'd expect to see than the sample voltage.

What do you see at the junction of R36 and R37?

You could rig up a quick and dirty LFO test with a 1K resistor and a diode...  Connect the resistor in series with the positive side of the diode.  Ground the negative side of the diode and the use the unconnected resistor end as a probe.  Touch it to the emitter of Q4 -- in theory, the LED should pulse with the LFO -- vary the speed control and depth control to see if they are working.

What may be happening is that your LFO isn't working and that Q4 is just sitting there at 3.x volts -- perhaps because the LFO isn't oscillating.  Q4 drives the clock input for the BBD driver, so if it is sitting there flat, everything is flat.

[mills]

Thanks for taking the time to read and help out.

I found the problem in the LFO section... A lifted pad that didn't come up enough to see until I wiggled the right part.  So, the two big problems I had with the build (getting signal and getting effect) were lifted pads.  Time to work on my soldering and maybe upgrade from my radioshack iron.   

One last question before I go tweak capacitors to get it set the way I want,   I still feel like there's some volume drop; would I be better off decreasing the resistor from the delated signal into the output stage of the 4558, or increasing the gain of the 4558? 

Thanks again, I'm pretty excited about getting this working!

[PerroGrande]

Which resistor are you contemplating changing?

[mills]

I figured that decreasing R22 would help, but wasn't really sure.

[mills]

And, hopefully one last question for the build...  The LFO seems a little choppy or square-ish, is there an easy way to make it less choppy... I was hoping for a smoother vibrato.  Or, if there's any reading about simple LFO's that I haven't stumbled across, I'd try and work through it myself...

[Mark Hammer]

R35 and C21 form a lowpass filter that keeps any "click-i-ness" of the LFO minimal.  You can use that path to smooth out the LFO waveform by adding some lag.  Since I'm not sure what hanging a bigger cap off the base of Q4 might do, try splitting R35 into a 120k and 100k resistor pair in series (in that order).  Then, hang a .47uf cap off their junction to ground.  That will start adding lag for anything that tries to rise and fall more than 3.4 times per second.

[PerroGrande]

You can try dropping R22 a bit.  It is the input resistor to an inverting op amp stage, so reducing it would increase the gain.

The only caveat is that it also sets the input impedance of the stage.  If you drop it too far, you might run into problems.

[mills]

Thanks for both the thoughts.  I'll play around with the values there and see what happens.

[PerroGrande]

I think you're safe increasing the cap a bit (c21). Q4 is just a follower that gets its DC bias from the previous stage.  The only thing that worries me about making the resistor *too* large is that you'll not have enough base current for Q4. Otherwise, I think you're fairly safe to experiment with both R and C in this area to see if you can get the LFO output more to your liking.

[mills]

Got it boxed up and working.  Sounds great.

Splitting R35 worked really well, there's a jumper right after it on the PCB so there was alreadya pad for the second resistor and, with a bit of filing, the 1uF cap to ground.  At really low ratess, there's sort of a hint of choppiness, but not enough to worry about.

Volume drop isn't that serious, and its pretty balanced when in the chorus mode, so I left that all alone.

Thanks again for the help!