Ross Phaser- not oscillating

[Ponchus]

Hey guys
Any ideas on what I should look for?

I just finished building a Ross Phaser. The hardest part (I thought) was fitting everything in the enclosure. I took my time, cut wires to exact lengths, and of the 4 boxes I've built thus far, I did my best job yet at neatly fitting everything in the enclosure. I was so excited...until I fired it up. LOL, chalk it up to experience, but from now on I might try actually testing my creations before all the complex wiring and all that.

Anyway, problem is this: I build a Ross Phaser from Tonepad's layout (the simple layout, no extra stages or mods). The LED does light up, and sound does pass through in both bypassed and active modes. The problem is that the pedal isn't oscillating. In other words, the LFO does not seem to be working. The sound, though, is getting affected by the pedal somewhat. If I feed a signal in, and twiddle the knobs, it sounds as if it's phasing to some extent. By turning one of the knobs, it almost has a wah effect or something. But once I stop turning the knobs, the sound stays at that setting, so there is no phase.

I read on another post that if I take an LED and attach it to a 10k resistor, I could test the LFO stage by probing it and seeing if the LED blinks (after grounding the negative end). I tried this trick, and the LED does light up, but does not blink. I've checked continuity on ground, as well as the +9v traces on the PCB, and all looks good. So I'm getting 9Vs to hit the correct pins of all the ICs, and all the correct pins of the ICs are grounded properly.

When I get home from work, I was planning on trying to touch up all my solder joints, but do any of you geniuses have any ideas on what to look for besides a cold joint? Thanks in advance!

[Ponchus]

bump

[Sir H C]

Link to schematic and voltages on the oscillator pins.  From there, boom, help.

[Ponchus]

Here's a link: http://www.tonepad.com/getFileInfo.asp?id=99

Here are the voltages at all the pins of the ICs

IC1
1- 4
2- 4
3- 3.5
4- 0
5- 4
6- 4
7- 2.86
8- 8.06

IC2
1- 1.26
2- 0
3- 4
4- 4
5- .645
6- 0
7- .645
8- 1.706
9- 6.11
10- 7.32
11- 8.04
12- 7.32
13- 3.99
14- 4
15- 0
16- 1.26

IC3
1- 1.26
2- 0
3- 4
4- 4
5- 3.04
6- 0
7- 3.04
8- 1.918
9- 6.07
10- 7.29
11- 8.04
12- 7.29
13- 4
14- 4
15- 0
16- 1.26

IC4
1- 1.186
2- 0
3- 7.15
4- 5.85
5- 7.35
6- 0
7- 7.35
8- 7.35
9- 5.85
10- 7.15
11- 8.03
12- 7.15
13- 5.85
14- 7.15
15- .108
16- 1.382

Hope this helps you help me

[Ponchus]

bump

[gez]

I haven't checked over the LFO yet, but the main part of the circuit that deals with audio is a little out of whack.

The OTAs need a little attention.  The outputs of the OTAs' buffers should be around the same voltage as the +&- pins (4V in your case).  The actual outputs of the OTAs should be a couple of diode drops above this, about 5.2V.  In the first stage the output is too low in the second it's too high.  The same thing is replicated in the next chip.

When I've had this problem with LM13700s it's often the result of a hairline bridge between the outputs of the OTA/base of buffer and one of the power rails - often can't see it but the resistance, despite being really high, is low enough to pull the outputs up/down (depending on which rail is nearest). Get a scalpel and give things a scrape between the traces.

Output of IC1 looks a little low too, are you sure the reading is right?

[gez]

Pins 3&14 of IC4 are too high.  Check your resistor values for the divider (270k and 100k).  Hope I'm using the right schematic (there were two PDFs to choose from, I chose the most recent)

[Ponchus]

Thanks for the reply! I am about to go out to do some Valentine's Day eating, so I can't check this out right now, but later on tonight I definitely will...

Question though, should ICs 2 and 3 have identical voltages to the pins? I see that they match pretty closely, except pins 5 & 7, which are only .645 volts on IC 2, but are around 3 volts on IC 3.

Also, you mentioned the BUFFERS of the OTA output etc...I'm kind of a newb...which pins are we talking about? And I assume by OTAs, you mean IC 2 & 3?

You mentioned the ACTUAL OUPUT OF THE OTAs...again, which pins are we talking about?

Thanks for the info...

[Ponchus]

Oh, one more thing...just so we're looking at the same project file...the link is at http://www.tonepad.com/project.asp?id=25

There are 3 choices: the old layout, the new layout, and the Extra Stages layout...I'm using the middle choice, the new layout with no mods or extra stages.

Thanks again.

[gez]

Question though, should ICs 2 and 3 have identical voltages to the pins? I see that they match pretty closely, except pins 5 & 7, which are only .645 volts on IC 2, but are around 3 volts on IC 3.

Yeah, the chips should mirror each other.

The buffers' outputs (pins 8 and 9) should be at around half supply, same as the + and - inputs.  The OTAs outputs (Pins 5 and 12) directly connect to the bases of the buffers (pins 7 and 10) and are, therefore, at the same voltage - about a couple of diode drops above the buffers outputs, ie 5.2V (in your case)...or they should be!

Like I said, give the traces a scrape between the ones connecting pins 5&7 and 10&12 and the power rails.

[Ponchus]

GEZ, I just wanted to say thanks. I did get the Phaser working, and it sounds f----ing great! First off, you were spot on about there being a short somewhere. Even before you mentioned that, I had been checking continuity on various traces. I didn't do the best job at etching this board, to tell you the truth, so I was doublechecking my work. I was detecting a short that was killing me because even under a magnifying glass, I could not see what was creating this short. And on top of this, in some places my DMM would beep for a split second as if it detected a short but would then stop. After giving the traces a scrape with a razor, that problem disappeared and my voltages on all the ICs were perfect (as per what you said they should be at). But the pedal still didn't oscillate. I touched up all the solder joints in the LFO section (again), and for some reason it decided to work that time.

However, now I realized that my RATE pot is an audio pot, rather than a linear one, so I'll have to replace that soon :-) I have no control over the rate until the last fifth of a rotation, which isn't that great.

I think that after a month or so of rest, I will rebuild this pedal a lot cleaner. Thanks for all your wisdom man!

[Mark Hammer]

Several suggestions:

• Where the 10k resistor feeds the Iabc pins on the 13600 chips (pins 1 and 16), stick a 4.7uf to 10uf capacitor to ground.  The '+' side of the cap goes to the Iabc pin, and the '-' side of the cap to ground.  This will have the effect of "rounding off" the sharp upward spike in the sweep cycle when you turn the speed up fast.  It's what the Small Stone used, and having stuck one on my Rpoez, I can confirm that it was a terrific idea.
• That 10k resistor where it says "LFO" over on the right, sets the amount of current driving the phase shift stages.  It is like a sweep width preset.  If you want to experiment with having subtler, more shimmery sweeps for some of the middle and faster speeds, change it for a 6k8 value in series with a 50k to 100k pot, wired up as a variable resistor (i.e., use the wiper and one side lug).  This will give a sweep width a little wider than what you presently have (nice for long slow sweeps), and permit shallower sweeps for when you turn the speed up.
• Along with the regen 500k pot, the 27k resistor to the right of the Regeneration control shown in the Tonepad project pdf determines the amount of feedback to earlier stages in the phase-shift path.  You can actually drop it safely to about 22k.  This will increase the risk of breaking out in oscillation (which I haven't experienced yet in mine) but will also let you dial in sharper resonances than the stock version, while at the same time not preventing low resonance settings at all.
• As always, with any phaser, you can get a nice vibrato sound by simply lifting the clean signal from the mixer stage, as shown in the Tonepad document.  You can also produce subtler less-intense phase-shifted sounds by increasing the value of the mixing resistor for the wet signal.  In this case, it is the 27k resistor "coming down" from the end of the phase-shift stages to the mixer.  If you look at the Ropez parts layout, that will be the 27k resistor just below the 470k resistor, which is just below the 4558 dual op-amp.  Having a perfect 27k/27k match between the dry and wet mixing resistors will achieve deepest possible notches.  Increasing the value of the wet mixing resistor will move the phase shift signal a little more into the background.  Since you can't always assure that 5% resistors are as matched as their color-code would suggest, I recommend replacing that 27k mixing resistor for the wet signal with a 22k or 24k unit, in series with a 100k pot; again, wired up as a variable resistor.

With the listed changes, the pedal will still be able to achieve in-your-face slow sweeps (especially with the increased sweep width and resonance potential), but it will also be able to achieve nice bubbly pseudo-Leslie sounds, and subtler shimery sounds for when you want your rhythm playing to by more out-of-your-face.  In short, less of a one-trick-pony, and more of an all-purpose phaser.

We can discuss the Univibe and phase-filter mods another time.

[gez]

Where the 10k resistor feeds the Iabc pins on the 13600 chips (pins 1 and 16), stick a 4.7uf to 10uf capacitor to ground.  The '+' side of the cap goes to the Iabc pin, and the '-' side of the cap to ground.  This will have the effect of "rounding off" the sharp upward spike in the sweep cycle when you turn the speed up fast.  It's what the Small Stone used

That's interesting Mark, I posted this mod for the Ross last year sometime but was unaware that the Small Stone used it!  Probably one of those things where I've seen it on the Small Stone schematic so many times 'without' actually noticing it, then it pops into my head as 'my own idea'! 

Actually, for the Ross I used a larger value so that it attenuates the LFO waveform with increasing frequency, in effect reducing the depth as the rate gets cranked up.  It caused distortion of the wave form at higher pot settings - you end up with something resembling a relaxation waveform - but this seemed to suit the effect.  The 'hyper-triangle' shape is retained at low frequencies and still gives you that great sweep, but 'straightens out' as things get friskier, which seems to sound less comical.

[gez]

in some places my DMM would beep for a split second as if it detected a short but would then stop.

Glad you got it working.  One thing to do from now on is test your boards before you solder (I've learnt the hard way!  ).  As well as a continuity check, test for resistance between traces (set your meter for the highest setting).  This will save you a lot of tears in the long run, believe me! 

[Mark Hammer]

That is exactly how it works.  It is quite possible you "didn't notice" the use on the Small Stone because of diferences between SS issues.  The earliest issue used a pair of OTAs for the LFO, with a 10k series resistance and 10uf cap to ground on the LFO output.  When E-H converted to a single OTA oscillator in later issues, that 10k/10uf combo was replaced by a 100R resistor on the output, withuot any cap.  Since they have used that for some time, and Francisco Pena simply implemented it in his Piedrita clone project, t is understandable that it looked both familiar and novel at the same time!

Note that the variable resistance I suggested on the LFO output changes the properties of this simply RC filter, such that waveshape not only depends on LFO rate, but on sweep width as well.  A 10k/10uf combo gives a "rolloff" (which essentially involves a rounding off of the "pointiness of the top part of the sweep cycle) starting around 1.6hz.  If a variable resistance changes that to, say, a 50k/10uf combo, then the filtering action is imposed at lower sweep speeds, in this case around 0.3hz.  On mine, I went with a 4.7uf cap instead of 10uf, to essentially compensate for what would occur with the inclusion of the variable series resistance.  I looked on the scope and there is still a pronounced hypertriangularity in the LFO waveform output (at the RC junction) at medium and medium-fast sweep rates, though the last part of the upward sweep doesn't seem to zip past you like a Harley on the highway quite as much..