F....'n Ross Phaser Univibe Mods

[modsquad]

Okay so I built another Ross Phaser after finally getting the first one working.  I applied Mark's mods and can't get the darn thing to ouput anything.  The voltages on the 13600's look good but the ones on the 4558 look weird to me:

1    3.92
2    3.92
3    2.68
4    0

To me these look okay, its 5-8 that are off to me:

5   3.85
6   3.89
7   .68
8   7.73

Shouldn't 7 be around 4.0 with the input and inverting voltages of 5,6 around 3.89.  Could it be a bad IC?

[gez]

I deleted the schematic after helping you on the last one and I can't find the damn thing over at GGG (I've always had difficulties finding stuff there).  Could you link to it again?

Bearing in mind a lot of your problems were with your soldering in the last one, it's possible that's the culprit here.  What size tip are you using for your iron?  Might be time to invest in something a little slinkier if it's on the large size.

[modsquad]

http://www.tonepad.com/getFile.asp?id=99

I just figure that the values for the 4558 should be like the values for the phaser.   

Another funny thing is I have a 510 ohm resistor on my LED which pulls down the voltage from 7.9 to .72 and 1ma.  Which is not enough to light the LED.  On the Phaser board I have the same setup and the voltage goes from 7.9 to 3.41 and 6ma which lights the Led.  My concern is the drop in current.  Shouldn't the current remain the same since the resistance and voltage is the same? And it was working.  Back to looking at the solder joins again. 

I have a fine tip but these small boards are driving me nuts.  What I am finding is that the copper layer is not very thick and heats up and off easily if you are not real careful.

[gez]

Pin 7 should be roughly the same voltage as pins 5&6.

Check for DC on the 150k to ground following the output - could be that you've got the 1u cap the wrong way round and it's gone leaky.  Also scrape around pin 7 to get rid of any PCB bridges (and check for solder splashes). 

If you've shorted the output it's possible you've damaged the chip - I can't remember if this chip has protection - so you may have to replace it if things still look the same after a clean up of the area.

[modsquad]

Let's all say Stan's an idiot together...1,2,3..."Stan's an idiot"

I soldered the legs on the 1uf cap together before the 150k resistor and output.  All 4 of them got soldered together.  I will fix that and see what happens.  These tightly coupled boards make it tough to keep the pads from connecting when soldering.  Of course it could be my lack of skill also. 

[Mark Hammer]

Magnifying glasses are wonderful things to have around the bench, when soldering.

Let's all say together "Stan is now older and wiser".

[modsquad]

actually use a magnifying glass...I lost my eyesight reading cap labels.  Okay so I found the problem with pin 7...bad solder bridge between 27k  resistors going to pin 7.  Now the voltages shake out to:

1    4.14
2    4.18
3    2.85
4    0
5    4.15
6    4.18
7    4.06
8    8.29

Which looks reasonable based on what I have read.  I still however have no sound. I also have 4.06v going into the 1uf cap before the output.  BTW don't eat pizza flavored goldfish while you solder.  It leads to errors and burnt things on the workbench.

[gez]

Slight offset, but I wouldn't worry about it.  If all the voltages work out then the only thing to do next is audio probe it to find out where the break is.  If you don't have a probe made up (I don't) just use my 'Lazy B*stard Audio Probe Tm': stick a cap on the breadboard, send one leg to your amp and croc-clip a meter probe to the other leg...bit precarious but it works.

[modsquad]

Okay audio probed the circuit.  It appears that the last issue is that there is no signal getting to the LFO (IC4).  I have probed all the leads in the signal path and the LFO is dead as an armadillo in the road at night. 

[gez]

You're not going to hear any audio at the output of the LFO, it's just there to modulate the allpass stages.

The following is a list of points that the audio path travels through, in order:

IC1a pin 1 Output
IC2a pin 8      "
IC2b pin 9      "
IC3a pin 8      "
IC3b pin 9      "
IC1b pin 7      "

At what point don't you hear anything??  Example: no sound at pin9 IC3b...IC3 is the chip to check and tidy up (look for PCB bridges, solder splashes etc).

[gez]

PS  Do the readings for your LFO fluctuate or are they static?

[modsquad]

Checked the pins on IC3b and am getting signal on those.

[gez]

Checked the pins on IC3b and am getting signal on those.

I only gave IC3b as an example.  What I was trying to get across is that you have to go thru the audio chain until you find the break in the chain.

Well, if you're getting signal upto and including IC3b there's only IC1b left.  So are you getting audio at its output?  If not, there's your problem.  If you are getting signal but no phasing, then your LFO (IC4) isn't working.

[modsquad]

Just curious, what would cause the LFO not to phase (steady voltages)

[gez]

Just curious, what would cause the LFO not to phase (steady voltages)

Bad soldering, wrong component values, wrong polarity of polarised components, tiny PCB bridges, damaged chip, PMT...such is the wonder of screw ups. 

[modsquad]

Okay, so now I have some vibe going on.  However, the Regen and sweep controls have no effect.  I have a 500k audio pot on the rate and get a narrow band of change with it.  

[gez]

Can't help with the pots I'm afraid (I only ever breadboarded this circuit and it was some time ago).  Probably a different taper is required, unless some connections are a little dodgy.

[gez]

What do you mean by 'sweep control'?  Can't see that on the schematic.  If you mean the knob that sets the rate then you have a problem there.

[modsquad]

Sorry, I pulled the 270k resistor connected to IC4a pin 1 and replaced with a 100k pot and 180k resistor in series.  I think its on of Mark Hammer's suggestions.  I have it on the phaser board also and it seems to work well.

[gez]

Does it (should it) alter depth then (by shifting sweep start point)?