Super Phaser question (resistor replacement)

[jfrabat]

Hi, guys.  I am building a Super Phaser from a JMK PCB.  Here is the schematic:



Now, my question is specific to R23; it is supposed to be 150K, but I cannot find any 150K locally (and I already used all the 150K I had; I had 5, but this build requires 6!).  Would it be OK to use a 180K (I found some with 173.4K measured resistance)?  I know that's more than 10% difference, but still...  will it make it not work?

[Mark Hammer]

Use a 100k and 47k in series.

[jfrabat]

Use a 100k and 47k in series.

The thing is that there is not enough space in the board...  Maybe in parallel they would fit, but I would not get the required Ohms.

[stringsthings]

Looks like there's enough space on the board.  Just stand them on end to make a triangle.
Solder one end of each to the board and then solder the two together at the top of the triangle.
We've all done it at one time or another.

[Fender3D]

Why don't you just use 3x 100K~120K instead of R19, R20 and R23?
Gain and mix will remain the same...

BTW
R26 can be any other higher value, if saving 150Ks is a must

[Mark Hammer]

Looks like there's enough space on the board.  Just stand them on end to make a triangle.
Solder one end of each to the board and then solder the two together at the top of the triangle.
We've all done it at one time or another.

I do this all the time.  The only caveat I would add is that the solder joint between the two resistors at the "top" of the triangle can sometimes short out against the underside of a pot of switch, depending on how you  mount boards and how much space there is inside the enclosure.

That's no reason to avoid doing it.  Just a reason to be mindful about what might be unintentionally in contact with something else when the almost inevitable it-worked-before-I-boxed-it-up thread appears.

[jfrabat]

I ended up finding some locally, so that's done.  But now I am having issues with the pedal.  Quick question; on the other 5 pedals I used the 1/8W resistors (the tiny little ones); could that be my issue?  Sound is coming out of the pedal, but it is not phasing, and there is a bit of distortion-like sound coming out (not through all the sound; just on certain parts).

[Mark Hammer]

1/8W resistors are fine.  Especially when you consider that current issue Phase 90 pedals and their budget clones (I have a Behringer clone) use surface-mount resistors that are even smaller than 1/8W.

Proper setting of the trimpot is absolutely essential to getting a phasing sound..

[jfrabat]

Ah hah!  That could be the issue...  Especially considering I have not touched it!

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[jfrabat]

Proper setting of the trimpot is absolutely essential to getting a phasing sound..

I tried fixing it with the trimpot, but no difference...

[Mark Hammer]

Verify your JFET pinouts.  It never ceases to amaze me just how many different pinouts there can.  Sometimes even for the same transistor number.

[StephenGiles]

Have you checked that there are no solder bridges with a magnifying glass, and that no IC legs are bent and therefore not making contact? Also check for clean audio on IC output pins in audio path.

[jfrabat]

No solder bridges or bent/broken legs I can see...  Voltages are as follows:

Q1 (2N5952):
4.24
4.06
0

Q2 (2N5952):
4.06
4.06
0

Q3 (2N5952):
4.06
4.06
0

Q4  (2N5952):
4.06
4.06
0

Q5 (2N5087)
4.08
8.9 (not sure, as it was difficult to reach the pin)
0

IC1 (TL072):
1: 1.36
2: 1.36
3: 1.10
4: 0
5: 0.92
6: 1.33
7: 1.38
8: 8.9

IC2 (TL074)
1: 1.48
2: 1.42
3: 1.15
4: 8.9
5: 1.15
6: 1.49
7: 1.49
8: 1.48
9: 1.49
10: 1.15
11: 0
12: 1.15
13: 1.48
14: 1.48

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[reddesert]

You need to specify which pins the voltages are on for the transistors, or it's not possible to interpret. lso, be careful of the pinout - I think 2N5952 have a different pinout than other JFETs, so if you don't check the pinout it is possible to get them in backwards.

Nevertheless, we cal already tell that something is wrong. Typically, the DC voltage of the op-amp input and output pins in a circuit will be around half of Vsupply, so about +4 volts, and yours are all about +1.5. That's suspicious. 

Next, if you look at the schematic, the source of each JFET is directly connected to the non-inverting input of each of the quad opamps (pins 3,5,10,12).  I don't know which pin of your JFETs is the source, but they're all at either 0 or +4 volts (the source is probably at +4).  But the pins 3,5,10,12 of the quad opamp are at +1.15 volts. That should be impossible since they are directly connected by a wire. So something is clearly wrong - not sure if it's voltage readings, or bad solder/socket connections, or some transistor or chip is in the wrong way round and shorting to ground, etc.

[jfrabat]

You need to specify which pins the voltages are on for the transistors, or it's not possible to interpret. lso, be careful of the pinout - I think 2N5952 have a different pinout than other JFETs, so if you don't check the pinout it is possible to get them in backwards.

Q1 to Q4 are all the same, so its reading Drain, Source, Gate (in that order).  Q5 is reading Collector, Base, Emitter (IIRC; I will double check tomorrow).

Next, if you look at the schematic, the source of each JFET is directly connected to the non-inverting input of each of the quad opamps (pins 3,5,10,12).  I don't know which pin of your JFETs is the source, but they're all at either 0 or +4 volts (the source is probably at +4).  But the pins 3,5,10,12 of the quad opamp are at +1.15 volts. That should be impossible since they are directly connected by a wire. So something is clearly wrong - not sure if it's voltage readings, or bad solder/socket connections, or some transistor or chip is in the wrong way round and shorting to ground, etc.

I will double check all the reading tomorrow; I can confirm that transistors and IC's are in the correct way, though (at least according to the board!).  I used the exact same transistors and IC's so the direction should not have changed.

Here's a pic of the board and other components (as long a photobucket lets me direct link them, that is!)

[duck_arse]

is there any chance you've put the bias trimpot in the wrong way? check the voltage on the zener, and at the trimmer wiper, and on the width pot, they should all vary from zener voltage to 0V as you vary the trimmer setting. the trimmer also applies bias to the fet gates, which you show as 0V.

[jfrabat]

is there any chance you've put the bias trimpot in the wrong way? check the voltage on the zener, and at the trimmer wiper, and on the width pot, they should all vary from zener voltage to 0V as you vary the trimmer setting. the trimmer also applies bias to the fet gates, which you show as 0V.

Looks to me that is the issue.  I cannot say which leg is which (it only fits one way), but I get 0V at the square pad, 1.14 V at the right of it, and 1.14 dropping to 1.13V at the top regardless of trimpot position.  So how do I figure which leg goes where?  I know...  Rookie question.  I am just learning this stuff...

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[jfrabat]

By the way, Zenner in 1.14...

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[jfrabat]

Took out the trimpot.  Tried testing it, but it seems to be shot.  I will try to get a new one sometime this week (or next, as I have to travel this week). I have another in stock, but it is reading 564K max.  I guess I COULD use it, but trimming may prove much more difficult, right?

[Mark Hammer]

If you have a 250k pot (not a trimmer), you could see what the two resistances are on each side of the wiper, and substitute fixed resistors for the trimmer...assuming you have those values on hand.