MXR Flanger Clone - Running Into Problems

[Bainzy]

I've finished wiring this thing up, the switching works, the 9v > 17v > 15v power supply circuit works and the voltage is being read all over the circuit.

The problem is that I'm getting a sound when the flanger is on, but it's just the normal guitar sound - ie no flange. Turning trimpots has no effect.

I've tried troubleshooting it and found this discrepancy in schematics. The one with the 68k resistor is the one I used, but the one with the 62k resistor is the original MXR factory schematic. This is the only circuit difference between the two schematics (the other differences are just slightly different values used). Could wiring the flanger up like the schem with the 68k resistor stop it working? Is that my problem?

[George Giblet]

Are you are referring to the 68k between the bases of the PNP and NPN in the bottom right hand side of Stellan's schematic?

I actually thought that resistor should connect from the base of the NPN transistor to +15V, not the base of the PNP.   I haven't analysed the effect of connecting it to the PNP base.  Where does the original schematic show it to be connected?

It could stop the high frequency clock oscillating.  If you have an oscilloscope that would be trivial to check.

You have set the trimpots to the mid position at least?  If the delay output trimmer is set to zero you won't get any delayed signal.

[Bainzy]

Are you are referring to the 68k between the bases of the PNP and NPN in the bottom right hand side of Stellan's schematic?

yep that's the one

I actually thought that resistor should connect from the base of the NPN transistor to +15V, not the base of the PNP.   I haven't analysed the effect of connecting it to the PNP base.  Where does the original schematic show it to be connected?

That's what I thought too - I've shown the original schematic excerpt in the first post (it's the first, darker coloured image)

It could stop the high frequency clock oscillating.  If you have an oscilloscope that would be trivial to check.

You have set the trimpots to the mid position at least?  If the delay output trimmer is set to zero you won't get any delayed signal.

I don't have an oscilloscope, but I've tried putting all 3 trimpots in different places and I can't hear any delayed signal whatsoever.


I've tried taking out the opamps/chips from their sockets, and the only one that actually cuts out the signal is the first one, the NE5532.

Do you know if Stellan's schematic has ever been tested?

[George Giblet]

> I've shown the original schematic excerpt in the first post (it's the first, darker coloured image)

I check a PCB picture and it matches you original schematic.
I'm 100% sure now that the resistor should go to +15V. 

For the sake of correctness it would be a good idea to wire that resistor to +15V - there's not point making something wrong.
You might want to use 56k instead of 68k in that position.  The VOX flanger uses 24k, given that works it would be best to go lower for R44 rather than higher.

I noticed is R45 is 10k on the original schematic (and my PCB pic), not the 22k as shown on Stellan's schematic.  I'd fix that as well.

>I don't have an oscilloscope

One trick is to put a 10k resistor in series with the + lead on your multimeter, and probe pins 1 and 2/5 of the 4013 IC with the other end.  Set your multimeter to 20V DC and connect the - lead to ground.  You should see about 7.5V on each pin.  If the voltages are at +15V or 0V that means the high frequency clock isn't working. (I'm assuming a digital multimeter).

Be carefuly not to short anything since pins 1 and 2/5 of the 4013 connect to the SAD delay chip - you don't want to fry that!

>Do you know if Stellan's schematic has ever been tested?
I was under the impression Stellan built it but it's an assumption.

[Bainzy]

When measuring that I get between 0.05V and 0.1V for pins 1 2 and 5 on the 4013 chip.

The only substitution I've made from Stellan's schematic is replacing a 22k resistor with a 15k one in the bottom right hand corner of the schematic (the one near the 500k trim pot). Surely that can't have affected it though? 

[George Giblet]

Since *both* pin 1 and pins 2/5 are 0V I suspect there's something wrong with 4013 chip, or perhaps something around the SAD chip.

Here's some stuff to check,

1) To make sure there's nothing silly going on first check: (I don't think this is the problem by the way).
    Assuming 68k and 15k you should be seeing around 2.5V at the base of the transistor marked BC547 on Stellans schematic.
    On the emitter of the same transistor you should be seeing about 0.6 volts below that, say 1.9V.
    If the voltages are pulled down a lot lower that this try adjusting the 500k trimpot that to the right of the transistors.
    If these voltages are off it's highly likely the transistors are in wrong  ie. a pinout issue.

2) Check pin 1 is grounded on the SAD chip.  This appears to be missing on Stellan's schematic!

3) Check power is getting to the supply pins on SAD chip, pins 9 and 7.

4) Check power is getting to the 4013 chip.

5) If any problems in 2, 3, 4 do the 10k resistor thing again and see if you are getting better voltages.

6) At this point I'd be pulling out the SAD chip and debugging the 4013 oscillator section.  If you have socket it that is - if it's soldered don't pull it since you could fry it in the process.  An alternative is to disconnect the wires going from the 4013 (pin 1 and pins 2/5) to the SAD chip, connect these line going to the SAD chip to ground. 

With the SAD chip out do the 10k resistor thing again.

7) Now the 4013 ckt is "free" I'd start forcing signals into the 4013 set/reset lines to see if the 4013 is OK.  An alternative here is to just replace it.

[Bainzy]

2) Check pin 1 is grounded on the SAD chip.  This appears to be missing on Stellan's schematic!

Yeh I noticed that too - good thing I have the original MXR one! Pin 1 is definately grounded.

3) Check power is getting to the supply pins on SAD chip, pins 9 and 7.

15.1v - check 

4) Check power is getting to the 4013 chip.

15.1v - check 


I'll get onto checking the rest of that stuff today. I have sockets for all the chips on the board, so taking them out isn't a problem. Fortunately I had the hindsight to buy a spare 4013 chip, which is still in it's protective anti-static bubble.

[Bainzy]

Assuming 68k and 15k you should be seeing around 2.5V at the base of the transistor marked BC547 on Stellans schematic.

I don't know why it says "BC547" there - there isn't supposed to be a transistor between the 68k and 22k resistors. I've got 1 BC557 and 3 BC548's.