XC Phase, no effect and dropped volume

[Raspymcnasty]

I finished building this a couple weeks ago.
It worked totally fine and now it doesn’t and I’m not sure why.

Here are the issues
1. When i adjusted the trim pot to find a good spot it would hiss-hiss-hiss in bold mode but not in Script.
2. While i was adjusting the trimmer the led would sometimes blink
3. Now when i adjust the trimmer it has 0 effect and volume decreased as well.

Not sure where to go from here any help would be greatly appreciated.

 

[Mcknib]

What's your voltages on the zener cathode and Q1 to 4 gate pins

 

[zgrav]

are the transistors oriented correctly? does not look like they are positioned as shown on the PCB outline, but I realize you may be substituting different ones.

 

[Robert]

They're oriented correctly (assuming they're 2N5952), the first revision PCB silkscreen was incorrect for Q1 - Q4.

 

[zgrav]

thanks for the clarification.

 

[Raspymcnasty]

What's your voltages on the zener cathode and Q1 to 4 gate pins

I'll go ahead and check this:
I'm new to electronics so I just wanted to clarify the steps to do this.
1. Set DMM to V DC
2. stick ground stick into ground or enclosure hole
3. stick red stick onto cathode of zener and measure max voltage?
4. stick red stick onto furthest left leg of Q1-Q4 and measure max voltage?

and to confirm yes, these are 2n5952s (purchased here)

 

[Mcknib]

Yes that sounds right if it's autoranging you don't need to set the voltage level if not set it to 20 vdc

It's just to see if the power sections ok and the trimmer

You want around 5v on the zener and just over 2v on the gate pins which basically tells you everything prior to them is ok

 

[Raspymcnasty]

Yes that sounds right if it's autoranging you don't need to set the voltage level if not set it to 20 vdc

It's just to see if the power sections ok and the trimmer

You want around 5v on the zener and just over 2v on the gate pins which basically tells you everything prior to them is ok

Firstly, thank you so much for your response.

I just measured the gate pins and zener cathodes.


Q1-Q4 roughly around 3.7
Zeners roughly 4.6

 

[Mcknib]

Try turning the trimmer to see if it alters the gate voltages if so that would mean it's ok

I'd set the gate voltages to around 2.5v

Any phase 90 circuits I've built the phasing sweet spot is around 2.2v on the fet gates

Just to be clear what effect are you actually getting through the circuit I've assumed you are getting audio but with the problems you've listed

 

[Raspymcnasty]

Try turning the trimmer to see if it alters the gate voltages if so that would mean it's ok

I'd set the gate voltages to around 2.5v

Any phase 90 circuits I've built the phasing sweet spot is around 2.2v on the fet gates

Just to be clear what effect are you actually getting through the circuit I've assumed you are getting audio but with the problems you've listed

It’s the weirdest thing, it works again. But the effect isn’t as deep and warbly?

But thank you for the knowledge of setting it to around 2.2-2.5v, strangely the trimmer is at a different spot than it was before

 

[Mcknib]

Try turning the trimmer in very small increments I mean tiny amounts to see if you can increase the warble

Then perhaps check all voltages to see if anything's way out

I normally go to general guitar gadgets if I can't work out what voltages to expect most of their build docs have voltages listed

The P90 build instructions doc has all the voltages here

MXR™ Phase 90™ (Script Logo) | General Guitar Gadgets

www.generalguitargadgets.com

The difference being this uses 6 x LM741 single op amps like the original rather than 3 x TL072 dual op amps but if you compare schematics you can work out what part is what and which pins are used

In the ggg P90 the buffer is IC1 the XC it's IC1.1

The LFO is IC6 the XC IC3.2

The phase stages as you can see ICs 2,3,4 and 5 and XC ICs 2.1, 3.1, 2.2 and 1.2

Some pins are different power pin on the TL072 is 8 LM741 it's 7

+ and - Input and the output pin on the LM741 are all the same pins 2,3 and 6 with only 1 internal circuit and as you know the TL072 has 2 internal circuits so two of each input and output pins 1,2 and 3 and pins 5,6 and 7

So easy enough to work out and compare your voltages