Troubleshooting Phase 45 build

[bex97]

Hello,

This is my first post on this forum so please let me know if I get the etiquette wrong..

I've been trying to build a phase pedal for a while now, I think this is my second or third complete rebuild of this circuit. This time, I get clean signal through the pedal both on and bypassed, but no phase effect.

Here's all the relevant links:
Layout: http://tagboardeffects.blogspot.com/2012/07/mxr-phase-45.html
Kit: https://www.bitsbox.co.uk/index.php?main_page=product_info&cPath=280_281&products_id=2044
Thread I posted on gearspace with a little info on what I've already tried and a few more pictures: https://gearspace.com/board/diy-electronic-build-refurbishment-photo-diaries/1362996-diy-pedal-troubleshooting.html

The JFETs supplied in the kit have a different pinout to the ones in the layout, hence the flipped orientation.
The trimmer is the second one I have tried. The plastic bit fell off but this is the closest one I had to 200K, the rest all measure around 170K. It still turns so hopefully this is still usable.

Voltages (tested with 9V battery):
Q1 (left)
D = 4.16
S = 4.16
G = 2.7-3.2V (fluctuates)

Q2 (right)
D= 4.16
S= 4.16
G= 2.7-3.1 (fluctuates)

IC1 (top)
1 1OUT = 0V
2 1IN- = 2.06V
3 1IN+ = 4.13V
4 VCC- = 0V
5 2IN+ = 4.13V
6 2IN- = 3.55V
7 2OUT = 7.10V
8 VCC+ = 7.90V

as noted in the the gearspace thread, when I remove pin 1 from the socket I get a voltage of 7.47V
The IC's were tested at a different time with a different battery, in case that makes a difference.

IC2 (bottom)
1 1OUT = 4.12V
2 1IN- = 4.13V
3 1IN+ = 3.93V
4 VCC- = 0V
5 2IN+ fluctuates between 3.7-4.1V
6 2IN- = 3.93V
7 2OUT fluctuates between 3.4-4.6V
8 VCC+ = 7.88V

D1
top = 0V
bottom (negative) = 4.16V

The read out on the diode seems like it should be the other way round to me, but I'm no expert. Should I try replacing this?

Thanks for reading,
Bex

[aleks_tedstone]

I had a very similar problem very recently! The nice folks here on the forum pointed me towards in the right direction. I found that my trimmer potentiometer had quite a narrow range in which it would bias the transistors correctly. It is quite noticeable when you find it, and almost completely clean up to that point.

Here's the thread:

https://www.diystompboxes.com/smfforum/index.php?topic=127844.msg1227760#msg1227760

And these are the voltages my JFETS were at the correct bias point, BUT note that your JFETs may have different hfe values, so use your ear.
D= 5.08V
S= 5.08V
G= osc between 3.47 and 3.75 V

Best of luck!

[aleks_tedstone]

PS your diode looks like it is oriented correctly, and the cathode (striped end) should indeed have a positive voltage on it in this case, as it is also a biased component. Hard to tell in your photo, but make sure it goes to pin 2 of the IC at the bottom of the photo.

[bex97]

Thanks Aleks!

I've tried very slowly going through the trimmer, sitting at 5.09V and at the same relative percentage of the battery voltage. No luck unfortunately.

I think the problem is probably rooted in that 0V output from pin 1 of IC1..
I've downloaded some software that should give me an estimate of voltages at each point, hopefully that will shed some light on things

[aleks_tedstone]

At least your IC's are socketed in case you need to switch out a TL072, but it could be as simple as a short that is grounding pin 1 of IC1. Check continuity between pin1 IC1 and ground, by having a good look for any possible solder bridges, and using the continuity setting on your multimeter if it has one.

Two of the op-amps in the ICs are part of all-pass filters in the circuit, so these need to be functioning correctly to create the right amount of phase shift. The other op-amps handle the LFO and the input buffer.

[bex97]

It works!!

I took the trimmer out and soldered sockets in to make it easier to swap them out, switched the trimpot out for a random untested one, and it worked as soon as I plugged it in. I'm not inclined to adjust anything or take measurements because that's one guaranteed way that I'll break it again lol. Perhap I just got really unlucky and had two or three bad trimmers back to back.

Thanks for all your time and help Aleks!