Phase 90 pedal problems

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Spencerleehorton

Spencerleehorton

Well-known member
Joined
May 12, 2012
Messages
4,080
Location
Felixstowe, Suffolk, UK
Hi Guys,

have tried to find what the problem is but cant seem to track it down!!
i have built a phase 90 pedal using 741 chips and have used SK30 or J201 as FETs, not at the same time, along with the 4125.
I am getting no dry signal and the pedal is just acting as a tremolo, no phasing?
I have printed out this pcb:

http://effectslayouts.blogspot.co.uk/2015/04/mxr-script-phase-90.html

all values are correct apart from the 47nf caps i have 33nf in there but i have tried 47nf and it sound the same, as there is no phasing to it at all?

voltages wise on the 741 im getting:

pin1 = 0v
pin2 = 4.97v
pin3 = 4.74
pin4 = 0v
pin5 = 0v
pin6 = 4.97v
pin7 = 8.41v
pin8 = 0v

all of the 741 are about the same.

the 4125 is E = 4.88, B = 4.24, C = 1.8 - 2.6v (varies up and down)
Q1,2,3,4 = D = 4.80, S = 4.60 - 4.80 (varies), G = 2.8 - 3.3 (varies)

I'm must be missing something obvious, i dont have a DPDT switch in, feel like im cursed again at the moment!!!

any help would be really appreciated.

regards

Spence.
 
The site you site is not showing a schematic.

Other sites have similar plans but the one I saw has no DPDT?

Which might relate to "no dry signal"?

The "phasing" is almost inaudible until you mix it with DRY. The dry path can't be complicated. Figure out where you are losing it.
 
Hi Spencer,
I built  a couple of Phase 90 in the past.

Like PRR said the Phase will only happen when the effect signal is mixed with the Dry Signal.
Having a kind of tremolo effect instead of phasing is what happens in this pedal when no dry signal is mixed in.
Some people use a SPDT switch to kill the dry signal and achieve the effect you're getting.

Do you have a scope? inject signal and follow the schematic, check were you are loosing the dry signal.

For guitar pedals I just use a signal audio probe for troubleshooting, I inject music at the input and following the schematic I probe before and after each component, you will reach a point were the signal disappears.

audio_probe.gif

 
Hi guys,

the schematic is on that page just down a bit:

http://www.generalguitargadgets.com/pdf/ggg_p90_sc.pdf

i have disconnected R24 and connected to D2 5.1v feed and looked at the phased signal and looks good, looking at the dry signal it has oscillation in it?
cant seem to find where its coming from but at pin6 IC1 is looks ok, no oscillation, pin6 IC6 has oscillation this is with R24 disconnected?
any ideas?

regards

Spence.
 
Spencerleehorton said:
Seems my problem is the Ener maybe

http://www.diystompboxes.com/smfforum/index.php?PHPSESSID=c3d3ab7a1a5096d8bba68e0c10954ff0&topic=98606.0
Click to expand...

yes,
try subbing the Zener like they advise in the thread.
I didn't follow the GGG project, I used Tonepad.
Although GGG is a great website I had a lot of problems when building and MXR Phase 100 unit with the GGG circuit, so when it was time for the Phase 90 I just went with the tonepad one. GGG eventually droped the Phase 100 project after so many people having problems.

In the Tonepad article, they say this:

"You must use matched FETs for this circuit to work.
Tweaking the value of the zener diode may be needed for some FETs. It's recommended to use sockets for the FETs and zener diode. "

http://tonepad.com/getFile.asp?id=42

A nice feature in Tonepad is that you can also  view other peoples Build Reports, with mods they've done or problems they've found to complete the project, click "view build reports" at the end of this page:

http://tonepad.com/project.asp?id=7
 
im pulling my hair out here!!, if i dont match the jfets will i not get phased signal?

ive got noise on the 5v rail, i did have noise on the 9v rail but that seems to have gone, i cant believe how much trouble im having with this simple circuit and i cant seem to track down what the hells going wrong!!!!

at this point in time i want to work out how to get it functioning, doesnt have to be correctly but if the jfets are not matched will i get signal?
 
Spencerleehorton said:
if i dont match the jfets will i not get phased signal?
Click to expand...

That or a weak phasing effect.

Spencerleehorton said:
i cant believe how much trouble im having with this simple circuit and i cant seem to track down what the hells going wrong!!!!
Click to expand...

It's a simple circuit, but you didn't follow the simple guidelines for the project like the need to match the JFETS, so you made it much more difficult. LOLOLOLOL

Spencerleehorton said:
at this point in time i want to work out how to get it functioning, doesnt have to be correctly but if the jfets are not matched will i get signal?
Click to expand...

You should get the Dry signal at least. if you inject signal at the input where do you loose it?
There's not much circuitry in the dry part, do you have dry signal at IC1 output?
Do you have Dry signal before the 150K resistor? and after?

Did you re-check your resistor values?
Is that a home etched pcb? re-check  solder traces continuity

Check for solder joints shorting to a neighbour pcb track, it's quite common in this fx pedals pcb's since the traces are  small and really close to each other




 
Some good reading:

MXR Phase 90 Circuit Analysis
http://www.electrosmash.com/mxr-phase90

RG Keen JFET Matching for Effects
http://www.geofex.com/article_folders/fetmatch/fetmatch.htm

If you need any help in matching JFETS I can do that for you, I have a Peak DCA75 that makes matching a breeze. I can match some and send them to you.
Let me know if you need it
 
well ive had a rest and come back to it and it still isnt right but i have to ask this.
there is ripple on the 5v rail at D2 could this ripple be bcause the 5v rail isnt delivering enough voltage? voltage is at 4.98v
does it need to be above 5.3v?
 
well ive made a Jfet tester and have tested all my jfets and got a couple of pairs that are pretty close, have put them in and ive changed the caps as i had 33nf in there cause i couldnt find the 47nf, ive found the 47nf now so they are in.
as far as i can see and test everything is correct now and i think it might be working, will check in morning!
 
Quote from: Whoops on January 09, 2017, 08:04:27 PM

    You should get the Dry signal at least. if you inject signal at the input where do you loose it?
    There's not much circuitry in the dry part, do you have dry signal at IC1 output?
    Do you have Dry signal before the 150K resistor? and after?

    Did you re-check your resistor values?
    Is that a home etched pcb? re-check  solder traces continuity

    Check for solder joints shorting to a neighbour pcb track, it's quite common in this fx pedals pcb's since the traces are  small and really close to each other


You never replied to these questions...

I will add some more,
How well are the JFets matched now?

Did you change the Zener as its advise in the tonepad document?
 
hi sorry this has been such a headache i thought i'd replied, yes ive changed zener to 1N914 and ive matched up 2 pairs of Jfets, its kind of working but i need to go through all resistors as ive made a couple of mistakes, its sort of working but i juts cant track down yet why the dry signal is oscillating?
Hopefully a bit more poking around should find it!!

regards

Spence.
 
> ive changed zener to 1N914

1N914 is not a Zener.

(Actually it will "Zener" but up around 80V-130V, nowhere near what you want.)
 
Hi PRR,

sorry i meant D1 not the zener!!

right ive spotted my problem wand its a pcb design fault, the 22k R3,5,6 dont seem to attach to 5v rail? R4 does but the others dont?
when i attach them to 5v rail something goes down to earth?
 
also spotted a point which wasnt attached on pcb and a bit where it was down to gnd and shouldnt have been.

works a treat now, i will redesign in eagle and post up a working pcb design if anyones interested?
 

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