LOUD SHREIK WITH MXR PHASE 90......

[Govmnt_Lacky]

Building a 3-in-one stompbox with a Tonepad MXR Phase 90, GGG Tycobrea Octavia, and a Tonepad Boss CE2 Chorus. Already wired up the Phase 90 and the Octavia. Having huge problems with the Phase 90. I used matched 2N5952 FETS, 5.1V Zener, TL072 Op Amps, and a 2N4125 PNP all according to the schematic. Added the level mod (250K Log pot in place of the 150K resistor. Also added the Depth mod (1M resistor in series with a 500K Log pot) in series with the 250K trim pot. Now for the issue.... when I ONLY PLUG IN MY AMP ON THE OUTPUT! That is to say NO GUITAR plugged in, I get a loud shreiking tone from the circuit. The controls "seem" to work because the speed knob seems to phase the shreik. I also notice other changes with the other mod knobs but I cannot figure out what is causing the shreik! I DO have the Octavia from GGG connected and it seems to work fine when the P90 is bypassed but I REALLY...REALLY...REALLY... need help to get rid of that shreik. I dont want to rip the board out and check solder runs on the board if I dont have to. HAS ANYONE ELSE HAD THIS PROBLEM?

ANY HELP WOULD BE APPRECIATED!!!

[cathexis]

I've produced a few good loud shrieks when confusing the input and the output of certain pedals (the Uni-Vibe comes to mind), like feedback. If there's anything before your phaser it could be feedbacking on noise only (even if you have nothing plugged into the input of the chain). Worth a check, anyway.
LARS

[Govmnt_Lacky]

Here is some more info about the project. I read that the voltages of the passives and such might help so here it goes:

I have also performed some of the mods as follows:
(Level mod) 250K Log pot installed between collector and base of PNP transistor with one wire going to lug 1, and the other wire going to lugs 2 and 3.
(Depth mod) 500K Log pot installed after 250K trim pot with one wire going to lug 3 and the other wire going to lug 1 with a 1M resistor soldered between lugs 1 and 2
(Classic/Modern mod) 22K resistor put onto SPDT switch to switch between classic/modern config.

Now, I have removed the wires form the PCB for the Level and Depth mods and replaced them with the appropriate resistors but no change.

Here are the voltage readings on the components:

IC1
Pin1 3.88
Pin2 3.88
Pin3 3.70
Pin4 0
Pin5 variable between 3.31 and 5.53
Pin6 variable between 3.5 and 5.5
Pin7 switches every few seconds from 1.35 to 12.00
Pin8 12.60

IC2
Pin1 3.88
Pin2 3.88
Pin3 3.88
Pin4 0
Pin5 3.88
Pin6 4.04
Pin7 3.06
Pin8 12.60

IC3
Pin1 9.35
Pin2 3.63
Pin3 4.17
Pin4 0
Pin5 7.80
Pin6 7.90
Pin7 6.42
Pin8 12.60

Q1
Gate 2.60
Source 3.88
Drain 3.88

Q2
Gate 2.60
Source 3.88
Drain 3.88

Q3
Gate 2.60
Source 3.88
Drain 4.18

Q4
Gate 2.60
Source 3.88
Drain 7.75

Q5
Collector 3.30
Base 3.30
Emitter 3.88

5.1V Zener
Black stripe side - 3.88
Other side - 0

Any...and I mean any help would be appreciated.

[Govmnt_Lacky]

Trying to keep this at the top of the list. ANY HELP WOULD BE GREATLY APPRECIATED!!!!!

[slacker]

There's a couple of things that look odd to me.

The voltage on the kathode (black band) of your zener diode should be about 5.1 volts. Check that the components around there are the correct values and make sure you haven't got any shorts or bridges of solder between any of the traces. Once you fix that all the voltages that are about 3.8volts should be about 5 volts.

Something is also wrong with IC3 pins 1,5,6 and 7 are all too high they should be the same as for IC2. Do the same checks round there as for the zener.
The other thing you could do if you've used sockets is swap ICs 2 and 3 over to see if the problem is being caused by a bad chip.

Hope this helps, and welcome to the forum

[Govmnt_Lacky]

Thanks slacker. I definitely appreciate the heads up and the effort. I will check those out first thing tomorrow morning. Please stay tuned as I will post a response tomorrow. Any other help will be appreciated for all of you knowledgable people.

By the way, is it OK that IC1 has such a wide variation on the indicated pins? Seems odd to me that those pins vary in voltage so much!

RG--- if you see this post, could you please give me your input? Everyone tells me that you are the expert on these matters.

Many thanks in advance 

[slacker]

The voltages on IC1 are fine. The second half (pins 5,6 and 7) make up the LFO this produces a voltage that swings up and down to create the phaser's sweep. The speed of the changes on these pins should is you turn the speed knob.

[Govmnt_Lacky]

Once again, thank you slacker. Tune in tomorrow as I will post my findings based on your input. Many thanks to all who assist me with this problem.

[Br4d13y]

the same thing happened to me but in a different situation. i was doing a mod of an existing stock phase 90 and i accidentaly bridged some components. when i plugged it in it bypassed fine, but as soon as i turned it on, it would oscillate, but the guitar signal was still coming through. it was wierd and awesome at the same time. i decided to return it to normal though, and am now no longer in possesion of the phase 90.

[Govmnt_Lacky]

Br4d13y,

Not exactly the same but close. This is a tonepad PCB. I have had bad experiences with their PCBs before. I built a Sansamp GT2 and found a bad board run that I had to bridge myself to get it to work. Took me 2 hours of reading out with a multimeter but I fixed it.
Anyway, this is a bit different. My guitar does NOT even come thru. It is just a loud solid squeal. I built this project as a 3-in-one with a Tonepad Phase 90, GGG Tycobreah Octavia, and a Tonepad Boss CE2 Chorus. Only have the Phase 90 and Octavia hooked up right now but the input and output wiring for the CE2 is wired to the 3PDT so the sound will run thru the box. Anyway, I found today that when I have power running to the Phase 90 AT ALL (bypassed or not), the squeal is there. The only way to get rid of it is to disconnect power from the board. I even went as far as to directly jumer the input jack tip to the output jack tip to bypass the circuit(s) entirely but with power on the box the squeal was still there. If I remove power from the Phase 90 the Octavia sounds great, but once I apply power to the Phase 90 it squeals. The power is set up as follows:

Wall wart going to DC input jack.
Power goes to terminal strip to power Phase 90, Octavia, CE2, and all three LEDs
Ground comes in on DC jack to output sleeve which is jumpered to input sleeve.
Phase 90 grounds go to 3PDT (for LED and input grounding), output sleeve, and jumper to Octavia ground.
Octavia grounds go to Phase 90 board, 3PDT (for LED and input grounding), and output jack sleeve,
CE2 grounds WILL GO TO input jack sleeve, and 3PDT (for LED and input grounding)

If anyone sees something wrong with this, please let me know or any other help would be appreciated.

[Zben3129]

have you checked to make sure there are no shorts from your voltage rail to the signal path on the mxr board? It sounds like you may be putting voltage into your signal path.

Zach

[Govmnt_Lacky]

Zben,

Going to do some debugging tomorrow. I will add that suggestion to my list along with slackers suggestions. Thanks for the input. Any other suggestions are welcome cause I am the type to keep pushing until it works or until I have to go home. Will post my results tomorrow so stay tuned. Thanks to all who made suggestions.

[Zben3129]

Good luck, I'll check tomorrow,

I haven't been on in about a year, so if I say something stupid, I'm just a little rusty 

Zach

[Govmnt_Lacky]

SUCCESS!!!!!!!

I found the problem. Turns out that I was stupid. What I did was I added a SPDT switch for the 22K feedback resistor. The stupid thing was that I wired the resistor DIRECTLY to the center and bottom lug of the switch along with the wires to the PCB. In effect, no matter which position the switch was in, the resistor was still in the loop. I dosconnected the resistor and VIOLA.... worked like a champ!

So now I have a new question.... Is there any way I can change the value of the resistor so that when the switch IS IN FACT engaged it will not scream bloody murder thru my amp? Should I experiment with a higher resistance value until it is OK? What should it sound like or what is a good tone to have withmatched transistors and a feedback resistor? The posts on the Tonepad build reports say that people have matched their transistors (FETs) and put in the feedback resistor but no values were listed.

Thanks to all who helped along the way and to all who will help on my NEW problem.

[Govmnt_Lacky]

I have some new updated info about this project and I think it is still not right. Let me get out the basics:

IC1:
Pin1 3.88
Pin2 3.88
Pin3 3.88
Pin4 0
Pin5 3.31 to 5.53 varying
Pin6 3.50 to 5.50 varying
Pin7 Switches from 1.35 to 12 every few seconds
Pin8 12.90

IC2:
Pin1 3.88
Pin2 3.88
Pin3 3.88
Pin4 0
Pin5 3.88
Pin6 3.88
Pin7 3.88
Pin8 12.90

IC3:
Pin1 3.88
Pin2 3.88
Pin3 3.88
Pin4 0
Pin5 3.88
Pin6 3.88
Pin7 3.88
Pin8 12.90

Q1:
Gate Varies form 1.85 to 2.00
Source 3.88
Drain 3.88

Q2:
Gate Varies form 1.85 to 2.00
Source 3.88
Drain 3.88

Q3:
Gate Varies form 1.85 to 2.00
Source 3.88
Drain 3.88

Q4:
Gate Varies form 1.85 to 2.00
Source 3.88
Drain 3.88

Q5:
Collector 3.30
Base 3.30
Emitter 3.88

Zener diode
Cathode (Black stripe) 3.88
Anode 0

These are the new readings. As posted before, when I remove the 22K feedback resistor from the loop entirely, the circuit works but here is the weird part. Aren't my voltages that are 3.88V supposed to be closer to 5.1V? Shouldn't the Zener diode Cathode read 5.1V or closer to it?
Why is it that when I put the 22K resistor in the loop it causes the circuit to shreak?

Any ideas on the low voltages or the feedback resistor are appreciated.