Original MXR Phase 90 doesn't work

[Elijah-Baley]

Hello!
A friend of mine has an old MXR Phase 90, Rev. D. Year 2004.
This is the PCB:



It works in bypass, but no sound when engaged. The LED works fine. I noticed that the PCB, around the TL062 warm up, just a bit. When it is engaged it's not really muted, I can hear just a bit of hum.
I can't see what is the problem. The pedal it's been unsued for a while, and it was just borrowed to a friend of this my friend, but he said it doesn't work.

I measured the voltages of the ICs and of the 2N4125. I guess are very far from a working effect.
PSU 8.99v about. Speed pot at minimum. Guitar and amp connected to the PCB.

TL0762
1 8.94v
2 8.86v
3 1.07v
4 0.00v
5 1.87v
6 5.50v, but it decrese quickly and then slowly until about 4.47v
7 7.86v
8 8.99v

TL064
1 0.98v
2 0.93v
3 0.16v
4 0.00v
5 0.16v
6 0.20v
7 0.89v
8  1.03v
9 4.96v
10 0.47v
11 8.99v
12 0.03v
13 1.03v
14 1.08v

2N4125
E 0.16v
B 3.47v
C 0.95v

There's a thing I can't explain. The two diodes in the bottome right  corner.



I used my tester to measure the forward voltage. The diode below measures about 0.215v, the upper one 0.076v. But I can measured even if I reverse the pointer of my multimeter. Pretty same value, just the below diode it was 0.225v.
Then, the upper on, you can barely see from the picture, seems to have a small fracture.
I can tell that the left side of these two diodes are connected each other and goes to the ground of the circuit, so the below diode is across the ground. The upper one (the just suspected cracked one) is across 9v and the ground.
I can't imagine nothing else that a wrong polarity voltage or maybe too much voltage could be caused a damge. But I don't know.

Can someone get some hint about this problem?
Thanks!

[GibsonGM]

Strange, huh?  If it were me, I'd remove the ICs and the 2 diodes, and measure again to see what you get.  Might then get normal voltages in some places, and screwy ones to point in a direction.   I wonder if reverse polarity was applied?    Grab a schematic, too.

Is it me, or does that left-hand jack look like it's seen some heat??  Might be nothing I s'pose. 

(you mean TL062, right? Just checking.  Both chips are cents so no big deal to replace w/new)

[Mark Hammer]

A TL064 has the same standard quad op-amp pinout as an LM324, TL074, TL084, et al.  That form factor has pin 4 as V+ and pin 11 as ground.  Have you possibly misread which pin is which in your listing of voltages,? (i.e., you interpreted the chip "upside down", treating pin 8 as pin 1).

Alternatively, is the chip itself socketed and reinserted upside down?

[Elijah-Baley]

Strange, huh?  If it were me, I'd remove the ICs and the 2 diodes, and measure again to see what you get.  Might then get normal voltages in some places, and screwy ones to point in a direction.   I wonder if reverse polarity was applied?    Grab a schematic, too.

Is it me, or does that left-hand jack look like it's seen some heat??  Might be nothing I s'pose. 

(you mean TL062, right? Just checking.  Both chips are cents so no big deal to replace w/new)

I'd think twice before desolder the two ICs because the pedal is not mine, and I want to be really cautious.

There's some schematic around, like this, for example:

But, though I didn't check avery parts, I tried to follow the signal helping with this schematic, and I found that C5 (looking at the schematic because I can't see the name of the parts on the PCB) goes in the pin 3 of the TL061, but pin 1 and pin 2 are not connected each other, but there's a 10k resistor across them.

Then, there's something difference with the didoes probably, that I can't get. I'm sure that are not in parallel, and probably early I was wrong about them. And my multimeter is not so good, currently!
No diode in line to the 9v, I guess. But the resistor close to the upper diode is the 10k, though I can measures aroun 200R. It's in parallel with that diode.

And yeah, I also noticed that burn or whatever on that part of the switch of the jack. What it could means?

A TL064 has the same standard quad op-amp pinout as an LM324, TL074, TL084, et al.  That form factor has pin 4 as V+ and pin 11 as ground.  Have you possibly misread which pin is which in your listing of voltages,? (i.e., you interpreted the chip "upside down", treating pin 8 as pin 1).

Alternatively, is the chip itself socketed and reinserted upside down?

Thanks Mark, I misread the orientation of the TL064, it was under the pot and I confused the orientation. I still think there's something wrong.
Before work on it I would like to know what is faulty to this pedal.
If there's something damaged or something damages it?

[Govmnt_Lacky]

That form factor has pin 4 as V+ and pin 11 as ground.  Have you possibly misread which pin is which in your listing of voltages,? (i.e., you interpreted the chip "upside down", treating pin 8 as pin 1).

Agreed!

Also, why are you seeing full V+ on Pins 1 & 2 of the TL062? Those voltages should be closer to Vref or around 4.5-5V

[Elijah-Baley]

Here the right pin voltage measurement. It should be right.

TL062
1 8.94v
2 8.86v
3 1.07v
4 0.00v
5 1.87v
6 5.50v, but it decrese quickly and then slowly until about 4.47v
7 7.86v
8 8.99v

TL064
1 1.03v
2 4.96v
3 0.47v
4 8.99v
5 0.03v
6 1.03v
7 1.08v
8 0.98v
9 0.93v
10 0.16v
11 0.00v
12 0.16v
13 0.20v
14 0.89v

2N4125
E 0.16v
B 3.47v
C 0.95v

[Mark Hammer]

The voltages on pin 1 and 2 of the TL062 look high.  Under what conditions are those voltages being read?

[Govmnt_Lacky]

Here the right pin voltage measurement. It should be right.

TL062
1 8.94v
2 8.86v

Still a problem here. Should not be this close to V+

EDIT: Unless your meter is measuring only the high side of the LFO but, this voltage should be shifting between V+ and around 1.5VDC.

[Elijah-Baley]

Thanks, guys!

Indeed, of course there's something wrong.

I hope I did the measurement well.
- I plugged the PCB to the guitar and the amp.
- The PSU is a 9v Boss.
- The pot is at zero.
- The black pointer of the multimeter is on the sleeve of the output jack. The ground.
- The red pointer I touched the pins.

Anyway, in less than a minute the TL062 get definitely hot.

[Govmnt_Lacky]

One thing to note...

It does appear that the design is using the Ring switches on both of the jacks to switch on/off something. In this case, when a plug is inserted into either of the jacks, it forces the tab to make contact with the other side. If you look at the difference between the Ring and the Sleeve/Tip you will see.

[Elijah-Baley]

Yes, I noticed that. I don't know why it's like so. I neve seen reverse switch jack like these.
I don't think we should care about it. I mean it should work.

[GibsonGM]

Could someone have plugged in a wal wart of too high a voltage for the pedal?  Arced on contact? Lightning strike/surge?  Anyway, I'd look for solder bridging under the TL062, see if something 'in line' to pin 1, 2 is shorted....otherwise, time to pull 'em carefully.    I'd put in sockets so it is a 1-time solder.    That way you can see what all the power supply & bias voltages are, AND the ICs are not affecting things downstream.    I can't evaluate your skill in doing this tho, Elijah...sometimes it has more to do with 'trying things' than a plan you can logically follow.  This is 'easter egging', but here, taking the ICs doesn't seem so harsh (to me).   No clue if the FETs are ok.  I would not substitute new chips until I got readings from the sockets and opinions first.

Put it this way; your friend's pedal will never work again if you can't work on it, because I doubt you can find a specific little cause for this puppy.  Go over it really well; if you find someone 'did something' in there and made solder bridges, that's one thing.  Otherwise, I suspect the ICs (at least 1) are screwed up.   Either the board shorted to the case, or over-voltage did it, or......cap failed....?   But only a hunch, I'm not there like you are.  I could just be looking at it wrong, tho I usually have a higher success than failure rate   

[Elijah-Baley]

I inlude in my shop cart a socket IC 8 pin, a TL062.
I'm not sure about the diodes. The schematic is a bit different from the PCB. I'm pretty sure that the two didoes are not in parallel, and I can't identify them.

[GibsonGM]

The only diodes I can find on ANY schematic are the ones in the power supply, and one is a zener.they ARE right next to the DC jack, perhaps that is what they are?....HTH!

[Elijah-Baley]

Yes, they must be in the power supply.
Look at the schematic I posted, while...



...I checked the connections of the upper and below diodes and the 10k resistor, just there.

10k resistor
Left side to the positive of the 15uF
Right side to the 9v

Upper Diode
Left side, positive to the Ground
Right side, negative to the 9v

Below diode
Left side, positive to the Ground
Right side, negative to the positive of the 15uF

And, of course, the negative of the 15uF tantalum cap is on the ground.

Did someone can solve this enigma?

[GibsonGM]

Upper diode appears to be D1, 1N914.   This diode is for power supply reverse-polarity protection

Lower diode appears to be Zener diode, 5.1V, which is REVERSE BIASED (this is how they operate)...the cathode goes to the "+".   This provides a 5.1V reference voltage, "Vref".     What voltage do you measure at the top of the 15uF cap??

If they are not broken, do not fix them     We will test this when the chips are no longer in the circuit.   (I believe a bad chip could affect the output of the Vref and give us a false reading).

[pinkjimiphoton]

the voltages on the chip make me suspect it's toast. only time i ever seen pins 1 and 2 that high are with a shorted chip, which would explain the hum and the heat.

[Elijah-Baley]

Thanks. In a few days I desolder it to check the voltage. Later I'll solder a socket IC to try some chip, probably a TL072, I think I have some.
Anyway the hum is really really feeble. My friend said it was mute, but I heard the hum just because I raised up the volume of my amp, so I heard the difference between bypass and engaged. When it was engaged it is a bit noisy, but it doesn't sound.

[Elijah-Baley]

I desolder the TL062.
Now this is the voltage I get:

TL062
1 0.00v
2 0.00v
3 0.06v
4 0.00v
5 0.09v
6 0.07v
7 0.07v
8 9.00v

I think this is promising, right?

[Mark Hammer]

Looks closer to how things should look.