Help repairing MXR Micro Chorus M148 SMD version (post 2008)

[rAdAdA]

Hi there,

At first, I'd like to thank the community for reading this post, I'm a newbie in electronics and please excuse me for asking maybe silly questions. Also apologies for my broken english...

Here's the story :

• I'm trying to fix a friend's MXR Micro Chorus mostly equipped with SMD components (rev B).
• my friends did power by mistake the pedal with 18V or 22V power supply one or two times, bad bad !
• since then, the pedal led briefly lits on time to time, when switching on the DPDT gently (like a faulty contact behaviour "kind of effect"), but, from a sound aspect, you can't hear the chorus effect really, what ever the knob position. You get the guitar sound though. bypass position or not. You head the guitar sound when switched on, sometimes the led doesn't lit.... Same behavious if I use a 9v power supply or a battery.
• I unsoldered and tested the DPDT, and it's working pretty fine.

Here are some pics, I can't provide better now sorry:



I can't find the schematic on the web, but Fender3D's old model one. This has nothing to do with this current version unfortunately....

The pics included in this post are also identical to my model : https://music-electronics-forum.com/...me-boost-issue.

How could I test the 9V input buffers transistors or diods? Without a diagram, it's very difficult as I don't have an experience (yet , I don't see/smell any burnt component neither.

Do you have any idea/clue/hints where I could orient my investigation please ?

Thank you for your precious help !

-R

[The Dude]

Disclaimer: It's hard to say for sure without a schematic, so this is sort of a guess.

Since the issue came from improper power supply, I'd check D2 and D5. It LOOKS LIKE one is a protection diode to ground off the DC input and the other might be a series diode. You could also check the supply pins of the op amps to make sure the full 9V is getting there. Normally, -V pin is ground and +V is supply pin of the op amp.

[rAdAdA]

Thank you so much Mr Dude !

Indeed, I tested *in circuit* the diodes with a multimeter - in diode mode-:

• D2 looks dead ( returns 0.147V Anode to Cathode, but also 1,578V in reverse!)
• D5 looks good (gives 0.147V Anode to Cathode, and an expected OL in the other way!)

I could get their references by taking pictures, D2 and D5 are both B340A (SCHOTTKY BARRIER RECTIFIER apparently). Their Vf is given for 0.5V, very far from 0.147V. However I might misread the datasheet
I don't have the exact reference in stock, I need to order them... I have SS54, SS34 and SS310, Do you know if I can't replace them with one of these refs? SS34 looks the closest, its voltage support even higher (I_FSM 100A vs 40A)

Many thanks !!

-R

[Helmholtz]

Test diodes out of circuit.
A forward voltage of 0.15V looks ok for a Schottky at mA currents.
There are no amperes flowing in a pedal.

[The Dude]

Schottky diodes will have a lower forward biased voltage drop, so I suspect D5 is probably ok. D2 is likely bad and your SS34 should work fine. As Helmholtz says, an out of circuit check would be best.

[Helmholtz]

No doubt D5 is good, so no reason to disconnect.
Not sure if D2 is bad, measuring out of circuit will show.

[rAdAdA]

You were right, D2 is actually good when tested out of circuit. Checking the V+ V- on :

• U4 (MC33178) and I get ~1,4-1,38V
• U1 (HEF4013BT) : V+ V- = ~6.7-6.6V
• U3 (061C ST EZ017 , same as a TL061C) V+ V- = ~1.41-1,42V
• U2 (V3204D) V+ (pin 5) V- (pin 1) = ~7,1V

The red LED partially lits rarely sometimes, coming up and down slowly, very weird. and it has not died (tested OK out of circuit, diode mode was saying ~1.7V). Voltage mesured in circuit is 1.1V

Any ideas, transistors maybe ? Again without a schematic, it's hard....

[g1]

Where are you putting the black probe of your meter? What you think is ground may not be.
Also, in electronics, ~ is the symbol used for AC (rather than DC), so if you are using it to mean 'approximately', just say 'approx.' to avoid confusion.

[rAdAdA]

Hehe , Thank you g1, indeed I totally forgot about the meaning of the ~ when dealing with electronics (!!), I'll use "approx"next time indeed !

I was using putting the black probe on the IC's Ground pin, and again you're right, sorry my electronics school studies are 25 years behind me....

Here are the reviewed and corrected values using the correct ground this time :

• The power supply delivers a stable 8.9V
• U4 (MC33178) : I get approx 1.6V between Pin5 and real ground)
• U1 (HEF4013BT) : V+ V- = approx 8.1V
• U3 (061C ST EZ017 , same as a TL061C) V+ V- = approx 1.54V
• U2 (V3204D) V+ (pin 5) V- (pin 1) = approx 8,12V

[g1]

Does U4 have 8V between pins 4 and 8?
U3 should have 8V between pins 4 and 7.
Sounds like some IC's are not getting supply voltage.

[rAdAdA]

none of them have 8V...
U4 Vee-Vcc: slides from 3.2 to 3.8, sometimes up to 4v2. No real difference when the pedal is switched on and off.
U3 Vcc- Vcc+ : slides from 3.7V to 2.5V up and down, sort of yo-yo effect... Again difference when the pedal is switched on and off.

Now the LED lits as expected when the pedal is switch on after a long period of being power supplied. The led won't lit right away if you unplug/plug it from the power supply, however after tens of minutes it lits.... Wiked.

-Y

[Grrrg]

So you have to follow the supply voltage which can be tricky without schematic. I would start at c26 and c27 or directly at the power input to see if there is 9V anywhere at all.

[rAdAdA]

Good catch Grrrg! Power supply returns 8.9V, C26 8.3V and.... C27 has approx 4.7V on its positive leg!
And by looking closer, this capacitor is a 16V !! Based on the fact my friend told me he powered the pedal with a 18v or 22V tension! It should be be it ! Pretty clever from MXR to have put a 16V capacitor here actually.
It looks blowed up on the paint, that would explain the sticky dirt on the power supply plug, What do you think ?


Now my concern is.... I can't read the farad value on the cap, and no schematics... How do you figure/measure a capacitor value, when it has blown up already ((laugh)) ?

Thank you so much for all your inputs !!!

-Y

[rAdAdA]

C27 was dead indeed! I replaced it by a 100uF 16v (I checked on Jackcon's LMK datasheet, and based on its physical size, it could only be a 100uF.)

I still get 4.7V on c27's positive leg, but now the led lits normally (Yeahh!) However the chorus effect is still not audible -the guitar sound is-, and turning the rate knob makes no difference.... Damn it!

I also unsoldered and checked c26, just in case, and it's correct.

We're getting there !

-Y

[rAdAdA]

Good news! I actually turn both trim pots TR1 and TR2... The chorus effect actually re-appeared ! I haven't touch them personally, maybe my friend did.... Well end of story.

Anyway THANK YOU very much again for this fixing party journey, I leaned a lot, and we have a working pedal now ! Thanks for sharing

Take care,

-Yann