Build Your Own Clone Message Board

Hi all! I assembled my Mimosa and found that it didn't work. Diagnosis:

-signal passes in bypass, and the LED turns on when engaged
-the pedal only puts out white noise, intermittent oscillations
-the level knob functions, the blend knob does not appear to

I started troubleshooting
-I looked at the PCB and noticed the op-amp was flipped the wrong way. removed power, flipped it the other way, plugged it back in, still nothing. I should say that it was extremely hot and burned my fingers a little...didn't feel normal! could I have fried it by plugging it in the wrong way?
-next I checked the forums and took the advice to re-flow every solder joint. still didn't work. I even tried re-flowing it AGAIN just to make sure (because I was hoping it would be that simple!)
-I read through the recent "Mimosa No sound when engaged..." thread, some great advice there!

I started trying to trace the signal back. I don't have a signal probe, just a digital multimeter with a consistency tester. with the effect on I'm getting continuity from poles 4 and 5 on the switch, and from 7 to 8, and from pole 5 to C1, but after that nothing (I tried resoldering C1 already).

I took readings from the IC and various places in the circuit, they are way way off compared to the "ideal" Mimosa readings from that thread:
1) mine=10.64 ideal =4.51
2) mine=8.54 ideal=4.51
3) mine=1.31 ideal=4.51
4) mine=14.85 ideal=9.02
5) mine=7.84 ideal=4.51
6) mine=1.61 ideal=4.52
7) mine=1.20 ideal=4.51
mine=0.27 ideal=4.94
9) mine=0.25 ideal=4.94
10) mine=7.90 ideal=4.84
11) mine=0 ideal=0 (ground)
12) mine=7.75 ideal=4.51
13) mine=7.95 ideal=4.51
14) mine=8.40 ideal=4.50

Right side of R4 - mine=8.74 ideal=9.02
Left side of R4 - mine=16.36 ideal=4.84 (same is pin 10 of IC1)
Right side of Q1 (Drain) - mine=16.60 ideal=9.02
Center of Q1 (Source) - mine=3.96 ideal=2.60
Left side of Q1 (Gate) - mine=2.07 ideal=1.67
Bottom of R22 - mine=16.78 ideal=9.02
Top of R22 - mine=8.23= ideal=4.51
Bottom of R8 - mine=8.71 ideal=4.51

some pictures:


(I should reiterate that I reflowed it twice and every joint looks solid under a magnifying glass. any that look gnarly in the photo are a function of the overhead lighting)
thank you all for your help trouble-shooting!

Installing the IC backwards very likely fried it. Contact BYOC about a replacement.

What are you using for a Power Supply? Your pin 4 reading on the op-amp is way too high for a 9V PS. It’s still within the chip’s tolerance but it will skew the other voltage readings providing the IC isn’t dead.

Cut off all those little pigtails of component lead at the very top of the solder joints. They are all short circuit risks.

_________________
"It’s your soldering."

great advice Stephen!!

I asked BYOC about a replacement and they're sending me a new one. excellent customer support! I still want to get these voltages closer before it arrives.

those initial measurements were with a One Spot. I followed your advice and trimmed every lead as close as possible to avoid short-circuits, and reflowed everything for the third time just to be sure.

I also realized my multimeter's battery was running low so I replaced the battery before re-checking the voltages. and I biased Q2 to 1.6V first (I have to say, this has been a great education in how to use a multimeter). they were way different this time, now they're super low!

new readings:
1) mine=2.20 ideal =4.51
2) mine=1.50 ideal=4.51
3) mine=1.31 ideal=4.51
4) mine=9.35, the One Spot is supplying 9.36V so this is great
5) mine=2.53 ideal=4.51
6) mine=1.32 ideal=4.52
7) mine=1.20 ideal=4.51
mine=0.96 ideal=4.94
9) mine=0.66 ideal=4.94
10) mine=3.53 ideal=4.84
11) mine=0.01 ideal=0 (ground) - should I be worried about this?
12) mine=2.80 ideal=4.51
13) mine=2.42 ideal=4.51
14) mine=2.19 ideal=4.50

Also, the IC is generating a massive amount of heat, my multimeter's thermometer says 160 degrees! this is with either a PS or battery. is this normal?

I checked that all the caps are correct and polarized correctly, so now I'm about to sit down and recheck all the resistors. what else can I check?

The low voltages and heat generation point to a bad IC with a power short in it somewhere. I wouldn't run the pedal anymore until the replacement IC comes. See how it runs with that. Be sure to check the new IC for heat build-up, in case there's some other problem somewhere in the circuit. But I suspect the pedal will work fine with a new IC.

_________________
“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

awesome! thank you for the wisdom, it's super useful to be able to bounce this stuff off of much more experienced builders

update: I received the replacement IC from BYOC, lightning fast! I popped it in (the correct way this time) and now I am getting the dry signal with no problems, and the level and blend controls are working as they're supposed to.

However, instead of any compression I'm getting wild oscillations. when the blend control is full CCW I have clean guitar, and fully CW I have dry guitar plus oscillating. thoughts?

Check component placement carefully, something might be swapped...

*EDIT* Also, have you adjusted the trim pot?