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PostPosted: Tue Feb 15, 2022 12:48 pm 
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Hello. I'm having some trouble with my Mimosa.

Bypass works fine. When I engage the footswitch, the light comes on and I get sound. The level knob works as expected, but as the blend knob is turned clockwise, it gets quieter and quieter until there's nothing at the end of its travel, as if there is no wet signal being blended in.

I have reflowed. Which always makes me sad...it was so pristine before. :)
The trim pot is set to that the center leg of the transistor next to it is as close to 1.6v as I can get it.

This is the same problem described in this thread, so I've tried to do some preemptive troubleshooting based on what was discussed there:

Voltage Measurements from Q1:
D 9.13
S 4.24
G 1.59

Voltage Measurements from Q2:
D 1.59
S 1.59
G 0

Here are my voltage measurements from the IC pins:

1 4.55
2 4.55
3 4.56
4 9.13
5 4.56
6 4.58
7 4.56
8 4.99
9 4.99
10 4.90
11 0
12 4.56
13 4.57
14 4.57

Any thoughts? I can make an audio probe if necessary, but I may need very elementary guidance on what to do with it.


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PostPosted: Tue Feb 15, 2022 3:05 pm 
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The IC voltages look right on the money, but I'm concerned that something is off with the voltage biasing between the trimpot and the two JFETs. I notice the following differences between your Mimosa and my own, which operates perfectly:

  • The trimpot in my pedal is set almost exactly at the middle of its sweep, while yours appears to be fully CW in order to achieve ~1.6V on the source of Q2. That kind of difference is very suspicious. These trimpot circuits are generally set up so that the expected optimal setting will be near the middle of the pot sweep.
  • You report 4.24V on the source of Q1 where mine shows only 2.5V there, which also seems odd.
  • The voltage on the drain of Q2 in my pedal continually fluctuates between 1.3 and 1.6V; you have reported no such fluctuation with your pedal.

If I have misrepresented any of your findings, please let me know.

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PostPosted: Tue Feb 15, 2022 3:54 pm 
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Thanks for writing back.

I also thought it was odd that I had to go so far on the trimpot, but in that other thread, it looked like the person with the same issue had it set similarly. Not sure what that indicates though.

I'll recheck the voltages that you mentioned. When you were seeing fluctuations, were you running signal through it? I'm not, and not sure if that matters. All I had plugged into it when I tested was power and a 6" patch cable on the input that goes to nothing, just to complete the power circuit.


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PostPosted: Tue Feb 15, 2022 4:04 pm 
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I have confirmed:

- Voltage still reading 4.24V on the source of Q1
- The only fluctuation that I see on the drain of Q2 (still with no signal present though) is in the thousandths place on my meter


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PostPosted: Tue Feb 15, 2022 4:17 pm 
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I have no idea if this is a valid test, but I don't have enough hands to test it and play an actual guitar. I played a 440Hz sine wave on my laptop, plugged in an 1/8" to 1/4" wire to the headphone jack, adjusted the volume until it was putting out around 100mV AC (?), and plugged that in to the pedal. I also monitored on an amp. That Q2 value still doesn't fluctuate more than 0.001V, even when moving the knobs.


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PostPosted: Tue Feb 15, 2022 4:49 pm 
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Sounds like a bad JFET in Q2, same issue that the builder in your referenced thread had. Please contact sales@buildyourownclone.com and request a replacement at no charge.

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PostPosted: Tue Feb 15, 2022 4:55 pm 
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Cool, thank you.

It's so weird that it's the same problem in the same spot, but it sure seems like it to me too.


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PostPosted: Tue Feb 15, 2022 8:00 pm 
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chip wrote:
It's so weird that it's the same problem in the same spot, but it sure seems like it to me too.

Actually, it doesn't seem that weird to me. As thru-hole JFET stocks dwindle and the unit price goes up, there seems to be a higher and higher incidence of quality/reliability issues. Unfortunate for us DIY'ers!

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PostPosted: Tue Feb 15, 2022 9:12 pm 
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Ah, ok.

This is a potentially dumb question and will likely raise more questions, but I'm trying to learn this stuff...

Should a 2N5458 work in that spot, or would that throw everything off? (the specified part is a 2N5457). Because I maybe tried that and nothing changed...all the behavior is the same, the trimpot has to be adjusted the about the same position to get 1.6V on source of Q2, etc. Maybe that's not a valid test though since it's not the right part.

Also, I noticed that when I pull the jack out of the input (but the pedal still has power), the bias voltage goes to the full voltage (9.13V). Does that sound right?


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PostPosted: Tue Feb 15, 2022 9:46 pm 
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I would think the 2N5458 would work in this application as well. Very similar to the 5457 overall.

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PostPosted: Tue Feb 15, 2022 11:02 pm 
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Ok, well, that didn't seem to fix it.

I just built an audio probe and it works. Like I can touch the input with it and the parts of the switch where the signal goes and I get noise. I'm not sure which points would be interesting from there. Is there a map somewhere that shows what component number (like R2, C4, etc.) is where on the pcb? I think I could follow it from the schematic easier than the traces on the pcb.


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PostPosted: Wed Feb 16, 2022 11:38 am 
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Start at pin 7 of the op amp. Then pin 10, then 8 and then 14.

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PostPosted: Wed Feb 16, 2022 2:47 pm 
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I do not have signal on pin 7.

Assuming they're numbered like this:
Attachment:
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PostPosted: Wed Feb 16, 2022 4:00 pm 
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Yes, that's the correct pin numbering.

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PostPosted: Wed Feb 16, 2022 5:17 pm 
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DISREGARD PREVIOUS TEST

Sorry, I'm new to this sort of test. I just did more testing and tried follow it all the way from the input and realized I had the switched bypassed. :oops:

Actual results:

Has Tone?
  1. Yes
  2. Yes, but quieter than 1
  3. Yes, but quieter than 1
  4. [V+, did not test]
  5. No
  6. No
  7. Yes, but quieter than 1
  8. Yes, but VERY QUIET
  9. No
  10. No
  11. [V-, did not test]
  12. No
  13. No
  14. Yes, but gets quieter as BLEND goes clockwise

I tested each of these with both the level and blend in motion, and the only one that seemed to react to the knobs was pin 14.


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PostPosted: Wed Feb 16, 2022 6:33 pm 
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I'd have to guess that Q2 is bleeding off your signal before it can get to pin 10. I don't think a 2N5458 will work as a replacement in this circuit. You should probably email sales@buildyourownclone.com for a pair of replacement 2N5457.

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PostPosted: Wed Feb 16, 2022 7:21 pm 
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OK. I really appreciate the support! Thanks for helping me troubleshoot. And now I have a cool audio probe too.


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PostPosted: Tue Feb 22, 2022 9:31 pm 
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Ok, I got the new JFET and installed it (thank you).

It doesn't appear to have changed anything. Am I missing something dumb here?

I did some additional tests...I hooked it up to my DAW, because I can make it make noise wet (see below), and I wanted to see if it was compressing, and sometimes that's easier for me to see visually.

When I have the bias set to spec (which I'm doing my connecting the negative terminal of my meter to ground and the positive to the Source (center pin) of Q2) it's still way at the low end of the range of the trim pot. When I initially posted this, I said that there was no wet signal. It turns out that there is a tiny bit. When I have both the blend and volume set all the way to max (clockwise) and play a guitar into it with the bias at 1.6V DC, it will technically move the meters in my DAW, from a noise floor of -56dB to like -42dB. I couldn't hear it my shop amp, but it is technically doing something.

When I set the trim pot to the center its rotation, the bias reads like 5.0 to 5.5V DC, but the passes plenty of signal. Still at 100% wet on the blend knob, I have to turn the volume down to even test it or it overwhelms the inputs on my interface. If I lower the volume to where it's not peaking out, the guitar comes through normally, but it doesn't seem to be compressed, neither by ear or by looking at the wave forms.

I tried to find a sweet spot between 5.5V and 1.6V, where the compression is happening but the signal isn't so low that I can't hear it, but I'm not sure I'm detecting any. Sometimes I think I do, but I then I bypass the pedal and I'm not. I think I may just want to hear it and am psyching myself into it. It certainly isn't obviously squashing my guitar sound like my Keeley will on extreme settings.

Does this troubleshooting help? Is there some other test that I should do? I have the audio probe now and can take readings with that or a meter as needed.

Sorry I'm not getting this.


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PostPosted: Thu Feb 24, 2022 10:53 am 
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My diode is in the right way, right? I'm not missing something about the way the Ge ones are marked?

Please let me know if there's anything else I can do to help troubleshoot.


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PostPosted: Fri Feb 25, 2022 2:28 pm 
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Your diode is correct. It's only the diodes with the white dot that you need to be careful about.

If you're expecting it to have the same "sound" as your Keeley compressor, maybe this is just a case of you expecting it to sound differently. The Dynacomp (what the Keeley is based on) has an extremely fast attack, so you will notice the compression more. It will sound like an actual effect. It's going to have that snappy pick attack that makes your strings feel like rubber bands. The Orange Squeezer has a slower attack, so it's not going to have that feel or sound. It's going to be more like a studio compressor where you almost don't really even know it's on. 1.6V isn't always going to be the exact bias voltage. Just go by ear and adjust the bias trim pot so that the wet signal volume level is exactly the same as dry signal or as close as you can get it. See if you notice it then. You'll notice it more on the peaks rather than the attack, so if you continuously strum really hard, it would be about the same as if you're continuously strummer softer....to a certain degree...it's not going to squish all the dynamics. If you actually want it to have the same sort of attack as the Keeley, you can try changing the 2.2uF capacitor to a 1uF or smaller.

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PostPosted: Mon Feb 28, 2022 11:04 am 
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Ah, ok. I messed with it for a loooong time this weekend, and I guess I have it dialed in.

I was definitely looking for something different than the Keeley, so that's not an issue. I'm still not convinced that I can actually hear it beyond my wanting to hear it (I've been doing audio stuff for like 20 years, and at this point I feel like I know what compression sounds like) but after playing with it for a while, I can "feel" it in the way the amp responds, etc. It's pretty cool in that regard. Not what I was expecting, but maybe what I actually needed.

The bias is set way higher than spec, but not all the way up to 5V, where it was in the middle of the range. I don't remember where it wound up, but probably around 2.5-3V. In the old days of the MIMF, I remember an argument that arose when guitar tuning machines with high gear ratios started to become popular. The primary advocate for using the old, lower ratios made the argument "I just don't want to have to turn the knob hundreds of times when I change strings, and don't care if it's a little more finicky when actually arriving at the pitch. We're musicians: we're used to making extremely small adjustments." That has always stuck with me for some reason, and I tend to agree in principal. But I don't think I've ever made such extremely small adjustments to anything as I did with that trim pot. :) I have a set of precision screwdrivers, so I was able to find one that pretty exactly matched the slot of the trimmer, but found myself not so much "turning" the trimmer as delicately bouncing the blade of the screwdriver within the microscopic amount of play so that its inertia would nudge it to where I needed it to go.

As always, I appreciate the guidance and patience. This is resolved.


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