Tap Tempo Tremolo signal lost in the path

[Zoot]

Hi Everyone,
I hope you are staying safe (and calm), especially those in the US and other countries where COVID-19 and trouble is the norm these days.

I'm wrestling a (MUSICPCB) TAP TEMPO TREMOLO, for some time now, every now and then I bring it to the table, but it's kicking my ass most often.

The Tremolo LED doesn't lighten up, unless I touch it's negative leg or path with a probe...
I checked all the voltages that are explicit in the diagram, and they are all OK. Checked as well the voltages of the ICs and Vactrol, as per picture below. I also checked my components's values more than once, I tried more than one TL072, changed the Vactrol... No luck.

The sound signal goes until the first leg of the Vactrol and dies there. What is quite strange is that before the TL072 input, after R11 220K, sounds get lost. But it is present after R12 220K, when it goes into Pin 1. I don't understand that. The RED Xs show in the circuit where signal gets lost.

I would really appreciate a guidance on how to go about it next. Can some one please help?!

Thanks so much in advance,
Zoot

[Digital Larry]

The signal is not "lost" at the op-amp input.  It is typical for an inverting op-amp stage as shown to show zero voltage fluctuation at the (-) input.  It should measure very close to Vref.  This is referred to as "virtual ground" and is the result of being inside a feedback loop with very high gain. 

Also, just to be clear, reading your post, it sounds like the problem is that the tremolo LED doesn't come on unless you touch it?  I'd look for a cold solder joint or broken/cracked trace there or nearby.

Are you getting sound out of the output, or not?

[duck_arse]

as DL sez. also, photos of your build, please. what type # transistor are you using for TR1?

[Zoot]

Thank you Digital Larry and duck_arse for your replies. Appreciated.

Thanks for explaining that it's normal to have signal « disappearing » in the input of the TL072 (I was referring to the audio signal, sorry if unclear)

The issue is not the LED not lightening up, this is probably part of the issue. I reflowed all the joints and it didn't fix the LED not lightening.

Using an audio probe, the signal goes to the Vactrol, and it seems to die there, as on the PIN 7 of the TL072 there's no output. There's not audio signal after the 1st leg of the LDR. That's the issue, as I'm not getting any sound in the output.

For TR1 I'm using a 2N3904, which I replaced yesterday, just to be sure. It didn't change anything.

I'm attaching pictures, wiring is a bit all over, as I've been dealing with it since a while. As soon as it works I'll rewire most of it. If you see what I should do next, please help me! It's been a while this thing should be working.

Thanks again guys.

Abrazo,
Zoot

[Digital Larry]

Here's a debugging suggestion.  Use tweezers or needle nose pliers to short the C and E on TR1.  The LED should light and you should get signal.  With no current going through TR1 collector, there's no LED light and the LDR is at maximum resistance, therefore the op amp stage is at minimum gain.  If shorting TR1 C to E solves the problem then you need to look at the TR1 circuit carefully.

Pin 5 of the PIC at 70 mV, if I am reading that correctly, is not enough to turn on the transistor.  Something seems weird in that area.  Do you have any evidence that the LFO PIC is working at all?  I'd expect to see a varying voltage at pin 5.

[Zoot]

Digital Larry: spot on, thank you.

If I short C & E on TR1,  LED lights and audio goes through.

Indeed, measuring again, PIN5 of the PIC varies, from 70 to 115mV.

well, I'll keep working on it and report back here if any progress or stuck.

Thanks
Zoot

[ElectricDruid]

Which version of the PIC have you been sent?

The board is designed for the TAPLFO 2D. The TAPLFO 3C can be used with some simple changes, but if you've built the circuit for the 2D (you have - the 3C doesn't need the crystal) then you need to use the 2D.

If the PIC isn't working correctly, it might be because you've got a bad pot. Check the voltage on pins 9 to 13 to see if you've got a solid voltage from 0-5V and that it responds to the pot.

[Zoot]

Hi Taylor,
Thanks for jumping in. I'm using a TAPLFO 2D I received with the PCB.
Will check the voltages and pot response tomorrow for sure, it's 0am here where I live (CH)... and I'm going back to the office tomorrow for the first time.
I'll report back here when done.
Thanks again!
Zoot

[Zoot]

Hi Taylor,
Solid voltage values on all pins you mentioned, and all 5 responding to the correspondent pot. It seems all cool here.

PIN 09: 0 to 5V
PIN 10: 0 to 5V
PIN 11: 0 to 3.7V
PIN 12: 0 to 5V
PIN 13: 0 to 5V

My suspicion lies between R1 10K until the Vactrol, but still didn't find it.

@Digital Larry: when you mention that I should look carefully to the TR1 circuit, you mean the Vactrol/LED/Trims zone?

Thanks again,
Zoot

[ElectricDruid]

Hi Taylor,
Solid voltage values on all pins you mentioned, and all 5 responding to the correspondent pot. It seems all cool here.

PIN 09: 0 to 5V
PIN 10: 0 to 5V
PIN 11: 0 to 3.7V
PIN 12: 0 to 5V
PIN 13: 0 to 5V

I'm not Taylor, but those voltages all look good. Should be fine.

My suspicion lies between R1 10K until the Vactrol, but still didn't find it.

@Digital Larry: when you mention that I should look carefully to the TR1 circuit, you mean the Vactrol/LED/Trims zone?

You could try taking a basic LED and series resistor (series resistor for 5V) on long leads and connecting it between ground and pin 5. If you see the LED lighting up and following the Rate control, then the PIC is fine and the problem is further on. The only way this might not be true would be if TR1 was fried and shorting the output to ground (possible, but not very likely). Assuming pin 5 is ok, then you need to look at TR1, the Output LED, and  both presets. If any of those are seriously messed up, you might get the result you're seeing.

HTH,
Tom

[Digital Larry]

@Digital Larry: when you mention that I should look carefully to the TR1 circuit, you mean the Vactrol/LED/Trims zone?

I mean mostly in the circuit driving the base of TR1.  If there was a dead short between the PIC and TR1's base, I'd expect the voltage to at least go to 0.6 or 0.7 volts depending on how much current the PIC pin can output.  Since it's not even getting to that level it sorta makes me think the PIC pin's shorted out, but that doesn't make sense either as it varies between 70 and 115 (?) millivolts.  Suppose you lift the end of the resistor connected to the PIC.  If you connect that loose end to 5 volts does the LED come on?  Is the PIC pin still down at about 100 millivolts?  If so, something is either wrong with the PIC or it is shorted out somehow.  IMO.

DL

[Zoot]

Tom, yes, Tom... (why am I calling you Taylor?)
And Digital Larry.
Thank you both! Will try what you suggest.
Report back soon.
Zoot

[Zoot]

Hi Tom, hi Digital Larry,
This is becoming very messy. I destroyed the Vactrol, one leg came out, and the rail connecting R1 to TR1 popped when I unsoldered the resistor. I'm thinking this have been the issue all along (R1 rail). This whole issue happened right by the end of the build. I though this would work seamless, as it's a pretty simple build. Quite fragile this board to bear with my technique. Anyway... I will take a break and purchase another board...  The only issue is that I couldn't test the PIC. I guess I'll buy a second one and start over.

I have some other sister projects arriving and others on the build, a Wavelord from Madbean (to use my TAPLFO 3C) which seems like a better project than the MPCB TTT. Also I just populated a Flangelicious I received in a letter from Portugal
In any case I wanted to thank you for your patience and judgement less support.
Best,
Zoot 

[Digital Larry]

Well, good luck.  To me, it's sort of fun to try to problem solve these things and it tickles the part of my brain going back to when I first learned this stuff.  If I build things (which I mostly don't any more) I tend to build circuits in sections and test them 1 by 1.  So for example in this case, power supply first, PIC second, then LED/vactrol and finally the op amp section.  Sometimes it's a little overwhelming otherwise.  Of course, sometimes you can't do that.

Good luck, I'll be interested to hear what happens.

DL

[Zoot]

Digital Larry, I will definitely follow your advice to go by sections in my next built. Sounds like a good idea.
Thanks,
Zoot