VFE Choral Reef Build - Distorted Output - [SOLVED]

[Cybercow]

Got the Choral Reef up and running today but I cannot seem to ditch the distortion. All the voltage measurements seem to match the build document voltages. And I've gone back and forth between the two trimmers to try and get rid of the distortion. No parts substitutions - except that 65K resistor where I tented two resistors that equal 65K in series.

I followed the build docs for adjusting the BBD, but it sounds very distorted. There is a sweet spot at just about midway on the BBD trimmer where the distortion is at a minimum. But I don't believe it should be that distorted.

All parts were sourced from SmallBear and Mouser.

Does anyone have any experience with the VFE Choral Reef that has a distorted output? Or are the trimpots just that fussy with only a tiny sweep section of each of the trimmers? I spent over an hour already futzing with them in very tiny increments.

TIA

[Scruffie]

What sort of pickups are you using? It's not exactly designed for high headroom.

Could you please also post your MN3008 voltages regardless, just so I can double check them.

[Cybercow]

What sort of pickups are you using? It's not exactly designed for high headroom.

Could you please also post your MN3008 voltages regardless, just so I can double check them.

Thanks Scruffie . . . . I use a '79 Strat, so the pups are not hot. I remeasured all the voltages as shown in the chart, and the MN3008 is spot on. However, I think I've found the source of the distortion.  Pins 12, 13 & 14 of the TLE2074 are reading much lower than they ought. Instead reading 3.86, 4.88 & 4.88 respectively, they are at 0.93, 1.04 & 1.04 - also respectively to pins 12, 13 & 14 of the TLE2074. That's the first opamp section in the circuit. All other voltages are within 5% of the voltages listed in the build doc. Except pin 2 of the 4047 which measures 34mV instead of 115-118mV.

When I try to measure the 2.2MΩ resistor on pin 12 of the TLE2074 (with the TLE2074 removed) I cannot get a reading between pin 12 and the VB. Theoretically, with the TLE2074 removed, I should be able to read 2M2Ω between pins 12 and 5 on the TLE2074 socket. But it reads as 'open'.

Also, there seems to be no source of "VB" on the schematic and as such I have no reference for what VB is supposed to be. I can only guess it's supposed to be 1/2 of the supply voltage. I can easily find 4 locations on the schematic where "VB" connects to specify points, but no reference to a "VB" in the power distribution section.

[madbean]

Remember that for the Choral Reef the VB source is on the switching board. This is explained in the SB3 doc. It's the only project that requires it.

[Scruffie]

That'll do it, I was expecting that the BBD output op amp was going to be the problem, given how close to the rails it ends up biased.

[Cybercow]

Remember that for the Choral Reef the VB source is on the switching board. This is explained in the SB3 doc. It's the only project that requires it.

I just sorted that bit out. And I did carry over the -V line from the SB3 to the PCB. I also followed the explicit instructions in the "In the Choral Reef the following exceptions are applied" section, leaving off the charge pump IC and C5.

[Cybercow]

That'll do it, I was expecting that the BBD output op amp was going to be the problem, given how close to the rails it ends up biased.

What doesn't make sense is why all the other voltages are correct. I even swapped the TLE2074 with a fresh one - same result. (I got both TLE2074 chips from Mouser, so I should be able to assume they're both good. Or at least one of them.)

I'll look closer at the SB3 switching board to see what I can find. Without a schematic for it, I'm left to tracing from the layouts in the SB3 build doc. And I did catch the bits in that build that are exclusive to the Choral Reef.

Back to the bench . . .. .

[danfrank]

Did you add the 2 resistors and reverse the one cap on the switching board? This is part of the "choral reef" difference on the switching board.

[Cybercow]

Did you add the 2 resistors and reverse the one cap on the switching board? This is part of the "choral reef" difference on the switching board.

Yes - R5 & R6 installed and C6 installed reverse of the polarity markings on the SB3.

[Scruffie]

Oh I see, I thought you'd resolved the problem.

I think photos might be of use in this case, seems like there might be a short if the VB is working elsewhere.

[Cybercow]

Bench update . . . .

With the TLE2074 removed:
1) pins 12 & 13 now read 4.88 volts - but pin 1 reads only 1.2v.
2) unable to measure the 2M2Ω resistor between pins #12 & #5 of the socket - but there is continuity between pin 12 and the 2M2Ω resistor; there is also continuity between pin 5 and VB - AND continuity between the other side of the 2M2Ω resistor and VB.
3) the 2M2Ω resistor measures 2.2M Ω (with the TLE2072 out of the socket.

On the SB3 board:
1) R5 & R6 are in place
2) C6 is installed reverse of the PCB markings (as per the build doc)
3) there is ~4.88 volts on the V- point and ~9v on the V+ point

Other points:
1) all 3 16mm pots have SmallBear pot condoms on them
2) with all chips in place, powered up and plugged in, all the controls have functionality. The output is just quite distorted. (The first opamp [TLE2074] section has a low-voltage condition on pins 12, 13 & 14.)
3) C18 (.22µF) is not leaky (from Mouser)
4) C12 (10µF BP) is not leaky (from Mouser)
5) Tried a few different quad opamps (TL074, TL084, OPA4134 & OPA4132) and all exhibit the same low voltage on pins 12, 13 & 14 - and all the pots still have functionality - just dirty output
6) With all chips in place and the circuit powered up, there is 4.88v at the VB points, including pin 5 of the TLE2074

I've not started pulling my hair out yet, but it's close to that pointless prospect. Clear photos coming shortly.

Any further suggestions or things to check will be appreciated.

[Cybercow]

Here are the build photos . . .







During the build process, I carefully examine the progress with a strong light and magnifying visor - double checking for any bridges and\or solder splashes. Especially before soldering the pots into place; as they cover up much of the work. I suspect there may a micro fracture (possible a scratch) through a trace near the first opamp section.

Thanks for all the suggestions thus far.

[Cybercow]

PROBLEM SOLVED!

To cut to the chase - it was the 2M2Ω resistor off of pin 12 on the TLE2074. When measuring from pin 12 of the empty socket to the 2M2Ω resistor, I got continuity. When measuring across the 2M2Ω resistor I got 2.2M Ω resistance. When measuring the from VB board input to the 2M2Ω resistor I got continuity. That is what told me the 2M2Ω resistor was good.

HOWEVER . . . ..  with the TLE2074 still out of socket, I should have been able to measure 2.2M Ω resistance from pin 12 to the VB input - but it read open. That should have been my troubleshooting "AHA!" moment. But I overlooked it because all the other readings suggested all was well.

As it turns out, the pressure of applying the DMM probes to each side of the 2M2Ω resistor was putting enough pressure to make the resistor measure properly. Without any pressure, it measured 'open'. I had reflowed that resistor twice and all the above measuring and testing remained true. So, I pulled out the 2M2Ω resistor and put in a fresh one and THEN I could read continuity from pin 12 to the VB input. I then replaced the TLE2074 chip and pins 12, 13 & 14 then gave correct voltage measurements. After which I hooked it up to my amp and guitar and VOILA! I had crystal clear chorusing with all controls doing fantastic things to the chorus settings.

Finally, I went back to the 2M2Ω resistor I had pulled and gently connected alligator clips to the leads and re-measured it. It read 'open'. I took the gator clips off and applied the DMM probes to either side and it measured 2.2M Ω. Apparently, that specific 2M2Ω resistor has an internal micro-fracture. It's in the bin and I'm happily onto finishing the enclosure for my new VFE Choral Reef pedal.

Thank you to everyone who participated.

[jimilee]

Daaaaamn nice job.


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[Cybercow]

Daaaaamn nice job.
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Thanks JimiLee !