July 24, 2020 at 1:56 am #13732
I’ve built about 10 pedals to my endless enjoyment and have always managed to complete them without incident. As a result, I’m a newbie to troubleshooting, so bear with me. I recently finished a D’lay pedal, and no sound is passing through when the pedal is engaged. I can trace the audio path back to the output for IC1 and still get signal though the output, but when I send signal through pin 2 it starts to get wonky (sometime a highly distorted signal will register). Testing voltage gives high readings. 8.9V are coming into the board, but pin readings beginning at first pin are 8.29, 7.79, 6.97, 0, 6.82, 6.92, 7.57, 8.94. I’ve attached a few pics in case anything stands out. Note, the TL072 chip I used does not have the notch I am used to to mark pin orientation, but it does have what I assume is a marker near pin #1. However, I’m not 100% confident I have the chip oriented correctly.
Any suggestions or guidance would be greatly appreciated!
ColinJuly 24, 2020 at 2:43 am #13733
The dot would indicate pin 1 Colin, sometimes it’s either a notch or dot and sometimes both if it’s both you’d go with the notch
Can’t really see the component side pictures they’re really dark with the shadow, we’d need to be able to clearly see all components, resistor colour bands and your offboard wiring so that it can be traced
Your TL072 readings are too high VR going into pin 3 should read half your power supply check what voltage you get at the junction of R17 and 18 the power supply voltage divider you can use your meters continuity check to see where they join
What are your voltages on the PT2399 pins 1, 2, 3 and 4
Pin 1 should be around 5v with 2 half of pin 1s voltage and 3 and 4 are grounds or 0v
I think looking at your solder side R22 appears to lack solder I don’t have the build doc for the tap tempo which I’m assuming is your other board so don’t know what you leave out etc
*EDIT just found the tap tempo doc and see the notes on R22 so ignore that!July 24, 2020 at 3:40 am #13734BarryKeymaster
D’Lay Tap Tempo Doc added to Master List on Guides Page.
Here is an image for Resistor color bands (not considering mods) as well as orientation sensitive components.
Zoom in if needed.
I hope that helps.July 25, 2020 at 8:05 pm #13744
Sorry- forgot to attach photos!July 25, 2020 at 8:40 pm #13745
Hmmm, looks like the post I wrote prior to posting the above two photo got lost. At any rate, thanks Billy and Barry for the quick response! I attached above photos that provide a better view of the front of the board. I believe the resistor values are correct (Note- Barry’s diagram above lists 10k resistors for the voltage divider but the spec sheet lists 12k which is what I used. As long as the value of both resistors is the same, it shouldn’t make a difference, correct?).
I measured voltage at the divider. There is 8.94V going in and 6.50V at the split. Pin 3 on the op amp reads 7.0V (why is the voltage increasing?). Something is obviously not right with the voltage divider.
I measures the pins on the PT2399 and they all read close to the recommended voltages. Any suggestions on next steps would be appreciated!
ColinJuly 26, 2020 at 2:58 am #13748
If you haven’t already reflow the solder joints on R17 and 18 just place your iron on the joints for around 4 seconds till it flows nicely
What are your voltages on pins 4 and 8 of IC1July 26, 2020 at 10:49 am #13750
I re-flowed all four joints but am getting the same readings. One more bit of information- I took readings at all four joints of the voltage divider. There is ~9V going into R17, ~6.5V at the junction (reading the same on both R17 and R18) and 0V coming out of R18. I assume that is not how it should be? I tested continuity for both R17 and R18 (while on the board). R17 is reading right at 12k, R18 is reading 10K7
Pin 4 of IC1 is 0V, pin 8 is 894V
Thanks for your continued help!
ColinJuly 26, 2020 at 10:50 am #13751
EDIT: obviously I missed a decimal- pin 8 is 8.94VJuly 26, 2020 at 1:17 pm #13753
You can’t test resistor value in circuit accurately
R17 and 18 voltages should be 9v top side of R17 and 0v bottom of R18 where they join should be half of the power supply 4.5v
Check the bottom end of R18 has continuity with ground touch the resistor leg with your meter probe not the solder joint
Post a nice clear in focus photo of the solder side of the pcbJuly 26, 2020 at 4:04 pm #13754
The lower side of both R17 and R18 (the side on the outside of the board) are showing 10.7k resistance to ground. The upper side (toward middle of board) of R18 has continuity to ground and R17 shows 12K to ground.
Attached is a photo of the solder joints- let me know if you need a better image.
ColinJuly 26, 2020 at 7:01 pm #13755
That all checks out with R17 and 18 apart from obviously the junction voltage of 6.5
Looking at your solder side I’d reflow C21 joints, if that has no affect
Take voltages on IC1 empty socket
Then with no power to the circuit take resistances from VR to IC1 pins 3 and 5 again you don’t need the IC inJuly 26, 2020 at 8:11 pm #13756
Progress! Without chip in, pin 3 is reading 4.35V and pin 5 is reading 4.45V so the chip is creating the higher voltage. Do I have a bad chip or the wrong one? Here is what I am using: https://www.digikey.com/product-detail/en/texas-instruments/TL072CP/296-1775-5-ND/277421).
ThanksJuly 26, 2020 at 11:52 pm #13758Bruce RModerator
Sounds like you guys are making progress. The only solder joint that really raised a question in my mind is circled in red above. I think that’s one leg of the trimmer.
The IC is the correct model.July 27, 2020 at 7:47 am #13759
I don’t see any bridges anywhere
With the IC being an active device I have had occasion in the past where with it out of socket voltages were fine in socket way too high changed the IC no difference and I eventually found a minute solder bridge
Now we know yours is ok until IC1 is in circuit I’d try another TL072 if that doesn’t work then check and visually inspect for solder bridges or shorts under magnification
C18 I think it is, solder joint also looks dull / dry
Another check you can do is with no power to the circuit check resistance between pins 1 and 2 should be around 510K and 6 and 7 around 22K this will let you know the op amp feedback loops don’t have any shorts or open circuits etc the op amp output can affect the inputs so worth checking
Nice to see you too Bruce hope you and yours are all wellJuly 27, 2020 at 11:38 am #13761
Resistance between 1 and 2 is good. Resistance between 6 and 7 is about 66k. When you refer to feedback loops, is that in the op amp itself or in the circuit somewhere? Will see if I have another TL072 on hand tonight to try and will also re-flow C18 and inspect solder joints under a magnifying glass. Thanks Bruce, Billy.July 27, 2020 at 2:16 pm #13764
The feedback loop is between the op amp pins where it goes from output back to an input
E.g. IC1B from pin 7 output back to pin 6 inverted input with R13 and C16 in the loop known as negative feedback this alters gain, filters frequency etc
Your resistance I think would include R12 and 14, so 3 x 22K or 66K could be correct we’d need someone with better knowledge than me to confirm it Cybercow or Wilkie or Bruce or Barry orJuly 27, 2020 at 10:41 pm #13778wilkie1Moderator
The build doc has a great explanation of the relationship between R12, R13 and R14 to adjust the levels of the dry and wet signals.
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