July 28, 2020 at 8:30 am #13789
Thanks Wilkie I’ve learnt something I was trying to figure out why Colin was getting 66K resistance between pins 6 and 7 using the schematic I thought those 22Ks R12, 13 and 14 aren’t in parallel so why’s it 66K? Paralell connections always throw me
I’ve just looked again and see R13 is actually a 68K in reality, so the 66K is about right
I did read the notes honest!
Colin you can use any dual op amp to test 4558, NE5532 etc doesn’t need to be a TL072 to test itJuly 28, 2020 at 11:40 am #13791
Last night I switched out the op amp. Voltages are now reading at expected values. However, still no sound, so on to round 2…
I can now trace the audio path back to the output pad for C22. The output from here contains both the wet and dry signal, so we are definitely making progress! The is no output from the input pad for C22, nor from any pad before it. I measured continuity between the input pad and C22, and it is open. I also measure resistance between the two pads on R20 and there is none. This seems potentially problematic (doesn’t that mean my input signal is going straight to ground?). However, I am still figuring out how everything in the circuit works, and know that you can’t get reliable reading when measuring components on the board. Additional suggestions would be welcome- I’m getting close!
ColinJuly 28, 2020 at 12:22 pm #13792
I’m slightly confused Colin I thought from your initial post you had audio probed the circuit and were getting audio output at IC1A
Is that the caseJuly 28, 2020 at 12:49 pm #13793
So for clarification, I just checked the audio probe documentation and I am probing in what I think is the opposite direction of what is outlined in the instructions I saw- I have the probe hooked up to an input signal and the output jack on the pedal goes directly to the amp (I have never been able to get the probe to work the other way around). Thus, I am working backward from the output jack rather than the other way around. So that help explain? Sorry for the confusion.July 28, 2020 at 1:10 pm #13794
Normal set up would be to have your probe connected to out, so ground one side then plug your out cable into your amp and the other end to your probe then it’s just a case of following the audio path with the probe end
Audio in as normal to in jack with whatever you’re using phone, guitar, mp3 etc I use a looper pedal just continually playing a loop, so I’ve got 2 free hands
Obviously making sure it’s in effects mode I’ve done that more than once not switched it on only realising a few head scratches later
With you getting output at IC1 initially that would indicate everything before it was ok
Now that you have correct voltages probe pins 1 and 7 IC1 to see if you get audio
If you think audio goes from in pad straight to ground just continuity check your in pad to ground to check it
At this point I wouldn’t worry about resistance readings the initial resistance readings were just a process of elimination because your VR voltage was 6.5v
So we basically check from VR and up to IC1 to make sure everything’s correct value wise etc for example we can see on the schematic from VR to pin 3 we should have 220K resistance (R19) you can see from pin 1 going back to pin 2 we should have 510K (R16)
So you’re checking everything before you condemn anything in this case IC1 eventually
When I get home I’ll link you to Tonmanns guide on checking op amp circuitsJuly 28, 2020 at 3:25 pm #13795
Just to be clear- based on the way I was using the probe, I now feel confident that everything between R21 and the pedal output (including the TL092 chip now that I replaced it) is working well. All is in reverse because of the way I was using the probe (I was verifying probe to output on the circuit instead of input to probe). I just double checked, and there is continuity between my input pad and the ground, so I know that is a problem, and in fact means I won’t be able to get any signal out if I use the probe in its intended way (since my very first point in the audio path contains the issue). I know that the ground is happening somewhere between the input pad and R21 (or maybe somewhere between the switch and the board, though the pedal works fine in bypass mode).
I’m not entirely sure I’m communicating clearly what I am observing, so thanks for bearing with me!
-ColinJuly 28, 2020 at 3:50 pm #13796
Continuity check all your IN connections to ground make sure nothing’s shorting, there’s no wires touching or shorting on the enclosure
Check that only one side of R20 has continuity to ground
Also check your 3PDT switch continuity in effects mode top row with middle common row
The way you’ve explained your probing you may well have found your problem but with you getting audio initially at IC1 would mean audio was getting into the circuit of course with you testing and moving the circuit it may have developed another fault
Bypass signal just goes from the in jack via the switch to the out jack and not through the circuit so that tells us your jacks are wired correctly but doesn’t tell us your guitar signal is going into the circuit if you don’t get audio on the in T pad then it’s not going into the circuit
I tried setting up my probe with audio to the input
It did work on a confirmed working fzzz bxxx when I probed the IN pad it was a lot louder and fuzzier than the out pad but I did get audio
So there you go Colin weird but it works!
Here’s the link to Tonmanns guideJuly 29, 2020 at 2:10 am #13818
Yes- I could actually see where it would be useful to be able to work into the circuit from both ends with the probe in order better drill down on the problem area.
Just to clarify, I was never getting sound from IC1, I just wasn’t communicating clearly what was happening. I think the chip was bad from the get go.
To conclude the story, tonight I stared at the area around the input for about 10 minutes and realized I had reversed the hot and ground wires when I soldered them into the board, which certainly explains the readings I was getting. I reversed them and the pedal is now working!
I am most grateful for all the help and patience you provided! Building pedals for me has always been about just following directions carefully. I understood how the off board wiring all worked, but had no idea exactly what happened when the signal reached the board. This has been a great introduction toward beginning to understand circuits, and now I’m excited to dive in further. Here’s the final build:
ColinJuly 29, 2020 at 8:57 am #13823
Ah I actually thought you’d probed it when you initially said I can trace the audio back to the output of IC1
Well it was a good exercise if we learnt something it definitely confused me trying to figure out how you got audio with those voltages etc
Anyway glad you got it going Colin great looking pedalJuly 29, 2020 at 4:20 pm #13831BarryKeymaster
Excellent! I hope you feel that troubleshooting in its own way can also be a rewarding experience.
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