Problems with Boss OC-2

[diablochris6]

Posting on an older effect that doesn't seem like people make anymore. I had a board fabbed for the chopped OC-2 schematic that is floating around. The schematic looks like this.http://www.aronnelson.com/gallery/main.php/v/Processaurus/OC_2_chopped.gif

I found a couple of mistakes in my board and corrected them: I originally didn't add Vref to R4, D10, R49, or Q8, and didn't add R31 to C18. I used a J201 instead of a 2sk30 for Q8 and a 2n3904 for Q2. The only other changes I made were removing the first 1M from the input and changing C2 to 100n.

The problem is that it when it is powered, it makes almost no sound. I tried placing a buffer in front of the signal, but no dice. I used an Em-Drive boost pedal that I made, coil-tapped my pickups for a hotter sound, and picked very hard, but the sound was extremely quiet and gated; both dry and wet cut off very quickly. Sometimes if I attacked the string too hard, the effect would blast. Also, I had to turn both the Volume and Gain on the boost all the way up to just get a little signal. I fished around with my audio probe and found that my signal going into pin 3 of the TL074 was fine but the output at pin 1 was nonexistent and a little hissy. Both pots would hiss if I rotated them.

I measured voltages on the IC pins with a DMM and the pedal plugged into a wall adapter and found the following:

TL074	LM324	CD4013	j201	2n3904
1. 8.72 2. 1.86 3. 2.69 4. 9.6 5. 5.22 6. 5.65 7. 5.58 8. 5.62 9. 5.62 10. 3.35 11. 0 12. 5.17 13. 8.72 14. 8.72	1. 8.33 2. 7.95 3. 8.44 4. 9.6 5. 8.18 6. 7.96 7. 8.33 8. 8.33 9. 7.88 10. 8.18 11. 0 12. 7.87 13. 8.44 14. 0	1. 0 2. 9.6 3. 0 4. 0 5. 9.6 6. 0 7. 0 8. 0 9. 0 10. 8.15 11. 0 12. 9.6 13. 0 14. 9.6	G. 3.24 S. 5.36 D. 5.35	C. 9.6 B. 6.53 E. 6.22

I believe that pin 1 of TL074 should be half of what I measured. I DMM'd the components around that pin and found that R6 and C3 both measure 0V. Shouldn't there be some voltage coming out of the resistor to the grounding cap?

The output for pin 14 of the LM324 is odd, but here is what is weirder. I was foolishly playing around with my input signal and placed it at the resistor leading to pin 3 of TL074. I still had to crank up my boost pedal pretty loud, but I was getting the sweet octave sound along with a good clean signal. Using the DMM, I found no solder bridges, bad connections, or unwanted grounding. I checked and re-checked all my components, and they were all fine. I am really stumped here. My gut is telling me the problem is in the first opamp stage. Any suggestions or guidance?

[duck_arse]

pin14 of the lm324 is bad. check the resistance between it and pin8 of 4013, and both those pins to ground. pin2 and 3 and 10 of the tlo74 are bad, too.

any photos we can look at?

[diablochris6]

I replaced both the LM324 and TL074 but got the same readings. I checked the resistor from pin 14 of the LM324 to the 4013, and it was reading correctly. I also checked the resistors around the other problem pins, and they were fine as well. I checked to see if pin 14 of the LM324 and pin 8 of the 4013 was continuous with ground, and they were not. I checked for continuity between all the components surrounding and connecting to the problem pins and found no continuity or solder bridges between any of the components that weren't supposed to be connected.

Here are some photos that I took with my phone. If you need more, I can take more. The layout is similar to the old layout I found on musicpcb.com. It isn't laid out exactly the same, but it is similar. I might be a little preoccupied with family over July 4th, but I will keep you posted as soon as I can.

[duck_arse]

well, I'll clutch at a few straws for you.

there is no chance the pot bodies are shorting on some of your (too long) pins under? and the via under the middle IC, it is measured and continuous, yes? and sockets. take the IC's out of the sockets, then make absolutely 100% sure that the IC-pin-connecting-part is in contact with the leads of the appropriate nearby components. 100% sure.

is there a picture/layout diagram of the blank board, both sides, to look at?

[diablochris6]

The pots shouldn't cause any shorts; they have plastic caps over the metal pots. I know that many of my components have long leads coming out of the solder joints, but I trim/move/measure them to make sure they don't touch anything they aren't supposed to. The via underneath the LM324 measures 7.88, the same as pin 12 of the LM324 it is connected to. I measured the continuity of the sockets with their neighboring components and found that they all connect correctly. Here are some pictures of a unpopulated board and, in case the pictures aren't good enough, the Eagle pcb layout I made up. On the Eagle file, I have a ground plane that isn't shown to make the leads easier to follow. Since the pcb doesn't have the same part numbers as the Milner schematic, I will include my version.




I had to add a jumper to attach my Vref to R2,D1,R7,&Q1 since it wasn't in my pcb.

Thanks for taking a look at this. I am completely bumfuzzled.

[merlinb]

TL074
1. 8.72
2. 1.86
3. 2.69

The opamp is clearly saturated. This might be because R4 is too big, R3 is too small, C2 is faulty, or something else is not right. Check the components in that area. (I think I can see some dodgy soldering around there in the photo?)

The second opamp stage is OK, so you are getting a satisfactory Vref to that area, and probably to R2.

[diablochris6]

Merlin, I did some more measuring and found that R4 is actually measuring around 8.9K (that is with the component in the pcb and no power), and R3 was 2.6K. Since I knew that I was getting 0V from the connection between R3 and C2, it might just be a bad cap. I know you can't really get an accurate reading of a bad cap on a cheap DMM, but I did seem to find an unwavering continuity between the two leads. I will probably just replace C2, R3, and R4 and see if anything changes, unless someone else has any other bright ideas.

[diablochris6]

I swapped out R2-R4 and C2 but still have the same problems and readings. Any other suggestions before I chalk this up to a bad board and start from scratch?

[merlinb]

I swapped out R2-R4 and C2 but still have the same problems and readings. Any other suggestions before I chalk this up to a bad board and start from scratch?

It's not a bad board (and if it is, it will be a minor copper bridge that needs to be scraped away or something), don't give up on it!

Try swapping in a new TL074 and see if you still get the same voltages around the first opamp. (You probably will, as a bad opamp is very rare, but it's worth a shot).

Also try removing C2 and see if the voltages around the first opamp change. This will reduce the gain of course, but we only need to fix the DC voltages right now; the rest will follow.

Post some high-res photos of the offending area, preferably with the pots removed. Measure the voltages with the pots removed. The fault is there somewhere, no doubt about it!

[diablochris6]

Removed C2, took measurements, and they are the same as when the capacitor was in. Removing the pot also did not change any of the readings.

I have swapped the TL074 before, but I did it again and found no change in the situation. So, the IC is probably ok.

Here are a couple pics of the problem area with the pot removed, as requested. The janky wire is my jumper that gives R2, R7, D1, and Q1 their proper voltage. When I removed the pot, I had to reinsert the wire into the pad. It isn't pretty, but it works.

[Marcvv]

I checked your pics, they are big indeed! 
No offence but may i suggest to reflow most of your soldering connections as they do not look to great. Also clip of the wires ends of the parts a little shorter there are some that almost seem to touch eachother. Maybe this will help in solving your problem. Sometimes one bad soldered connection that you reflow makes the whole thing come alive.  

[merlinb]

Removed C2, took measurements, and they are the same as when the capacitor was in. Removing the pot also did not change any of the readings.

So you still get this even with the cap removed?
TL074
1. 8.72
2. 1.86
3. 2.69

This indicates the bottom of R3 is shorted to ground somehow (meaure the resistance between there and ground, I think you'll find it is zero!)
Now, I can't see any obvious solder short in your photos, but that doesn't mean its defnitely not solder. I suggest you remove R3, the circuit should work then, to some degree (i.e. you'll get clean audio out, but no octave since the pot is removed).
Check for continuity between the resistor pad and ground, is it still shorted once R3 is removed? Reflow or suck out the solder. Is it still shorted to ground?
If yes then this implies there is a copper bridge that hasn't etched away. You may have to get a scalpel blade and scrape around the pad and trace to break through the solder resist. Keep checking for continuity until the short is broken.

[diablochris6]

Well, there was an errant copper trace that led R3 to ground. After taking care of that, I have a perfectly functional board now. I noticed the problem with R3 in my initial post, and it was nice to have a few other pairs of eyes to help me validate my problem. And it is good to know that the problem wasn't due to a design or component error on my part. Yes, there are a few dodgy solder joints, partly due to a iron tip going bad, but that is an easy fix. Thanks again, Merlinb, duck_arse, Marcvv, and all you lurkers out there. Glad to be part of this community! Maybe one day, I can give some handy advice to somebody else!

[merlinb]

Another case closed! That was fun!

[duck_arse]

hooray! fixxe that board layout, then everyone will want one. nice work, even with the giant pictures.

[diablochris6]

I fixed the layout as soon as I found the problem. No more need for a jumper, I spaced out the trouble area to avoid another production error, and even added a couple extra pads for some goofy mods that are floating around the web.

[Marcvv]

Great you got it working. Well done!