SOLVED Caesar Chorus: No signal or effect

[jeffwhitfield]

Hey folks! Another one to debug. This time, a Caesar Chorus build. Not getting any signal when Blend is on full. When turned fully counter-clockwise, I get a very low signal. No effect at all regardless of settings. Here's a breakdown of the build.

Instead of the V3102D and V3207D IC's, I used the MN3102 and MN3207...which I understand to be fully compatible. However, I did buy these from a dealer on eBay. Can't say they're fake since there's plenty of reviews posted for the dealer on these exact items. So the question is whether or not these will work. I'm assuming so since the other ones are based on these exact IC's. Anyways...

I originally put in a TL072 in place of the RC4558, which I figured would be better in terms of lower noise and whatnot. I got lots of distortion so I switched it out with a JRC4558 op-amp. No distortion...but no sound either.

Solder joints look good. Nothing seems out of the ordinary. Being that this is a chorus pedal, I don't really know where to start in terms of testing. Are there any particular voltages I should be testing for off the IC's?

 

[jeffwhitfield]

Ok, quick update...

I replaced the trimmer and the good news is that it fixed one part of the problem. Adjusting the trim, I can now see changes in the voltage for the op-amp and can adjust it down to 6v pretty easily.

Unfortunately, still no effect when adjusting the Blend knob. Sound completely cuts off when Blend is on full. Gonna do more signal tracing to see.

Still not convinced it's a bad set of chips though. Being that the chips do seem questionable I'm gonna go ahead and return them to the eBay seller and go from there.

 

[Bio77]

Great news. So are you getting clean signal through? If so, and If you still have the chips, you can start to work on the delay line.

 

[jeffwhitfield]

Well, I'm sending back the chips to the eBay dealer I got them from. Based on what I'm seeing with these, I just don't trust that the MN3102's I got are legit. I ordered some V3102's from Banzai but still need to get some V3207's. If I can get my personal build working then I plan on building more. I can wait though. No rush. In the meantime, I think Fig is sending me a working 3102 and 3207 for testing.

 

[jeffwhitfield]

Alrighty, so I got the chips Fig sent me and popped them in. With Blend full CCW, I get a clean signal...no problem there. Once I move the Blend CW the signal gets lost. I did some audio tracing with the Blend full CW and I see a hot signal off pin 1 of the RC4558 op-amp. Tracing it down the circuit, I get a full hot signal at C6 (33n). However, at both R16 and R17 the signal stops. I checked all resistors and capacitors and all appears to be fine...but that doesn't mean it is all fine. So...what's next for debugging.

 

[Bio77]

First try moving the bias trim and see if you can get signal at pin 3 of the BBD. If not, you seem to be on the right track of solving this. You should be getting signal all the way from C6 to pin 3 of the BBD. Maybe C6 is bad. Are you getting signal at both leads of C6?

 

[jeffwhitfield]

Holy shit! FINALLY got it working! What a ride! And you'll never believe what it turned out to be.

The fact that I was getting a clear signal off both sides of C6 but nothing after that made me wonder. So, I did a continuity test on the leads for all those points. C6 was getting clear continuity to pin 1 of IC2 and R34. I got clear continuity between R16 and R17...but nothing between R17 and C6. Reflowing the solder point did nothing.

This led me to believe that the PCB traces between these points was either defective or potentially damaged (not likely though). Looking at the PCB, it's clear there's a lead going from C6 to R17. So, on a whim, I added a jumper to these leads and, BAM, everything started working!

Big thanks to @Bio77 for leading down the right path and to @fig for providing me replacement chips for the ones that were potentially counterfeit.

Now that I got it working, I'm gonna do the natural thing any good pedal builder would do in this case: I'm gonna build 10 more just like it!

 

[fig]

Hey glad it's working AND your chips are good, so bonus points! Glad to almost help

 

[jeffwhitfield]

Hey glad it's working AND your chips are good, so bonus points! Glad to almost help

Dude, you didn't almost help...you helped. I'm still not convinced those 3102's I got would have worked. Damn well could have been a completely different chip remarked as a 3102...possibly a cheaper one that works similar but not exactly like a 3102 (thus the reason why some people don't have any problems with them).

 

[Bio77]

Congrats! Good catch, I wouldn't have guessed a bad trace.

 

[andare]

I'm a sucker for a happy ending. This pedal is on my list too.

 

[kryptofunk]

Sure thing. This is a great circuit. The TL022 is supposed to fluctuate, thats your LFO, so, good news there.

Below are my voltages. Looks like you have a problem with your Op-Amp. Looking at the schematic the Vref on yours is set to max. First, measure the voltage on pin 5 of you op amp, adjust the trim pot until you get ~4-6V see if you get sound through. If yes, you can adjust the bias from there to minimize distortion in the delay path. If you still don't get sound, you can try swapping out your op-amp, any dual op-amp will work here TL072, 4558, OPA2134,NE5532, ect. If that doesn't help you have to start looking at components connected to the op-amp. You can re-flow or change out ones where the audio dies.

I'm really hoping the bias point is the problem. This circuit is based on the Boss CE-2, which has a weird design in that the bias trim sets both the bias of the BBD and virtual ground. I'm getting to the end of my knowledge here, but it makes sense that the op-amp wouldn't function without a voltage differential. Good luck.

Yours	Mine
IC1 (TL022)
Pin 1: 0.65 - 7.86	Varies
Pin 2: 4.08 - 4.47	4.3
Pin 3: 1.46 - 6.59	Varies
Pin 4: 0	0
Pin 5: 4.05 - 4.45	4.3
Pin 6: 4.57	4.3
Pin 7: 1 - 6	Varies
Pin 8: 8.52	8.5
IC2 (RC4558)
Pin 1: 8.73	6
Pin 2: 8.69	6
Pin 3: 8.74	6
Pin 4: 0	0
Pin 5: 8.75	6
Pin 6: 8.74	6
Pin 7: 8.70	6
Pin 8: 8.75	8.8
IC3 (V3207D)
Pin 1: 0	0
Pin 2: 3.89	4
Pin 3: 7.77	5.2
Pin 4: 7.95	8
Pin 5: 8.53	8.6
Pin 6: 3.79	3.9
Pin 7: 7.90	6.5
Pin 8: 7.89	6.5
IC4 (V3102D)
Pin 1: 8.53	8.6
Pin 2: 3.79	3.9
Pin 3: 0	0
Pin 4: 3.89	4
Pin 5: 0.80	0.8
Pin 6: 7.54	7.6
Pin 7: 2.64	2.9
Pin 8: 7.95	8

@Bio77 @jeffwhitfield
I was in the process of completing my Caesar build and this was incredibly helpful! The pedal had no sound when the blend was maxed CW. In my case it WAS the bias. I adjusted it from 4.3V to 6V and the pedal came to life. A heartfelt thank you for all the details supplied in this post!

 

[thehiggsboson1]

wow, thanks so much for sharing this project. I'm about to start one.

 

[SeaThruLens]

This thread continues to be extremely helpful. Looks like I fell victim to the eBay BBD combo. Just ordered new ones from a reputable online retailer. Everything seems to work, left led seems to follow the rate pot and the two red pulse when in sine mode but the voltages are way off on IC 3/4 and when the blend knob is full CCW I get a clean signal and lose it as I dial it CW.