SOLVED Caesar Chorus: No signal or effect

jeffwhitfield

jeffwhitfield

Well-known member
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?

IMG_0640.jpeg
IMG_0641.jpeg
IMG_0642.jpeg
 
Sort by date Sort by votes
Are you saying that the pedal worked fine with the TL072 (minus the distortion)? The distortion could be from improper biasing of the BBD.
 
Upvote 0 Downvote
Nope. Even with the other op-amp, no effect.

I did a signal trace and, at the MN3207, leg 3 is the only one getting sound. So I’m thinking that maybe the chips don’t work well in this circuit. Might be something different with the others. Could be that they‘re compatible…but not backwards compatble. 🤪
 
Upvote 0 Downvote
The chips you have are fine for this circuit, if they are working. Since you have an audio probe, put the blend knob full CCW, take a look at the schematic and trace the audio signal through the clean part and see where it dies. Since you have signal at pin 3 (input) of the BBD start with C5 and go through to the output. As long as the op-amp is working, you should get clean signal.
 
Upvote 0 Downvote
Ok, so R9 and R10 both have strong signals going through them with the tracer. There’s a significant drop right after C5 and R14. I’m getting a stron signal off leg 2 of the RC4558 but the same volume drop off leg 1. R34 gives an additional drop and so on. Very faint signal off leg 6 of the op-amp and virtually nothing off leg 7.
 
Upvote 0 Downvote
Are R34-37 12k? If you turn the blend knob full CW do you get signal at pin 6 of the op-amp? Also, can you take the voltages of your ICs? I have a working unit and can give you a set to compare to later.
 
Upvote 0 Downvote
Again, thanks so much for your help. Definitely lost with this one. :P

Alright, here's what I got...

With Blend at full CW, I get no signal. Full CCW, I get a faint signal...much lower than what's going into the op-amp for sure.

I checked the value of R34. It's 12K for sure. R34-37 all check out as 12K.

For the IC voltages, IC1 (TL022) tends to fluctuate on many of the pins so I just did a ballpark on them. I tested with everything full CCW.

IC1 (TL022)

Fluctuates

Pin 1: 0.65 - 7.86
Pin 2: 4.08 - 4.47
Pin 3: 1.46 - 6.59
Pin 4: 0
Pin 5: 4.05 - 4.45
Pin 6: 4.57
Pin 7: 1 - 6
Pin 8: 8.52

IC2 (RC4558)

Pin 1: 8.73
Pin 2: 8.69
Pin 3: 8.74
Pin 4: 0
Pin 5: 8.75
Pin 6: 8.74
Pin 7: 8.70
Pin 8: 8.75

IC3 (V3207D)

Pin 1: 0
Pin 2: 3.89
Pin 3: 7.77
Pin 4: 7.95
Pin 5: 8.53
Pin 6: 3.79
Pin 7: 7.90
Pin 8: 7.89

IC4 (V3102D)

Pin 1: 8.53
Pin 2: 3.79
Pin 3: 0
Pin 4: 3.89
Pin 5: 0.80
Pin 6: 7.54
Pin 7: 2.64
Pin 8: 7.95
 
Upvote 0 Downvote
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.

YoursMine
IC1 (TL022)
Pin 1: 0.65 - 7.86Varies
Pin 2: 4.08 - 4.474.3
Pin 3: 1.46 - 6.59Varies
Pin 4: 00
Pin 5: 4.05 - 4.454.3
Pin 6: 4.574.3
Pin 7: 1 - 6Varies
Pin 8: 8.528.5
IC2 (RC4558)
Pin 1: 8.736
Pin 2: 8.696
Pin 3: 8.746
Pin 4: 00
Pin 5: 8.756
Pin 6: 8.746
Pin 7: 8.706
Pin 8: 8.758.8
IC3 (V3207D)
Pin 1: 00
Pin 2: 3.894
Pin 3: 7.775.2
Pin 4: 7.958
Pin 5: 8.538.6
Pin 6: 3.793.9
Pin 7: 7.906.5
Pin 8: 7.896.5
IC4 (V3102D)
Pin 1: 8.538.6
Pin 2: 3.793.9
Pin 3: 00
Pin 4: 3.894
Pin 5: 0.800.8
Pin 6: 7.547.6
Pin 7: 2.642.9
Pin 8: 7.958
 
Upvote 0 Downvote
I think your MN3102 might be a fake. Normal Matsushita/Panasonic BBD & clock ICs will have an inner raised frame that is a different texture to the outer edge. Your MN3207 looks fine, but the MN3102 has signs of being sanded & rebadged: it lacks the raised inner frame and the pin 1 marking dot looks shallower than the legit BBD:

OK vs Sanded BBD.jpeg
 
Upvote 0 Downvote
bowanderror said:
I think your MN3102 might be a fake. Normal Matsushita/Panasonic BBD & clock ICs will have an inner raised frame that is a different texture to the outer edge. Your MN3207 looks fine, but the MN3102 has signs of being sanded & rebadged: it lacks the raised inner frame and the pin 1 marking dot looks shallower than the legit BBD:

View attachment 17323
Click to expand...
Crap. That's what I was afraid of.

@Bio77, would the wrong chip in the MN3102 spot possibly cause op-amp issues? I would think it would since anything other than a MN3102 or similar chip might cause weird signal issues. :P
 
Upvote 0 Downvote
Bio77 said:
No, even if you remove both of those chips you should get clean signal through. Did you try moving the bias trim?
Click to expand...

Bias trim doesn't seem to do squat. Lemme try with a different op-amp. I did try a few different 4558's I had in my stock. No difference. Gonna try a completely different op-amp.
 
Upvote 0 Downvote
You must log in or register to reply here.
Back
Top