[Solved] Cepheid Chorus - only the LEDs work

Z

zipfool

Member
This seems like it's going to be tricky to figure out. Bypass works fine, but the 2nd LED stays lit even in bypass. When I engage the switch, both LEDs light up, but I'm not hearing any of the Chorus effect no matter how much I crank the knobs/pots.

I opted to go with the MN3207 / MN3102 IC pairing, but I suspect that my source for these was bogus (seller on EBay). I've tried 4 out of 10 of the pairs they sold me, and I'm getting the same [lack of] effect with all 4 pairs. I could keep trying out the other pairs, but I'm suspicious that all of them will be the same.

Take a look at these photos and LMK if you see anything else suspicious.

There's a few things to this build that are new to me: not sure what the trim pot is supposed to control, and I've never had to solder an onboard jumper to signify which chip combo I bought. Did I do the right thing by putting a bead of solder to connect each of the 2 sets of pads?

If I dial the trim pot up too high, the sound cuts out. Same if dial it too low. Right in the middle and I get clear sound/tone. What does this trim pot do?
 

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Lol. Thanks for being patient with me. Yes, I had it backwards!

Now I'm getting a blinking LED on one side and the other LED operates as it should when engaged. The blinking LED changes speed when I'm turning the rate pot.

D100 is now getting 9.09v on one end and 8.85v on the other.

The sound is still not being affected though. I tried dialing the trim pot in small increments and nothing seems to affect the sound.
 
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TL022CP:
3-5v fluctuating
4.3v
3-4.8v fluctuating
0v
.004v
4.38v
3.8-4.8v fluctuating
8.67v

MN3207:
0v
8.69v
3.58v
8.1v
8.73v
0v
.06v
.02v

MN3102:
8.73v
0v
0v
8.74v
-0.6v
0v
8.6v
8.15v

TL072CP:
4.34v
4.34v
4.34v
0v
4.34v
4.35v
4.35v
8.84v

See anything noteworthy?
 
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Definitely Pin 5 on the TL022. That one's supposed to be directly tied to VREF_B, roughly 1/2 of input voltage.

That should be equal to pin 2, which you have at 4.3V. Maybe a bad solder joint...it's worth poking around in that area.
 
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Now I'm getting some different values:

TL023:
2.7-5.9 fluctuating
4.35
3.5-5.0 fluctuating
0
4.35
4.38
3.9-4.7 fluctuating
8.75

MN3207:
0
8.75
6.03
8.18
8.75
0
.002
.006

MN3102:
8.75
8.62
.002
8.75
-.835
0
8.62
8.18

TL072:
0
6.89
6.89
0
6.89
6.89
6.89
8.85

What does it all mean!?
 
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zipfool said:
What does it all mean!?
Click to expand...

Basically:

Normal CE-2 LFO (TL022)
1)variable
2)Around 1/2 input
3)variable
4)0
5)Around 1/2 input
6)Around 1/2 input
7)variable
8)input voltage


Normal CE-2 pre/de emphasis filter (TL072)
1)similar to 2 & 3, around 1/2ish V
2)--
3)--
4)0
5)similar to 6 and 7, around 1/2ish V
6)--
7)--
8)Input voltage

But...I'll have to get back to you on the BBD and clock. I'm more familiar with the MN3007 in this circuit than the MN3207.

music6000 makes a good point, it *could* be related to a counterfeit Bbd/clock. I'm usually one to recommend a parts change as a last resort unless it's immediately obvious...you end up learning more in the process of building by doing a proper debug, and end up spending a lot less money too.

Your IC1 PIN 1 is off now. Take a look at what's directly connected to that pin on the schematic...that value needs to be similar to pins 2 and 3.
 
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Stickman393 said:
Basically:

Normal CE-2 LFO (TL022)
1)variable
2)Around 1/2 input
3)variable
4)0
5)Around 1/2 input
6)Around 1/2 input
7)variable
8)input voltage


Normal CE-2 pre/de emphasis filter (TL072)
1)similar to 2 & 3, around 1/2ish V
2)--
3)--
4)0
5)similar to 6 and 7, around 1/2ish V
6)--
7)--
8)Input voltage

But...I'll have to get back to you on the BBD and clock. I'm more familiar with the MN3007 in this circuit than the MN3207.

music6000 makes a good point, it *could* be related to a counterfeit Bbd/clock. I'm usually one to recommend a parts change as a last resort unless it's immediately obvious...you end up learning more in the process of building by doing a proper debug, and end up spending a lot less money too.

Your IC1 PIN 1 is off now. Take a look at what's directly connected to that pin on the schematic...that value needs to be similar to pins 2 and 3.
Click to expand...
Thanks! Are you referring to pin 1 on the TL072 when you say pin 1 is off now? Based on the list you gave, that's what it looks like...
 
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I reflowed the joints for R13, R10, R9, C2, and C4. I also checked the circuit with the other MN32017/3102 pairs that I bought.

I found that only 2 pairs out of 8 pairs of MN ICs had any effect on the sound. When I have either of these 2 pair in the circuit, I get a very subtle modulation sound. It sounds more like a vibrato set to very low depth. That's the most I get even though the depth is turned all the way up and rate is set to 50%.

I did notice a very subtle difference when dialing the trim pot. But even at a "sweet spot" it's still an almost imperceptible effect.

Any thoughts on what to try next?
 
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Oh yeah, here's voltage check on ICs:

IC1 - TL022:
2.7-5.9 fluctuating
4.35
3.5-5.0 fluctuating
4.35
4.38
3.9-4.7 fluctuating
8.75

IC2 - TL072
5.65
5.65
5.65
0
5.65
5.65
5.65
8.85

IC3 - MN3207
0
4.26
4.87
8.18
8.75
4.27
5.28
5.28

IC4 - MN3102
8.75
4.27
0
4.26
.28
8.36
2.72
8.18
 
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music6000 said:
Have you cleaned the back side of the PCB board, there's a lot of flux residue & stray bits of Solder.
If not, you need to clean the PCB!!!
Click to expand...
Scrubbed the PCB nice and clean. It still doesn't work.

I re flowed all the joints, tried a few more/other ICs and still nothing. I get a decent tone, sounds almost exactly like how it does in bypass. Just a very subtle modulation/vibe.

I'm coming to the conclusion that all of these MN chips that I bought are bogus. I bought them from a seller on eBay, so I guess that's what happens 🤷‍♂️.

Anyone know a good source for MN3207/3201 pairs? I guess V3207 or BL3207 would work as well. Let me know where you all are getting the hard to find ICs.
 
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zipfool said:
Scrubbed the PCB nice and clean. It still doesn't work.

I re flowed all the joints, tried a few more/other ICs and still nothing. I get a decent tone, sounds almost exactly like how it does in bypass. Just a very subtle modulation/vibe.

I'm coming to the conclusion that all of these MN chips that I bought are bogus. I bought them from a seller on eBay, so I guess that's what happens 🤷‍♂️.

Anyone know a good source for MN3207/3201 pairs? I guess V3207 or BL3207 would work as well. Let me know where you all are getting the hard to find ICs.
Click to expand...
Where are you on planet Earth?
 
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zipfool said:
Scrubbed the PCB nice and clean. It still doesn't work.

I re flowed all the joints, tried a few more/other ICs and still nothing. I get a decent tone, sounds almost exactly like how it does in bypass. Just a very subtle modulation/vibe.

I'm coming to the conclusion that all of these MN chips that I bought are bogus. I bought them from a seller on eBay, so I guess that's what happens 🤷‍♂️.

Anyone know a good source for MN3207/3201 pairs? I guess V3207 or BL3207 would work as well. Let me know where you all are getting the hard to find ICs.
Click to expand...
MN3207 & MN3102 not MN3201!
 
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