Cepheid sounds great but has 10db drop

datmuffin

New member
I’m a little baffled about the significant volume drop of roughly 10db when my Cepheid build is activated. Everything seems to be set up correctly and the pedal does sound really good, it just clearly cuts the output volume.

I’ve been over the cap and resistor values, I’ve swapped all three ICs, heck, I even tried two different power sources, a 9V 200ma and a 400ma just to see if that was a factor.

Anyway, hoping for some helpful tips as I’ve just about run out of ideas. Thanks!

(Also I know some of my resistors look funny- R28, R34, R35, but they all test out correctly!)
 

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blackhatboojum

Well-known member
It looks like you’re missing r35. Do you have it on top of r33 or something? The picture gets blurry when I try to zoom in on it.
 

blackhatboojum

Well-known member
Also, your foot switch doesn’t look like it’s wired properly. Do you have the pcb sw wire soldered to the out lug on the foot switch? Again, hard for me to tell though.
 

datmuffin

New member
It looks like you’re missing r35. Do you have it on top of r33 or something? The picture gets blurry when I try to zoom in on it.
It looks like you’re missing r35. Do you have it on top of r33 or something? The picture gets blurry when I try to zoom in on it.
The leads are long on it, but it’s clear of other components. When I was first troubleshooting I was looking at the image on the product page, which coincidentally is wrong on the r34 and r35. I proceeded to clip them out, which was when I realized the value printed on the PCB was different. Then I looked at the parts list to confirm the correct values. In my haste and frustration I just soldered the leads to the old leads. The pedal sounded the same after and I went back to double checking values, aware of the fact that I couldn’t trust the product page PCB image.
 

datmuffin

New member
Also, your foot switch doesn’t look like it’s wired properly. Do you have the pcb sw wire soldered to the out lug on the foot switch? Again, hard for me to tell though.
I don’t think there is any problem with the switch. It powers on as expected and the LEDS both light up. Again, it does sound really good, which is the frustrating part because the volume drop really limits it’s use.
 

PJS

Active member
I haven't compared to the Boss schematic at all, but in general with pedalpcb boards, the parts values on the board silkscreen are the correct values where they differ from the documentation values. It usually means the board has been updated and the documentation hasn't yet been updated to match.
 

bowanderror

Active member
Here are some things to check:
  1. R2/47k - Hard to tell in the pic, but looks like it might be 4.7k (Yellow-Violet-Black-Brown-Brown) - Use the resistor color code rather than testing with a DMM as you can't check resistance once they're soldered
  2. R32/10k - Also hard to tell, but looks like it might be 1k (Brown-Black-Black-Brown-Brown) - Use the color bands again
  3. Have you adjusted the trimpot? - It sets the Vref bias voltage, and can have a VERY narrow working range. Start at halfway and just barely move it one direction then the other to see where the chorus signal has the least amount of distortion.
  4. What kind of zener diode did you use? - With the MN3207 it should be a 9.1V zener
  5. Where did you get your BBD & clock ICs? - I know they're super hard to find, but many people have had problems with eBay vendors in this build.
  6. Is IC1/TL022 seated correctly in the socket? - Try removing it and aligning it with the socket before pressing down completely
Beyond that, I would take voltage readings at all of the IC & transistor pins and post them here. That should let us know if everything is getting powered & biased correctly.
 

datmuffin

New member
Thanks for the advice bowsanderror and blackhatboojum.

1. I double checked and R2 is at the correct value
2. same for R32
3. Yeah, adjusted the trim pot. Fascinating how narrow the window is on this build!
4. It is a 9.1v zener diode
5. I bought them on ebay. I bought two of each, and same results when I swapped them out.
6. It does appear to be seated correctly

I did measure voltage on the ICs and the transistors. Maybe you'll spot something amiss?

IC1
1. 4 - 4.7 (this one kept moving between these values)
2. 4.4
3. 4.4
4. 0
5. 4.4
6. 4.7
7. 4.3
8. 8.59

IC2
1. 5.8
2. 5.8
3. 5.8
4. 0
5. 5.8
6. 5.8
7. 5.8
8. 9.1

IC3
1. 0
2. 4.3
3. 5
4. 8.3
5. 8.9
6. 4.3
7. 6.3
8. 6.3

IC4
1. 8.93
2. 4.3
3. 0
4. 4.3
5. 0.28
6. 8.6
7. 2.7
8. 8.3

Q1
E. 0.39
B. 9.11
C. 9.11

Q2
E. 0
B. 0.37
C. 7.6

Q3
E. 4.5
B. 5.0
C. 9.11

Q4
E. 3.5
B. 4.0
C. 8.93
 

blackhatboojum

Well-known member
Can you post a pic of the underside of your pcb? Looks like you might have a short or something around q1. You should not have 9v at the base. You should have around 3 volts there and at the emitter.
 

blackhatboojum

Well-known member
Are you getting static voltage readings on IC1 pins 2,3,5, and 7 or are they fluctuating? They should be changing as the LFO sweeps. Same way with pins 2,4,5,6,7 on your 3102 and pins 2,6,7,8 on your 3207. I’m pretty sure there should be voltage fluctuations on your transistors too. I’ll have to poke around my chorus to verify.
 
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datmuffin

New member
Are you getting static voltage readings on IC1 pins 2,3,5, and 7 or are they fluctuating? They should be changing as the LFO sweeps. Same way with pins 2,4,5,6,7 on your 3102 and pins 2,6,7,8 on your 3207. I’m pretty sure there should be voltage fluctuations on your transistors too. I’ll have to poke around my chorus to verify.
The values are fluctuating in IC1. The 3102 and 3207 are barely registering any fluctuations, though. There is just a little movement at a thousandth of a volt.
 

music6000

Well-known member
The values are fluctuating in IC1. The 3102 and 3207 are barely registering any fluctuations, though. There is just a little movement at a thousandth of a volt.
R34 & R35 - 47K effect Volume. This is a common issue depending on what you perceive as a drop in Volume.
You could try stacking 47K on both R34 & R35 resistors temporary & that will give you 23.5K & see if you like that.
You can bring them down to 10K if you want to try that.
 

datmuffin

New member
That did it! I almost changed those two resistors anyway because on the product page for the PCB it has both R34 and R35 as 12k as opposed to the 47k printed on the actual PCB and the parts list in the pdf. But that seems to have done the trick. I measure a slight 1-2Db decrease now, which is close enough for me. Thanks so much for the tip! And thanks everyone who has helped in this forum. I’m very grateful for how many of you provided quality feedback and suggestions.
 

music6000

Well-known member
Oh, I forgot to say I used 12k resistors. Almost wonder if 10k on each would be enough to get unity. Might try later, just stoked to have this working now. Such a great sounding circuit.
That's correct, It had been reported that it had too much volume with 12K so the value was changed by request!
Tremolo is another circuit that suffers this dilemma so it's always good to see a Volume control or Trimmer on the PCB!!!!
 
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