SOLVED Cepheid Chorus, sound but no chorus effect, any ideas?

[dayday]

Hi everyone,

Just built a Cepheid Chorus.

When I first tested it, I got a whooshing staticky sound. Bypass works. The led light comes on. The LFO light remains on, doesn't flash. I used a probe and got clean sound up to R24, right before the signal goes into the 3007. After the 3007 it was static.

The next morning, magically, the whooshing static was gone, but there was no chorus effect. I could hear it trying but really nuttin'. The LFO remains steady on.

The non-chorus sound happens boxed or un-boxed. So I don't think it's a something's touching the inside of the box and grounding something out situation.

I adjusted the trimmer pot and still nuttin'.

For voltage measurements see pic. I don't know if these are correct or not but power is getting through.

Please take a look and let me know what you think.

Thanks!

Dave

***********************************

Days later...
The "Fix". For those keeping score at home, the chorus now works but I didn't actually fix it. It sort of fixed itself... very unsatisfying. Maybe an IC was loose and as I was going through checking everything I accidentally fixed it. in the future I may reflow some suspect solder joints. but am going to box this dog up for now and use it. thanks for everyone's excellent comments and help.

 

[Locrian99]

Have you turned the trimmer very slowly

It’s a small window that it does the effect. If you have a person or looper to hit a string it’s easier. If you’ve confidently done that. We can look at other things.

Shoot sorry I should read. So the lfo led remains steady on you say.

 

[Locrian99]

You’re 4558 voltages look whack.

 

[chris.knudson]

Do you have another MN3007 that you can try? If your audio probe worked to that point and then went sideways, I'd start there.

 

[chris.knudson]

You’re 4558 voltages look whack.

I agree that the 4558 voltages look odd.

 

[dayday]

Hi Locrian,

thanks for the reply. I swapped out the 4558 and got the same voltages.

what should the voltages be? and how should I attempt to troubleshoot? thanks

 

[dayday]

Do you have another MN3007 that you can try? If your audio probe worked to that point and then went sideways, I'd start there.

Hi Chris,

thanks for the reply. I pulled the MN3007 out of a working chorus pedal (that had other issues) so I think it's good.

 

[Bio77]

You’re 4558 voltages look whack.

Those might look better if the bias is set more towards the center. The BBD Bias sets the reference voltage for the 4558 too.

Does your DMM have a frequency function? If so, check pins 2 and 6 of the BBD to check if your clock is working. You should measure ~35kHz.

 

[chris.knudson]

what should the voltages be? and how should I attempt to troubleshoot?

I don't know what the voltages should be, but it seems odd that you are getting 7.4V on pins 5, 6, & 7. Pin 5 is the (-) in, Pin 6 is the (+) in and Pin 7 is the output of the 2nd op amp. I haven't had a chance to look at the schematic to see what is actually going on in that part of the circuit, but I would have expected those voltages to be different.

 

[dayday]

Those might look better if the bias is set more towards the center. The BBD Bias sets the reference voltage for the 4558 too.

Does your DMM have a frequency function? If so, check pins 2 and 6 of the BBD to check if your clock is working. You should measure ~35kHz.

THanks for the reply, Bio77. Unfortunately, no freq on the multimeter.

 

[Bio77]

THanks for the reply, Bio77. Unfortunately, no freq on the multimeter.

That's a bummer. Don't suppose you have another 3101 laying around? You could also change the jumpers and try a 3201/3207 set if you have that?

 

[dayday]

That's a bummer. Don't suppose you have another 3101 laying around? You could also change the jumpers and try a 3201/3207 set if you have that?

that's the only 3101 I have right now.

 

[Bio77]

Did you measure the voltages on the pins or on the socket? Wondering if the clock might not be making contact with the socket.

 

[Bio77]

If you wanted to see the voltages, check out the build doc for the MadBean Porkbarrel, but remember the bias pin effects everything tied to VB, so they might look different depending on where your trim is set.

 

[dayday]

Did you measure the voltages on the pins or on the socket? Wondering if the clock might not be making contact with the socket.

on the pins. easier to get to.

 

[dayday]

should I reflow the IC jumper pads? or since I have power that is not an issue?

 

[dayday]

the staticy sound is back. the LFO led remains steady on. I reflowed the jumpers to no avail.

 

[Locrian99]

sounds like bio might be the guy to be the most help with this.

I will say this typically when your voltages are incorrect on an ic or transistor m. It’s not that the chip is bad it’s the circuit surrounding the chip. It’s usually helps to narrow down where the problem lies. More so than say your 4558 is bad. Typically an (this doesn’t sound like a typically situation) your op amp pins all are about 1/2 thr supply voltage. 1-3,5-7 will be around 4.5v while 4 is 0v and 8 is 9v

 

[dayday]

Update. I checked every resistor on the board and they were all good.

then I plugged in the power and suddenly the LFO led started flashing. At that point I could dial in chorus sounds via the trimmer pot. yay!

So it works now, but I didn't do anything to fix it. Which means there is a suspect solder joint somewhere... correct? being the anal retentive guy that I am I don't know if I'll be able to sleep not knowing the root cause.

if a bad solder joint is the case... i've got some non-shiny, suspect joints near the top of the board. what is proper technique to "reflow" a joint? just clean it, then flux it, then heat it up and add some solder?

thanks for your help

 

[chris.knudson]

Update. I checked every resistor on the board and they were all good.

then I plugged in the power and suddenly the LFO led started flashing. At that point I could dial in chorus sounds via the trimmer pot. yay!

So it works now, but I didn't do anything to fix it. Which means there is a suspect solder joint somewhere... correct? being the anal retentive guy that I am I don't know if I'll be able to sleep not knowing the root cause.

if a bad solder joint is the case... i've got some non-shiny, suspect joints near the top of the board. what is proper technique to "reflow" a joint? just clean it, then flux it, then heat it up and add some solder?

thanks for your help

Pretty much yes. I usually clean the board with some isopropyl alcohol and an old toothbrush. It usually takes a few baths. I usually brush on the alcohol, then I clean it off the toothbrush with a paper towel, and I keep repeating the process until the board doesn't feel sticky anymore. After the board is dry an clean. just touch the joints with the iron to melt them and get them to "re-flow." If you need to add soldier you can, but it isn't always necessary, and it isn't usually necessary to add flux unless you have a particularly troublesome joint -- in which case, I usually hit it again with some rosin core solder which has flux in the core.