Bass Chorus build not working, need some help please.

[flon_klar]

This is my first post, so I hope I’m in the right place.

I picked up a schematic for a Bass Chorus at another website (BYOC).  I cannot get this thing to work right, and I could use some troubleshooting help.

I’ve built it 3 times, and every time, I have the same problem:  I get a bass signal through the circuit from input to output, but there is no effect.  It uses a 3102/3207 pairing, but I am getting absolutely no oscillation or LFO.  The Blend pot functions; it goes from full clean bass sound to a big hole where the chorus should be.  If I probe the 2 ICs while operating the Depth and Rate pots, I can see where the voltage values are changing. 

On this last attempt, the LED did not light up when I fired up the circuit.  While probing the ICs,  the LED suddenly lit.  I’m sure I momentarily shorted something and sent power to the LED, but I thought that seemed a little odd.

I have found a few issues, like a bad jumper wire, e.g., but repairing these small issues did not change the status of the effect.  As far as I can tell, I have power where I need it, and nothing seems to be out of place or connected incorrectly.

I’m at a loss as to how to proceed; I just need some tips on finding that missing effect.  If anyone here has experience with this particular unit, I’d appreciate the assistance.  Thanks.

In response to ElectricDruid, here is more info:

BYOC Bass Chorus, https://buildyourownclone.com/collections/bassfx/products/bass-chorus-kit

This is all assembled on a breadboard, so it's not the easiest arrangement to see clearly (images here: https://imgur.com/a/dFnDWEi). 

Modifications/substitutions/issues:  I pretty much followed the schematic exactly.  >The parts list wasn't too specific about transistors, but the drawing shows 3904s; the first time I (unsuccessfully) built it, I used the  3904s.  The second time, I changed over to a different breadboard, as I was having some issues with the original board, and on the third try I changed from 3904s to 2222s.  >I started out with a 20k trimpot (because I didn't have a 25k), but later changed it to a B25K, as it was easier to work with.  >The LED lit for a while today, but was off the next time I put power to the circuit.  >At the breadboard stage,  I typically don't include the bypass or jack switches in the layout.  >Power comes from a bench power supply, so no batteries to worry about; regular old negative ground. 

Voltage list:

Power supply displaying 9.24V, 22 mA.

Power section outputs:  9V= 9.24
                                   V+= 8.55
                                   1/2V= 4.56
                                   Vb= 8.44

IC1 (RC4558):  1= 4.63
                       2= 4.56
                       3= 4.56
                       4= 0.00
                       5= 4.56
                       6= 4.56
                       7= 4.49
                       8= 8.56

IC2 (MN3207):  1= 0.00
                        2= 4.20
                        3= 3.90
                        4= 7.89
                        5= 8.46
                        6= 4.21
                        7= 5.25
                        8= 5.32

IC3 (TL022):  1= 7.82
                     2= 4.38
                     3= 7.73
                     4= 0.00
                     5= 4.38
                     6= 4.38
                     7= 4.54
                     8= 8.55

IC4 (MN3102):  1= 8.45
                        2= 4.20
                        3= 0.00
                        4= 4.20
                        5= 0.07
                        6= 0.34
                        7= 2.27
                        8= 7.88

D1 (Blue LED):  A= 0.00
                        K= 0.00
D2 (1N4148):  A= 2.27
                      K= 8.24
D3 (1N4148):  A= .84
                      K= .227
D4 (1N4001):  A= 9.24
                      K= 8.56

Q1 (all 2N2222):  E= 2.88
                          B= 3.18
                          C= 8.54

Q2:  E= 3.90
       B= 4.48
       C= 8.54

Q3:  E= 3.20
       B= 3.70
       C= 8.55

Q4:  E= 0.39
       B= 0.74
       C= 8.53

Q5:  E= 2.06
       B= 2.58
       C= 8.44

Q6:  E= 0.00
       B= 4.56
       C= 8.44

[ElectricDruid]

Welcome Flon!

Sure, we can help. Can you have a look at this page below and give us some more to work with, please?

https://www.diystompboxes.com/smfforum/index.php?topic=29816.0

You've covered some of it already in your post, but a link to the schematic and some voltages around the chips would help a lot. Pictures of the circuit is often useful too, so we can check your resistor values and so on.

Thanks!

[flon_klar]

Thanks Druid; see my edit above.

[ElectricDruid]

Thanks, that's excellent - nice and clear what's going on.

Ok, the voltages look ok to me. Some of the transistors are a little bit off, but that's probably meter loading rather than anything serious. Certainly nothing jumps out as massively wrong. Maybe someone else can confirm?

The transistor choice isn't going to be crucial in a circuit like this. They're all basic "jellybean" parts, so anything decent should do the job. The only question would be if you swapped something with a CBE pinout for something with a EBC pinout (a classic error!).

My next move would be to listen to various places in the circuit with an audio probe (http://diy-fever.com/misc/audio-probe/)

1) IC1 (4558), Pin 1: the output from the input buffer (with pre-emphasis!). You should have clean signal here.
2) IC1 (4558), pin 7: the output from the final mixer. You should have clean and delayed signal here (e.g. chorus!) but you've probably only got clean signal
3) Q2 Emitter: pre-delay filter output. You should have filtered clean signal here too
4) IC2 (MN3207), pin 7 or 8: Delay line output. You *should* have delayed signal here, but we suspect you've got nothing, but if you do then test..
5) Q3 Emitter: post-delay filter output. You should have delayed signal here, if the BBD is giving any output.

Good luck!

[flon_klar]

Druid-

I love the audio probe.  That could have helped me with SO many issues in the past!

Everything checked out as predicted.  Clean signal at IC1/1, IC1/7, and Q2/E.  I suspect this trail is going to end at the BBD.  Is there a way to test it, besides the obvious "It's not working?"  I have another one I can swap, but it's from the same ebay vendor, so if the first one is bad....

However, there is one other thing that's bugging me about this- the LED.  I assume this is merely to signal that the effect is indeed on?  As I mentioned previously, the LED lit up when I powered up the effect circuit after the first 2 builds.  It did not light on the third build- until I shorted pins 3 & 4 on the TL022, at which point it latches and stays on until power is removed.  (I also discovered that shorting pins 1 & 2 also turns on the LED, but shorting them a second time turns it off.  Fun with op amps!)  I don't know if this issue with the LED is a symptom of the bigger problem, or if it's something else.  I tried changing out both IC3 and Q4, but the LED issue persists. 

Anyway, I appreciate your time and knowledge, and if you have any other info that could help resolve this, I'd like to hear it!  Thanks.

John   

[ElectricDruid]

The LED thing is interesting, I agree. The reason it's interesting is because the LED is driven from the LFO, so it should be flashing if the effect is on and the LFO is working. So the fact that it's not suggests the LFO isn't happy. Since the LFO output goes to the clock, it's possible that it's locked the clock up and with no clock, we'd get nothing from the BBD.

I suggest trying the following:

Separate D3/1N4148 and R41/33K from the clock, so that there is only the C18/47p between pins 5 and 7. Then connect a resistor from pins 6 to 7. This sets up the clock as an unmodulated clock, and if the clock chip and BBD are good, it should get you some sound coming through the BBD.

As far as "what resistor?", you could try 100K. That'll give you a medium clock frequency of roughly 50KHz. If you've got a variable resistor on wires you can plug in, that'd be fun - it'll be a "delay time" control. Anything between 5K and 1M should do *something*.
By the time the resistor is as large as 1M, the clock frequency will come down into the audio range (and below the frequency of the filters that are supposed to remove it) so you'll hear it wailing on the audio probe.

[duck_arse]

first post? or third? welcome, or welcome back? either way.

if your
Q6:  E= 0.00
       B= 4.56
       C= 8.44
base is 4V5, your emitter can't be 0V unless it is shorted hard to ground. I think it should read about 4V. it is possible, no, that you have miswired your bypass switch, yes? note, it is drawn wrong on the schematic - it should show both/all poles in the same sense, either both up or both down, but one up and one down is mechanically impossible, and so, wrong. and therefore, bad. tsk tsk.

we were having discussions on the operation of this type oscillator elsewhere recently, and the use of a bypass cap on the osc Vref came into question. in this case, the 10uF at C15. as you are at the breader, you could try pulling that cap out and seeing what results.

[ElectricDruid]

if your
Q6:  E= 0.00
       B= 4.56
       C= 8.44
base is 4V5, your emitter can't be 0V unless it is shorted hard to ground.

It *is* shorted hard to ground. It caught me out at first look too. Check the schem:

we were having discussions on the operation of this type oscillator elsewhere recently, and the use of a bypass cap on the osc Vref came into question. in this case, the 10uF at C15. as you are at the breader, you could try pulling that cap out and seeing what results.

+1 agree. The LFO is definitely a question mark at this stage. That's why I suggested separating the clock from it and making sure that we'd got clock+BBD working ok. If that's the case, the only remaining place the problem can be is in that LFO circuit somewhere.

[antonis]

if your
Q6:  E= 0.00
       B= 4.56
       C= 8.44
base is 4V5, your emitter can't be 0V unless it is shorted hard to ground.

It IS shorted to GND, Stephen..



edit: Tom is by far faster...

[anotherjim]

The 4.5(ish) voltages on clock pins of the 3102 suggest it is clocking, not definitely but probably is. The DMM gives the average of the fast square waves switching from 9v to 0v. If it was not clocking, one clock pin will be 9v and the other 0v.

If you pull R7 out, it should kill the clean sound so you can only hear what comes out of the delay line. Work towards getting audio there. If it's on a breadboard, may as well pull the Blend pot too so you only get audio via R23.

[duck_arse]

when I confuse myself, I really go to town. probably not enough fish.

when I say "if your Q6:  E= 0.00 .... ", I was meaning to say Q4, the one driving the led. which brings up the question, what was I thinking? how come Q4:  E= 0.39, yet there is no voltage showing on D1?
D1 (Blue LED):  A= 0.00
                        K= 0.00

to the actual Q6, and what I should have thought at the time - if the emitter is hard to ground, the base can't be at more than 0V7, can it? even if the IC is driving it merrily, via a base resistor, the base pin of the transistor can't be 4V5-ish. so, eventually. or am I still confused?

[ElectricDruid]

Duck's right that if there's 0.39V on Q4 emitter, then we should see that on at least some bit of the D1 LED!

Could you check that, please? There might be a bad connection between the transistor and the LED.

The Q6 base thing I can't work out. It's not a "4.5V DC" that it's reading there but rather "a high speed square wave I can't keep up with but which averages to summat like 4.5V". That's another reason to separate the LFO off for a minute and test the clock and BBD alone. Once that's working, we can stick this LFO back on and see what we get.

[duck_arse]

ahh, yes, the IC might be hammering away at the base resistor, and your meter might read average on the IC side of the resistor, but at the transistor base lead, it can't go above 0V6, because emitter.

[anotherjim]

I too agree the base should read a diode drop above 0v. There is a resistor out of the chip pin to limit the base pin current so the B-E junction doesn't fry.

[ElectricDruid]

This is all great, but irrelevant (sorry people!) until we get the clock and BBD confirmed as working.

I know the Q6 thing is weird and interesting, but can we come back to it after we're sure that the MN3102 and MN3207 are doing what they're supposed to? It seems to me we're getting ahead of ourselves because the next bit is more exciting

I want to see a solid clock and a signal out of the BBD before we start discussing what Q6 is doing, sorry.
I mean, you might have guessed right and Q6 is at fault, and that would be cool, but I'm trying to following a reliable debugging process here, and thus far, we only know that the audio gets as far as the BBD and stops. The next logical step from that is not Q6, even if it is weird. We need to work our way in.

Is that unfair? Am I out of order?

[flon_klar]

 (see post below)

[flon_klar]

OK guys, after reading all your comments, I've been screwing with this thing all day trying to get a handle on it.  I'm not doing too well in that respect.

I removed C15.

I removed D3 and R41 from the 3102 and installed a 1M pot between pins 6 and 7 and connected the audio probe.  There was no signal to the amp- until I turned the pot.  Adjusting the pot gave me a variable squeal over the range of the pot.  Once I stopped turning the pot, the squeal would die out after a couple seconds.  Not sure what SHOULD have happened, but I don't think it did.  After this, I replaced D3 and R41.

The LED now comes on when I power up the effect circuit- not when turning on my bench power supply, but when I clip the live lead onto the breadboard   If I quickly snap the alligator clip from the power supply 2 times as I connect it to the breadboard, the LED will switch off.  Then I can clip it on to the lead again, once, and the LED will light up.  Somewhere, somehow, I'm activating a switch (Q4?) by the rapid powering on/powering off of the circuit. 

Going through the circuit with the audio probe, I get a strong clean bass signal at IC1 1/7 and the Q2/E.  At IC2, the signal at 7/8 is very muffled and distorted, but I get a very good signal at pin 3.  The signal again is very weak and distorted at Q3/E.

Here is a new list of voltages, and compared to the original list; some are different than before, some not.


           ORIGINAL                                                          LATEST READINGS

Power supply displaying 9.24V, 22 mA.                     9.20, 40 mA  (this seems odd...)

Power section outputs:  9V= 9.24                            9.20
                                   V+= 8.55                            8.50
                                   1/2V= 4.56                         4.63
                                   Vb= 8.44                            8.37

IC1 (RC4558):  1= 4.63                                          4.61
                       2= 4.56                                          4.61
                       3= 4.56                                          4.61
                       4= 0.00                                          0.00
                       5= 4.56                                          4.60
                       6= 4.56                                          4.60
                       7= 4.49                                          4.60
                       8= 8.56                                          8.46

IC2 (MN3207):  1= 0.00                                          0.00
                        2= 4.20                                          4.16
                        3= 3.90                                          3.82
                        4= 7.89                                          7.80
                        5= 8.46                                          8.37
                        6= 4.21                                          4.16
                        7= 5.25                                          5.58
                        8= 5.32                                          5.58

IC3 (TL022):  1= 7.82                                              1.38
                     2= 4.38                                             4.24         
                     3= 7.73                                             1.48
                     4= 0.00                                             0.00
                     5= 4.38                                             4.25
                     6= 4.38                                             2.98
                     7= 4.54                                             7.74
                     8= 8.55                                             8.46

IC4 (MN3102):  1= 8.45                                           8.37
                        2= 4.20                                           4.16
                        3= 0.00                                           0.00
                        4= 4.20                                           4.16
                        5= 0.07                                           0.09
                        6= 0.34                                           8.23
                        7= 2.27                                           2.91
                        8= 7.88                                           7.82

D1 (Blue LED):  A= 0.00                                            5.97
                        K= 0.00                                            3.31
D2 (1N4148):  A= 2.27                                              2.91
                      K= 8.24                                              8.10
D3 (1N4148):  A= .84                                                2.20
                      K= .227                                              2.91
D4 (1N4001):  A= 9.24                                               9.23
                      K= 8.56                                               8.52

Q1 (all 2N2222):  E= 2.88                                           [No Change]                                         
                          B= 3.18
                          C= 8.54

Q2:  E= 3.90                                                              [No Change]
       B= 4.48
       C= 8.54

Q3:  E= 3.20                                                               [No Change]
       B= 3.70
       C= 8.55

Q4:  E= 0.39                                                            5.98
       B= 0.74                                                             6.57
       C= 8.53                                                             8.48

Q5:  E= 2.06                                                              4.59
       B= 2.58                                                              5.08
       C= 8.44                                                              8.37

Q6:  E= 0.00                                                              0.00
       B= 4.56                                                              0.10
       C= 8.44                                                              8.10

[ElectricDruid]

I removed D3 and R41 from the 3102 and installed a 1M pot between pins 6 and 7 and connected the audio probe.  There was no signal to the amp- until I turned the pot.  Adjusting the pot gave me a variable squeal over the range of the pot.  Once I stopped turning the pot, the squeal would die out after a couple seconds.  Not sure what SHOULD have happened, but I don't think it did.  After this, I replaced D3 and R41.

To be honest, this sounds pretty close. What you should have for testing looks like this:



You need to break the connections to  R41 and the diodes (in red), and then put the 1M pot between 6 and 7 like you had it (in blue). At the "close to 1M" end of the spectrum, the clock frequency will get so low you'll hear it, so I'm not surprised you got a squeal. That at least means the clock is working. When the 1M pot is below 100-200K or so, the clock squeal should disappear and you should be able to hear the input signal going through the BBD and coming out of the other end. If you can't hear audio, or can only hear distorted audio, adjust VR4/25K the Bias voltage trimpot. BBDs are pretty fussy about where the input bias is set, so it needs to be right for the BBD to work properly. You should be able to tweak it to get clean sound coming through. That's what we're really after. If we can get that, then we know that the BBD chip is good, and that the clock is working. That would then *only* leave the LFO.

[duck_arse]

something's out of order.

D2 (1N4148):  A= 2.27                                              2.91
                      K= 8.24                                              8.10
D3 (1N4148):  A= .84                                                2.20
                      K= .227                                              2.91
Q6:  E= 0.00                                                              0.00
       B= 4.56                                                              0.10
       C= 8.44                                                              8.10
IC4 (MN3102):  7= 2.27                                           2.91

points in blue are supposed to be connected. points in red supposed to be connected.

I have no/little idea of the workings of the MN3101 - but it looks as tho your lfo is locking/latching - as indicated by the led, and it flips state with power cycles. so if the lfo latches and turns on Q5 hard [like with 5V on the base], what effect does it have on the MN operation? basically, as druid sez, see if the MN functions without the oscillator, which has a short or misconnection somewhere about it's person.

Power supply displaying 9.24V, 22 mA.                     9.20, 40 mA  (this seems odd...)

interesting. looking at the supply section suggests as good as no change to the current drawn thru R42 via things on Vb, so all that extra current is being gulped down by something connecting to the +V line.

[flon_klar]

So Druid's comment about the fussiness of the BBD got me thinking- If it's that fussy, why would the power supply to the ICs be fixed at ~8.40V?  The circuit is designed with a "Vb" of 8+ volts and a "1/2V" determined by the trimpot.  So- I pulled the Vb supply to the ICs and connected them to 1/2V, then dialed down the voltage.  I instantly started getting the squeal, and as I approached the bottom of the pot, the squeal became audible clock, and bottomed out around 1.6V with a clock pulse of roughly 1 tick/second.  So I definitely have clock.  This has no affect on the LED however.  Now, how do I get an LFO going?