Debugging MXR Bluebox (tonepad PCB)

[will_raymo2000]

Not getting any output when effect is turned on.

Readings as follows:

IC1:
1 8.94
2 8.6
3 8.6
4 0
5 0.54
6 0.46
7 8.77
8 9.44

IC2:
1 0
2 9.58
3 9.6
4 0
5 9.61
6 0
7 0
8 0
9 0
10 0
11 0.09
12 0
13 9.61
14 9.61

D1:
A: 9.64
K: 0

D2:
A: 0
K: 0.03

D3:
A: 0
K: 0

Q1:
C: 0.09
B: 0.65
E: 0

Q2:
C:  0
B: 0.08
E: 0

Q3:
C: 0
B: 0
E: 0

Any ideas?

I used 47k log for the blend and 47k log for the output.

Cheers!


PDF/schematic here: http://www.tonepad.com/project.asp?id=33#

[will_raymo2000]

bump

anyone got any ideas??

[Mark Hammer]

Since you clearly have a meter, what is the AC voltage at IC1-pin 1 when you plug in and strum?

What is the AC voltage at pin 7?

What is the AC voltage at the junction of R13/R14/D2?

[will_raymo2000]

I don't personally play but I have a function generator. The input is a sine wave at 360hz at 320mV

IC1:
1 38mV
7 52mV

Junction:
0V

[will_raymo2000]

Turns out I had lifted the solder pad off one end of R12 so with a few jumpers it works. The only problem now is that input is 320mV and output max is 170mV.. is this a volume drop to be expected or is something else wrong?

Many thanks,
Will

[Govmnt_Lacky]

Something definitely looks wrong with IC1 Pins 5 & 6 as well as with the readings at D2, D3, and Q3.

My first inclination would be to check the following:

1) Transistor, IC, and diode orientations (are they reversed?)
2) Ensure the CORRECT components were used for all things mentioned in #1
3) Solder joints. Especially around D2, D3, and Q3 (look good because it only takes a "hair-like" strand to cause problems)
4) Look for joints under the 4013. Around pins 13 and 14. You show V+ on ALOT of pins. Probably have some joints under there or possibly a bad CD4013.


Good Luck and report back what you find (if anything!!)

[will_raymo2000]

IC1:
1 4.84
2 4.94
3 4.54
4 0
5 4.7
6 4.92
7 1.48
8 9.66

IC2:
1 9.67
2 9.67
3 9.66
4 0
5 9.66
6 9.66
7 0
8 0
9 0
10 0
11 9.56
12 9.67
13 0
14 9.67

D1:
A: 9.67
K: 0.06

D2:
A: 0.02
K: 0.03

D3:
A: 0.03
K: 0

Q1:
C: 9.6
B: 0.22
E: 0

Q2:
C:  0.05
B: 0.53
E: 0

Q3:
C: 0.01
B: 0
E: 0


Those are the new readings. I have checked and double checked the IC, diode and transistor orientations and have used the stated components in the build.

[jkokura]

Q3 looks suspect to me, but I'm not familiar with the circuit. Also, I always, always suspect wiring when I don't get any signal. Have you thoroughly checked you wiring? Pictures may help us out if you can give good ones, and I would look for solderbridges and backwards parts too.

Do you have an audio probe? That could help lots.

Jacob

[will_raymo2000]

I now have signal coming out of the circuit, yet it is hugely dropped in volume. Upped the input to 600mV (which is more than a guitar I believe) and the output is roughly 50 to 60mV!

Will keep looking through the circuit but cant see anything obvious. No solder bridges or wrong orientations etc. I have also tried several op amps of different types and have swapped the transistors round a few times to no avail.

[Mark Hammer]

Since you have access to a signal generator of known output amplitude, and a meter for assessing expected or unexpected signal levels in various places, that bodes well for narrowing down where the problem or problems lie.

The unit has two major signal paths, which share a common source.  While the audio signal passes through IC1a, IC1b, and Q1 (and AC/signal voltage can be productively measured at each output point along there), the control signal parts ways at the output of IC1a.

Both Q2 and Q3 are effectively "controlled" by the envelope signal, taken from IC1a, and applied to the collectors of Q2/Q3 via R13/R14.  They serve as effective "gates" to squelch audio signal when it reaches a point where sputteriness is a high likelihood.  Of course, the envelope detected via the circuit comprised of C7/D2/D3/R15 and C8 has to be sufficiently strong enough to turn Q2/Q3 on, if you're going to be able to hear anything at all.

All of which suggests that the output of IC1a is critical to the functioning of the pedal, and you should focus your efforts there.  o you know that the chip is oriented properly?  in full contact with all solder pads or socket pins?  not burnt out?

If the chip is socketed, consider popping it out, and replacing it, or at least trying it out in another circuit where it could possibly work if it was functional.

[Govmnt_Lacky]

o you know that the chip is oriented properly?  in full contact with all solder pads or socket pins?  not burnt out?

If the chip is socketed, consider popping it out, and replacing it, or at least trying it out in another circuit where it could possibly work if it was functional.

I have also tried several op amps of different types and have swapped the transistors round a few times to no avail.

I agree with Mark in that it sounds as if your op amp is not doing its job properly. I know you said you checked orientation however, was that according to the Tonepad layout or was that according to your IC socket? Since you claimed to have swapped out ICs, I can only assume that you have used an IC socket. Was the socket soldered in according to the layout? Also, as Mark stated, check for solder runs/bridges or cold joints around IC1.

[will_raymo2000]

I have used the HCF4013BE from ST (datasheet: http://www.rapidonline.com/netalogue/specs/83-0266e.pdf)

For the op amps I have tested the JRC 4558D and also the TL072 which both have the same pinouts. All the transistors and ICs are socketed for this precise reason!

[Govmnt_Lacky]

I do not think it is a bad passive. I think you need to look at traces and solder joints/points. Being that you alreadt identified a bad solder pad, you may want to look further into your PCB. Especially around IC1