I fried something...

[climberboy]

Hello all,
I've been working on Tonepad's MXR Blue Box clone and I've run in to a problem. I'm using a 3DPT for the bypass and added a jack for an outlet. I've done this same offboard wiring on another pedal and it works fine, so I'm not concerned about the power/bypass stuff. My problem started when I was testing it. It worked great off a battery, so I figured I'd use my Digitech GNX4's wall wart and see how that worked. When I plugged it in, the power LED on th pedal lit up for a second or two and then went out. The pedal was totally dead and the only thing that worked was the bypass. The power supply was also shot. Thankfully it was just the fuse that went out in the power supply, but I have no idea what's wrong with the pedal or where to start looking. When I put a battery in the pedal now, it draws 1.2 amps even though there's no sound coming through. I looked for similar posts, but didn't find anything.

Thanks!
Nathaniel

[Jopn]

Your power is touching ground somewhere.

[armdnrdy]

Need a bit more information.

But in the meantime.....did you use a metal DC jack?

[climberboy]

Nope, I used a plastic DC jack on this one. What else do you need?  I'd be happy to take some measurements and stuff. Here's the link to the project: http://tonepad.com/project.asp?id=33

[armdnrdy]

Following these guidelines has helped many a wayward build:

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

[Pojo]

I don't know much about the GNX4 power supply. But maybe there's a chance that it fed reversed polarity current to what your pedal expects? Check D1 and make sure it's not shorted out. That diode's job is to protect the rest of the circuit from a reversed supply but so much current is drawn in the case that the supply is actually reversed that it can be too much for the diode to withstand and will need replaced.

[digi2t]

If I`m not mistaken, the GNX power supply provides 9VAC power, and not DC. Check the polarity protection diode. Hopefully, you didn`t do any more damage than that.

[GibsonGM]

Provide a link directly to the Blue Box schematic, and we'll have a look for you...plus, make sure of the specifications on that power supply...does is say AC?  How many volts?

[climberboy]

Alright, so I just tried hooking up the battery to do all the voltage checks and I noticed that all the terminals in the area around C11 are extremely hot. I didn't take any measurements because I didn't want to leave the battery plugged in. The battery also got really hot. I did check the D1 diode and it appears to be bad- I got a reading of 3. The other diodes had readings of around 650. Could this one diode be all the problem? I double checked the GNX power supply and it does in fact say 9V AC @ 2.1A  I can't believe I missed that. I should also note that before I used the GNX power supply, I also used a 9V DC @ 1A power supply, which it also blew.
Here's a direct link to the schematic http://tonepad.com/getFile.asp?id=73

[Pojo]

You can temporarily remove D1 and see if the circuit works without it. If it fires up, we know it's just the diode. If not, more investigation is needed.

[climberboy]

I had an extra diode laying around so I went ahead and swapped it out. It powers up just fine now, but it isn't jumping octaves. The blend knob just makes the distortion either sound bright or muddy. Also, there's a background noise if I'm playing really quietly that sounds kind of like a helicopter? I don't have time right now, but I'll get some voltage readings tomorrow and maybe that will help. Thanks for the ideas so far guys! It's definitely  closer to working than it was.

[GibsonGM]

It happens, ha ha!  ;o)   The opamp(s) may have been damaged.   Yes, get some voltages at the pins, and that will tell us all more!
Glad it's not shorted anymore, though!

[Jdansti]

Welcome to the forum.  Glad you got it worked out. I confess that I thought this thread was about food when I read the title.

[climberboy]

Alright, so here's some measurements. The batter was getting pretty warm toward the end of it, but none of the components were hot.

Battery starting voltage= 9.44 V
Battery ending voltage= 8.66 V
Pulling 22 mA

IC1
P1=4.77
P2=4.82
P3=2.47
P4=0
P5=4.4
P6=4.43
P7=5.02
P8=9.29

IC2
P1=0
P2=1.51
P3=5.44
P4=0
P5=1.49
P6=0
P7=0
P8=0
P9=5.49
P10=0
P11=3.25
P12=5.54
P13=5.55
P14=5.69

Q1
C=6.42
B=0.23
E=0

Q2
C=0.03
B=0.3
E=0

Q3
C=0
B=0
E=0

[Jdansti]

22mA seems kinda low for the battery to be getting hot. Did you measure the current with your meter in series with one of the battery's leads?  A hot battery on a pedal usually indicates a short, but 22mA doesn't look like a short.

[climberboy]

Yeah, I measured the current by connecting one terminal of the battery to the pedal and then measuring in series off the other terminal. 22mA seems like a fairly normal current draw so I'm not sure why it got hot either. I'm guessing that since the one transistor has 0V on all terminals it's bad? I'm not sure what to look for with the opamps.

[Jdansti]

Sounds like there's a problem with the 0V. Can you provide clear photos of both sides of the board and the connections to the off-board components?

[dwmorrin]

IC2 pin 14.  It's the Vdd, aka the power input for the chip... I would expect +9V there.  It's tied to +9 thru a 330Ω resistor (R24).  I see no reason for it not to be around +9.  Check R24, C12, C13.
5.6V seems like a very low reading.

The transistors look like they get turned on and off by the square wave signal voltages.  I wouldn't be surprised it 0V readings at idle is normal, but I've never built this circuit, so don't hold me to it.

[climberboy]

I checked R24 and it's 331 ohms, so that's not an issue. As far as C12 and C13, unfortunately my multimeter can't check capacitors  ...
Here are pics of the top and bottom of the board as well as the off board wiring that I did. I found the off board diagram on this forum actually. I went ahead and double checked the current from the battery as well as the voltages on IC2 and got similar readings. http://s38.photobucket.com/user/climberboy/library/Blue%20Box%20Pedal?sort=3&page=1

[dwmorrin]

Then that just leaves the 4013 as the potentially fried component.
I would pull that next and try a new one if you got it.
R24 is a jumper in the original schematic, so pin 14 must be at 9V normally.