Another Rat Debugging

[johnravacio]

Hey Guys,

After reading a couple of previously posted and didn't get the same problem as mine(or I am so dumb to understand it   ), I finally decided to create a new thread.

I used the Tonepad PCB layout for proco Rat. Below are the following voltage readings on a couple of components. Bypass is Okay but no signal when switched on. LED is okay too.

I've used 1 Spot adapter 9.5v using DMM measurements

IC - LM308

1 - 8.86
2 - 4.7
3 - 4.26 (This is my initial suspect, I've read the other thread that this is supposed to be around 2v some other post say it shout be around 4.5)
4 - 0
5 - 0
6 - 4.72
7 - 9.49
8 - 4.79

Transistor - 2N5458

D - 9.5
G - 1.97
S - 0


Solutions did but failed:

I've replaced the IC with LM741 but still it didn't work.

Please, can anyone help me? I'm not much knowledgeable on electronics but I know basic.

[johnravacio]

By the way, i've also measured the 3 Diodes and looks ok to me:

D1
A - 0
C - 0.9557

D2
A - 0.9557
C - 0

D3
A - 0
C - 9.49

Any ideas would be greatly appreciated. thanks!

[ashcat_lt]

Both inputs and the output of the opamp should sit right around half the supply voltage.  If it's not too early for me to remember the pinout the inverting input is a little low and the output is pulling to the positive rail.  This is almost always because the caps to ground in the feedback loop are not doing what they should.  One (or both) is either faulty or solder bridged/shorted.

[johnravacio]

Thanks a lot for the inputs ashcat!

which cap you are talking about on the feedback loop? is it the 4.7uF or the 2.2uF. By the way, I did use a tantalum cap for 2.2uF. There was a dash on the tant and i've connected it to the + side. I might be wrong thought but i did get voltage reading with that 2.2uf.

[chromesphere]

Hey John,

I usually do some preliminary checks before i start getting into voltages etc.  It's going to be easier for me to link to my video then type it all out here.  The info might be old news to you, or it might be useful.  Either way, of the 50+ builds i've built so far, I've found 99% of the problems are solved doing these steps.
Hope it leads to a solution for you and good luck!
Paul


http://www.youtube.com/watch?v=JnD4-_Fhd90

[R.G.]

I used the Tonepad PCB layout for proco Rat.

It really, really helps people to help you if you'll post a link to the schematic you used so they don't have to go searching for it.

1 - 8.86
2 - 4.7
3 - 4.26 (This is my initial suspect, I've read the other thread that this is supposed to be around 2v some other post say it shout be around 4.5)
4 - 0
5 - 0
6 - 4.72
7 - 9.49
8 - 4.79

Pins 2, 3 and 6 should be within millivolts of the voltage at Vb. With 9.5V  at the power supply, this comes to 9.75. We see that on the output pin (6) and on pin 2. Pin 3 would be there, excepting that it's a very high impedance point. An ordinary meter can actually load this point down through the 1M resistor going to the bias voltage. This appears to be happening here; it's fine. The opamp is doing the right thing for DC.

Transistor - 2N5458
D - 9.5
G - 1.97
S - 0

Is it possible that you've confused gate and source? The gate should be at 0V, and the source at a couple of volts higher. This comes in two flavors: (1) mixing up which one you read the voltages from and (1) mixing up which pin is which on the actual part. Since JFETs commonly switch S and G pins, it's easy to confuse. Getting it wrong on the PCB will mean a non working pedal; getting it wrong on measurements confuses the process.

I usually do some preliminary checks before i start getting into voltages etc.  It's going to be easier for me to link to my video then type it all out here.  The info might be old news to you, or it might be useful.  Either way, of the 50+ builds i've built so far, I've found 99% of the problems are solved doing these steps.

It is always good to verify your wiring is right and nothing's shorted to the case. A lot depends on whether the thing has been wired up and put into a box before the problem call comes in, though.

[ashcat_lt]

K.  Apparently it was too early for me to remember the pinout.  Looks like RG's got it, though I think he meant that Vb should be 4.75V. 

Either of those caps could cause the problem I was mentioning, but that's less likely to be the problem given that the voltages look good.

[johnravacio]

First of all, thank you very much for all of your suggestions and comments.

Hey John,

I usually do some preliminary checks before i start getting into voltages etc.  It's going to be easier for me to link to my video then type it all out here.  The info might be old news to you, or it might be useful.  Either way, of the 50+ builds i've built so far, I've found 99% of the problems are solved doing these steps.
Hope it leads to a solution for you and good luck!
Paul


http://www.youtube.com/watch?v=JnD4-_Fhd90

50+ builds? Wow! this is just my 7th build and 2nd project with failure. The 1st failure I got was caused by a bad solder. Luckily my MN3007 wasn't dead.

I'm still at work now, I'll watch it tonight.

It really, really helps people to help you if you'll post a link to the schematic you used so they don't have to go searching for it.

Sorry for that I'll do it next time. Btw, here you go http://tonepad.com/getFile.asp?id=89.

Pins 2, 3 and 6 should be within millivolts of the voltage at Vb. With 9.5V  at the power supply, this comes to 9.75. We see that on the output pin (6) and on pin 2. Pin 3 would be there, excepting that it's a very high impedance point. An ordinary meter can actually load this point down through the 1M resistor going to the bias voltage. This appears to be happening here; it's fine. The opamp is doing the right thing for DC.

Its a relief knowing the OpAmp is doing fine for DC. I did actually hear the audio on Pin 6 but on a low volume. I don't have an Audio probe instead I put in a wire on PIN 6 and on the other end on the output(Not sure if its the right thing to do). I'll create one of those audio probe one of these days.

Is it possible that you've confused gate and source? The gate should be at 0V, and the source at a couple of volts higher. This comes in two flavors: (1) mixing up which one you read the voltages from and (1) mixing up which pin is which on the actual part. Since JFETs commonly switch S and G pins, it's easy to confuse. Getting it wrong on the PCB will mean a non working pedal; getting it wrong on measurements confuses the process.

Good point, I may have confused the gate and the source. I'll re-check the placement and verify it again with the datasheet and the schematic diagram.

K.  Apparently it was too early for me to remember the pinout.  Looks like RG's got it, though I think he meant that Vb should be 4.75V. 

Either of those caps could cause the problem I was mentioning, but that's less likely to be the problem given that the voltages look good.

Thanks for pointing it out. It lessens my confusion too.

[R.G.]

Its a relief knowing the OpAmp is doing fine for DC. I did actually hear the audio on Pin 6 but on a low volume. I don't have an Audio probe instead I put in a wire on PIN 6 and on the other end on the output(Not sure if its the right thing to do). I'll create one of those audio probe one of these days.

The problem with taking a wire to the output is that the output is pulled to ground by resistors or the amp input. If the resistors are big, the opamp output pulls the amp input too high, and if they're low enough to pull the opamp output down, the opamp can't work right. There needs to be a capacitor in series with the output pin of the place you're testing.

It would be very worthwhile for you to make a temporary audio probe from your output jack and a spare capacitor for testing. You can unsolder and resolder when it's working right. See http://geofex.com/FX_images/audioprb.gif

[johnravacio]

Thanks a lot chromesphere, ashcat_lt, and R.G. My Rat now works!

When I got home yesterday, I did first watch the video chromsphere posted (http://www.youtube.com/watch?v=JnD4-_Fhd90) and followed every step of the way. I get into the part where I need to examine the PCB with a lamp and with a Magnifying glass. And Booom!!! I've found there was a hairline cooper connecting between resistors 10K, 1M and the OUT. Amazingly, I can't see it without a Magnifying glass.

Root-cause of the problem:
1. Poorly etch PCB caused by carelessness or just a human error.

Lessons learned:
1. Follow the steps shown at http://www.youtube.com/watch?v=JnD4-_Fhd90 before doing other debugging tips.
2. What is a feedback loop
3. What are inverting and non-inverting input and what does it do

The problem with taking a wire to the output is that the output is pulled to ground by resistors or the amp input. If the resistors are big, the opamp output pulls the amp input too high, and if they're low enough to pull the opamp output down, the opamp can't work right. There needs to be a capacitor in series with the output pin of the place you're testing.

It would be very worthwhile for you to make a temporary audio probe from your output jack and a spare capacitor for testing. You can unsolder and resolder when it's working right. See http://geofex.com/FX_images/audioprb.gif

Thanks for the link. Very well illustrated. I will definitely create one those.