Pulsar acting funny

Started by perfectsoundforever, December 10, 2010, 08:33:44 PM

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perfectsoundforever

Hi everyone. Recently I've tried my luck building a Pulsar (with the Tonepad schematic: http://www.tonepad.com/getFile.asp?id=29) but, even though I got it """working""", it's acting really strange. When I turn it on, I can't hear anything, though if I turn the amp volume up, a really really weak (and wet) signal appears. The funny part is, if I run some big noise through it (like some strong feedback with an eBow), I kind of unlock the Pulsar and it starts working normally for a second, and then it starts distorting the sound a bit (but at a normal level). After that, I can use the effect normally until I unplug it, but with a bit of distortion.
I've measured the voltages for you to see if there's some problem there...

(power source reads 9.58, 9.4 at the circuit board)
Q1
E 0
B unstable... goes from 0.36 to 0.58
C 0.01

Q2
E 0
B the same... from -0.11 to -2.2 (negative?! what...?!)
C from 3.85 to 8

Q3
E 0
B from -1.34 to 0.6
Cfrom 0 to 2.57

TL072
1- 1.42
2- 1.37
3- 1.27
4- 0
5- 0
6- 8.63
7- 8.63
8- 9.27

Something is really wrong with that values... hope you can help me sort it out! I've replaced the 2n5088's multiple times with no luck.
Thanks in advance!

PRR

The transistors do not pass signal, they only wobble.... they are not the problem for getting (unwobbulated) signal to go through. 

IC pins 1 2 3 should be near half voltage, not way low like that. That's a problem. Since a dud IC is unlikely, suspect the resistors and caps which tell the IC what to do.

What is the voltage on C5?

If not 4.5V-5.0V, check R8 R9 for values (3K instead of 30K?) and shorts. Check C5 polarity and shorts. Still low? Lift one end of C5 and see if that point comes up close to 4.5V. If so, replace C5.

If still odd, inspect C3 for shorts, then lift one end of C3 and see if IC voltages get good. If so, replace C3.
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perfectsoundforever

Voltage on C5 is OK... it reads 4.98v. R9 is 33k though, couldn't find the correct value but I guess it shouldn't make a difference here... On C3 I'm getting 1.38v, is that supposed to be OK? I checked for shorts and couldn't find any, the only problem I can see is the trace between R7 and C4... I ruined the copper and the solder is a bit strange (it was the only way to get them together). Could that be the problem?
Thanks for your help so far!


PRR

Did you get as far as my 5th paragraph?
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perfectsoundforever

Oh sorry, I thought that it depended on C5 (like, if C5's voltage was OK, the problem wouldn't had anything to do with C3).

So I replaced C3 and still nothing... I tried lifting the negative side as well and I'm still getting a low voltage at the first three pins of the IC.

:'( what am i going to doo I just can't see what's wrong with this damn pedallll sdasdawdoawd


thanks for your help!

PRR

#6
I hate to say this.

Pull the IC.

If pins 1 2 3 come up to 4.98V or so, replace the IC. It has a zapped input. That's rare, but in cold dry weather if you are getting shocks when you touch doorknobs you should boil water and steam-up the room before you handle FET chips like TL072.

If pins 1 2 3 do NOT come up to 4.98V or so, look for shorts in that area near pin 3. It appears to be either a 13K resistor (1.27V/96uA) or two diodes.

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