pedal troubleshooting

[xtian]

When in bypass mode, pedal passes signal. When engaged, LED lights, but no signal passes. Contains this dual op amp: http://www.ti.com/lit/ds/slos073f/slos073f.pdf

Op amp has +9.4 vdc on pin 8.

When I tap on Base of right-hand transistor, I can get a click in the output. Otherwise, no sounds at all. None of the connections seem to be intermittent.

How to troubleshoot the IC and transistors?

[Deric]

Looks to be a Tubescreamer from the layout at Tonepad.com

http://tonepad.com/project.asp?id=1

First things first: Did it work before?

If so: Hard to tell from the pic but since it doesn't have a battery or any kind of polarity protection on the DC jack there's a good chance that someone plugged in the wrong power supply. I've seen this blow the chip on several occasions....

Easiest thing to do is swap in any dual op-amp: TL072, 5532, 4558 etc... and see if that fixes it.

If it didn't: What is the voltage at each pin of the IC and at each pin of the 2 transistors?

[vibratoking]

Again, did it work at one time?

I agree with Deric. If it did, I would pull the IC and check the power and ground connections before replacing it. Make sure you aren't going to blow the replacement. Make sure the IC is not flipped. You can do the same with the transistors. Check the voltages to make sure power and ground are correct. You can also check the transistors out of circuit.

If it did not work, recheck the IC, all power and ground wiring, continuity between power and ground (this should always be done) and also all the flying leads around the jacks, switches and pots.

[xtian]

Deric, great eye, and thanks for the layout/schematic!!

We're going to assume this pedal was working; it's a JHS. Or at least, it's in a JHS chassis.

The IC is oriented correctly. With the IC removed, pin 8 has 9.4vdc and pin 4 has 0, as you'd expect with proper orientation of DC adapter. All other pins of the IC socket show 4.7vdc.

The pins on the transistors read, from left to right as shown in the layout: 3.1, 3.6, 9.4, 9.4, 3.5, 3.0.

Any clues from this?

[Deric]

Voltages appear to be in the ball park. Go thru all wire connections and especially the wire connections at the board - I've worked on a few JHS pedals that were just built shitty. If all checks out, swap in a new IC. If that works I'd consider adding some sort of polarity protection in case someone uses the wrong power supply.

[xtian]

By injecting a sine wave here and there, I observe that I have signal at pin 3 (the input of the first op amp) but not at pin 1 (the output of same). See schematic. Is there any way that the feedback/clipping circuit between pins 2 and 1 could be the culprit, or is it definitely a bad IC?

Also, if I inject the sine wave on pin 1, audio successfully exits the pedal. So at least I found where it stops.

I don't have a dual op amp to try replacing. I'll have to order.

[Reeltarded]

Any of you pedal monster want to take a look at my Dunlop Univibe? It has something .. up. heh

[xtian]

Fine, send me your pedal.

What's a simple scheme for power protection against reversed polarity?

[vibratoking]

By injecting a sine wave here and there, I observe that I have signal at pin 3 (the input of the first op amp) but not at pin 1 (the output of same).

I am assuming you have the IC inserted when doing this? Is the output at a rail, being pulled to ground or power? A properly working opamp circuit will adjust the output so that the inputs match. If the loop is closed (not open between pin 1 and pin 2) then pin 2 and pin 3 should be nearly identical. If loop is not open and the inputs don't match, then I would change the IC based on the voltages that you listed previously.

Also, if I inject the sine wave on pin 1, audio successfully exits the pedal. So at least I found where it stops.

Thats a good indication that the IC is the problem.

[cbass]

Fine, send me your pedal.

What's a simple scheme for power protection against reversed polarity?

I belive a zener diode will do the trick.It will blow if you hookup the PS backwards.

[vibratoking]

I prefer to put a 1N4001 type diode in parallel between the power and ground rail at the input. When power is connected properly, the diode is reversed biased and does basically nothing. When power is hooked up in reverse, the diode forward biases, conducts and clamps the voltage between power and ground to one diode drop. This is enough to protect your circuit. The 1N4001 is rated at 50V and 1A current, which is normally enough to keep it from blowing, and keep protecting, in most situations.

The series diode costs you a forward voltage drop, around 0.7V, on your supply. Your battery or supply looks 0.7V less than it really is.

[xtian]

I prefer to put a 1N4001 type diode in parallel between the power and ground rail at the input.

That's great. So, would you put the cathode (marked with band) toward the positive voltage?

[JazzGuitarGimp]

Yes, that's correct. Cathode to positive, anode to ground. Keep in mind that if an AC adaptor is connected backwards, the diode will act as a short to the adaptor, which will likely cause it to overheat and burn up if left in this state too long.

[xtian]

Replaced IC; all better.

[xtian]

Replaced IC; all better.