Debugging MXR Distortion, newbie theoretical question

[kapsel]

Hi everyone,

First post, although I am a long time lurker in the forum. I am not sure where or how i should ask this, so if my post needs to be moved please go ahead.

After building the Electra Distortion (success) and the EH Screaming Bird (works, treble is boosted but overall volume is a bit low, don't know what i did there), I build the MXR Ditortion + on stripboard, based on tonepad's schematic (http://www.tonepad.com/getFile.asp?id=115). Of course I couldn't make that work - i get no sound at all
So I tried to debug and read the FAQ. The thing is that being a newbie, I can not evaluate if the voltage readings I get at different test points indicate something wrong with my circuit or if they are normal.

So i would be grateful if you could please guide me to some pretty basic theory/ tutotrial/ whatever that will explain what "moves" in a circuit and how? I understand there is the signal from the guitar going from input to output and dc going from the battery to ground (??).

To make this more circuit specific, I measure c. 8,8V at the positive lead of C6 (8,8V was also the battery voltage when I measured it) but as I measure along from there I get 0v at all other components until output (R9, D1, D2, C7, R10).

I am rather confused so any help will be appreciated.

Cheers

[smallbearelec]

Hi--

The MXR Distortion + is a single stage. If it's wired correctly, it will work; if not, it doesn't.

I build...on stripboard

That's where your problem is. The process of going from a schem (even those that are known to work, like Tonepad's) to layout is loaded with minefields, and beginners should Not attempt it. To figure out what's wrong:

--Remove the stomp switch; it confuses matters.
--Make clean copies of schem and layout. Pick a point on the schem and use the continuity scale of your meter to make sure that every point that you think should be connected Really Is. Mark off connections with a highlighter as you test. You will find problems.
--When you think you have found all the bugs, wire the input and output jacks directly and connect power. If the effect works, then add the stomper.

In the future:
--Work from vetted layouts until you are sure of your skills.
--For doing layouts, learn to use a program like DIY Layout Creator (free) that is optimized for editing boards.

Have patience, and don't give up. IMO, you bit off a little too much of the learning curve at once, but you'll feel better once you "digest" your first build.

[kapsel]

Hi

Thanks so much for your answer.

I didn't want to bother you people a lot so I didn't give the whole story. My bad.
So, I have spent quite sometime "playing" with the (amazing) DIYLC before I started building actual circuits. And since a couple of early circuits that I made had mistakes that I could not fix (eg I made cuts in the wrong positions, designed the circuit having in mind the copper part of the board and only after I soldered everything I realised that the IC would actually need to be the other way round etc) i was very cautious. So now, in order to be on the safe side, I traced the circuit on the non copper side of the stripboard using a permanenent marker and double checked everything pre- and post- soldering. I am pretty sure everything connects to what it should connect.

Additionally, I made a "testbox" inspired by a video from chromesphere, which includes everything wired (jacks, stomps, led etc) and 4 wires (in, out, ground, 9v) that end in crocodile clips  to connect to the respective wires of individual boards and test. I thought it was a good idea to test boards before I box them and, since the testbox is wired correctly, if something goes wrong i assume that the problem lies on the board.

So regarding the MXR Dist +, my thought was that either something is "fried" (btw I did use a socket for the IC) or that there is a bad solder joint or short somewhere. Since I am not sure how to locate this problem, I thought that any advice regarding how circuits (in general or this particular one) work would help me to test it instead of just posting voltage measurements and asking people here to locate my problem

Anyway, thanks again for the advice and encouragement, I will double check everything and report back if I find something.

Cheers

[PRR]

> there is a bad solder joint or short somewhere.

+99!

Maybe 99% of problems are in connections. Wrong place. Solder did not "wet" the metal.

Practice to learn to solder GOOD. There's 62 joints on that board, and more off-bad. Every one is there for a reason. If your solder joints are good 99 out of 100, with 75 joints it is very likely the final product "dont work!" You need good joints 999 out of 1000 to get away from this headache. Just like you learned to play 64-note blues without an "oops!", this is practice-practice-practice. (And listen/look at your goofs, to know how and why you goofed.)

Stay organized. Pre-made PCB is less stress than strip, Vero, or self-made. Even so, a PCB without markings is not that hard to foul-up. (I dunno if TonePad silk-screens part and lead IDs.)

Go ahead and try stripboard. But it's a bit like switching from Guitar Hero to guitar, or guitar to viola: whole new sessions of practice-practice-practice.

> any advice regarding how circuits (in general or this particular one) work would help me to test it

That's an open-ended question. Circuits come in near-infinite variety.

This is an OpAmp circuit. *Usually* audio opamps volt-out like this.

First find the spec-sheet for the pinout. *Most* 1-opamp chips use one pinout, *most* 2-opamp chips use another pinout, *recent* 4-opamp chips use one pinout, but there's an older 4-opamp pinout you still find in stomp-land.

V+ = positive battery
V- = negative battery
Comp, Offset, Strobe = don't probe!
Other pins are *usually* half-way between positive and negative battery. In typical 9V stomps, -9V is grounded, other pins are near +4.5V
+In = +4.5V
-In = +4.5V
Out = +4.5V

These +4.5V points may vary. In particular your meter may load-down the circuit, some points more than others. Look for the "wrong!" voltage. 4.1V may be OK. 1.3V or 8.1V is usually way-wrong.

From there, with learning and practice-practice-practice, you can deduce what part is not working as intended, or what test will clarify the situation. In 9V/4.5V circuits, a shorted bypass cap on the -In network will throw Out to +300V in theory, +8.1V in a real 9V circuit. Shorted bias cap kills the 4.5V supply, and put one In at zero, but the other In and the Out may fling high or low depending how the chip reacts to wrong inputs.

[kapsel]

OK, that is very helpful.
Thanks a lot for your time, I' ll try to sort it out in the weekend.