BMP FAIL :-( I've never had this problem

[johnadon]

Howdy, folks!

I just finished building my 30 bazillionth Big Muff Pi Triangle, but I'm having so far what seems to be an intractable problem with this build. For some reason the effect itself is very quiet in the background while a 'highs rolled off clean sound' dominates the effect when engaged. It's like it's mixing a clean signal with the distortion signal, although the clean sounds like the top is rolled off completely with a tone control. Everything else works normally like the tone, volume, gain. Although you have to strain to hear the distortion in the background it sounds exactly like it's supposed to.

No mods on this one because the goal was to have one in my collection that was as close to spec as possible. The transistor voltages are all on par with those listed on the GGG BMP notes and elsewhere. I measured component values as I put them in and have also checked the ones that are easily measured in-circuit. I've also taken the board completely out and did a direct in/out through the board to eliminate the possibility that there was something weird going on with the switch or enclosure.

Edit: I also traced my circuit and used a highlighter on the schematic to make sure I filled in all the gaps. I didn't notice any shorts along the way either, but I'm suspecting that is probably the culprit?

I'm sure this has been covered before, but I can't find the magic combination of search terms in this forum or on Google. Apologies if this is something that comes up regularly!

Thanks for any help!

John

[R O Tiree]

I hesitate to ask, as you've done this 30 bazillion times... transistors the right way around, diodes the right way round, 9V and GND connected the right way round, polarised caps the right way round, correct pot values in the right places... transistors will work kinda if they're the wrong way round, but with nowhere near the right gain.  And it's dead easy to think you've done it right when you've done it 30 bazillion times before.  It's why pilots sometimes try to land with their undercarriage up...

After that I'd look for solder shorts and dry joints. Get some flux and dab each joint with flux and re-flow. Use solder braid to soak a little away from any "blobby" joints.

If it still doesn't work, I'd replace polarised caps, diodes and transistors.

Hope you find it

Mike

[R.G.]

Situational blindness is something that affects us all. That's one reason I harp on "what to do when it doesn't work" so much.

The dead-boring process there, if done purely by rote, turns up information that's different from what most people do when they build. In forcing a separate and different way of looking at the circuit, it's remarkably effective at flushing out even things like backwards and incorrectly labeled parts.

I think you might want to give it a try.

[johnadon]

Unfortunately, the transistors are in the correct orientation although I thank you for prompting me to double-check the pinout. That is one area where I've gotten REALLY %^&*y and even pig-headed with my assumptions! Strangely, after 10 years of doing this I still get the pinout wrong on even a basic pot sometimes. :-D (I'm serious about that.)

Here's a question though: Does that sound possibly symptomatic of a fried diode? I recall getting heavy handed with the soldering iron on one clipping stage.

Make no mistake, I'm going to get out the meter again with a fresh copy of the schematic and find my error. I was hoping someone had BMP'ed into this problem before and I love easy solutions. But... I love BMP's even better so I'm going to keep plugging away. I don't know what it is about those things.

Thanks much!!!

-J

[johnadon]

Holy.

Freaking.

Crap.

The pinout for the transistors I'm using are oriented exactly the opposite of nearly every other npn I've ever used. 2n5088's. I'm used to the 'belly in front C-B-E'. These are the opposite.

Me? %^&*y? Pig-headed?

*blush*

Many, many thanks and many, many apologies. :-D

[johnadon]

Mrm. Didn't work although I corrected at least one problem. Strange that it's exhibiting the exact same behavior with lower gain. I'm suspecting I connected one of the components incorrectly whenever I reversed the transistors. May have burned one out. Who knows? A problem to worry about next weekend. :-)

[R.G.]

Never assume there is only one problem. 

In fact, in the parlance of programmers, there is *always* one more bug.

[Electron Tornado]

Have you checked it with an audio probe? That might help you narrow down which stage the problem is in.

[R O Tiree]

Never assume there is only one problem. 

In fact, in the parlance of programmers, there is *always* one more bug.

I'll drink to that. As soon as you identify one bug and fix it, that opens up a can of worms in another part of the program... <sigh>

[johnadon]

You're right. There's always one more bug and this time it's the one I had in the first place. (Actually I discovered one other minor bug, as well, but it shouldn't have had that much of an effect since it was just a slightly off emitter resistor.)

It turns out I did have the transistors oriented correctly originally, but I just picked the wrong manufacturer's datasheet to double-check. This is definitely a weird one, and I've literally spent nearly three times more time troubleshooting than it usually takes to build the circuit from scratch. I'm afraid I'm going to have to put this one on the shelf and start from scratch again. Not a biggie since I love this simple *ahem* circuit. I AM however saving it for study later. I've just got to know what in the world I did!

[johnadon]

I did try an audio probe. The circuit craps out even at the final gain recovery stage. I totally redid that stage, but nada. I've only twice run into a bad part so I refuse to believe it's a bad part in the build. (I've fried a LOT of parts, but have had only two bad ones out of factory.) It's most certainly a mistake.

[R.G.]

This is definitely a weird one, and I've literally spent nearly three times more time troubleshooting than it usually takes to build the circuit from scratch. I'm afraid I'm going to have to put this one on the shelf and start from scratch again. Not a biggie since I love this simple *ahem* circuit. I AM however saving it for study later. I've just got to know what in the world I did!

At the risk of being repetitive, you've just passed over the most appropriate advice you got:

That's one reason I harp on "what to do when it doesn't work" so much.

The dead-boring process there, if done purely by rote, turns up information that's different from what most people do when they build. In forcing a separate and different way of looking at the circuit, it's remarkably effective at flushing out even things like backwards and incorrectly labeled parts.

I think you might want to give it a try.

I realize I gave the advice in a more subtle manner than I usually do, but if you want to fix it in an expeditious way instead of beating about the bush or dropping it, go read and follow "What to do when it doesn't work".

[johnadon]

Okay, you've convinced me. Frankly, I enjoy doing this as much for the challenge as I do for the sound. I've got a number of builds of this circuit that I love already so this should be the kind of thing that shouldn't be getting to me in such a bad way.

And I know your posts run deep, R.G. :-) I'm not in any hurry so I'm going to spend some more time with it, but not until the weekend (if I can keep my hands off of it) since our agency is having a three year accrediting survey through the rest of the week. Will post again since I'm becoming a poster child for overlooking all things obvious. :-)

[johnadon]

Well, I had to give up on troubleshooting the BMP build. I'm going to assume I fried a couple of components, including possibly my brain..

Good news is I built another in short order that works just fine. Even better, all the failed troubleshooting jogged a dormant memory, and I've *finally* rediscovered a mod I did an earlier BMP that sounds better than all the others I've ever made. The mod removes flabby base and makes the tone control more responsive. The mod is very similar to the Gagan-style cap-in-parallel-with-pot tone control except that I chose set values to put in series between the regular input cap and the 3.3k input resistor. In this case I used 22k in parallel with a tiny cap, .001 uF. I'm sure there is something equivalent i could do with the single cap, but i was so excited about remembering the mod that I just popped it in the most convenient place in the layout. The cap takes out some of the bass but the 22k passes enough signal through to keep signal loss at a minimum through to the first amplification stage.

So... All that failed work did have a great result in that it jogged my memory about that particular mod. Quite a happy camper since I know how to replicate the BMP circuit I've always used the most.

(I know I type way too much. You should hear my voice mails. :-) )