MORE BOBTAVIA PROBLEMS!

[aut0tek]

*sigh* I decided to give it another try... beforehand I got no output... NOW if you turn the gain up you get a really fuzzy gain... and the level is more of an on/off switch than a pot... But anyhow... very very noisy even with both knobs all the way down... Lots of hissing/buzzing...
I, once again, used a multimeter and ohmed out everything I could... Everything has continuity where it's supposed to, and doesn't have it where it's not supposed to.

So... At least there's output now!!! And if nothing else I've made a fuzzbox... however I would still like to get it to octave as intended... What part of the Bobtavia circuit should I look into? I was guessing one of the diodes might have failed? I dunno? Space/Time millenium got a ripple in it? Gravitational pull of the moon affecting my output gain on my sphygmomanometer?

LOL Any help appreciated!

http://www.diystompboxes.com/pedals/bobEZ.jpg

-aut0

[guitar_199]

aut0tek,
I don't know if someone is having fun....or what....
But I just compared the schematic you linked to with one that I saved a long time ago from another source.
They are the same EXCEPT......
If you follow the connection from the level pot down to the capacitor/footswitch you go into the switch on the upper left.  Just below it the drawing that you reference goes out of the middle connection and over to the ring on the output jack.  ON mine.... it goes to the tip.
If you think about it....on a two conductor guitar cord the signal travels on the tip.  I think that is what I would try.... move the wire from the output jack ring.... to the output jack tip.
The way your schematic has it .... when you plug in the output jack.... it is grounding the output signal.

Here is a stab at the link to the schematic I have.....
http://users.ev1.net/~boruth/bobtavia.jpg

Good luck,
Bob

[RLBJR65]

Fun project, stick with it you will figure it out! Wiring is wrong on the schematic you posted. Use the one Bob posted you can also find it at G.G.G. http://www.generalguitargadgets.com/diagrams/bobtavia.jpg
Also another mod at G.G.G. http://www.generalguitargadgets.com/diagrams/bobtavia_spinal_ez.jpg
I don't know if this will help you out but I did a quick redrew of it. http://aronnelson.com/gallery/album10/Bobtavia

Here are some clips at Home Wrecker (old R.O.G. site) http://www.home-wrecker.com/salvo.html I think those are of the mod. version you are looking at. It's not a very clean octave anyway but put your Fuzz or an OD in front of it and it gets very very sick.

Richard

[aut0tek]

Hey thanks for the replies... As far as the wiring, yes I noticed that when I first built it:

http://www.elixant.com/~stompbox/smfforum/index.php?topic=36372.0

That was when it had no output at all, so I saw that, changed the ring/tip and still no output... Messed with it for a couple days and gave up... so I recently got another LM386... put it in there and now I have the fuzzbox, just no octave effect. :-/

-aut0

[RLBJR65]

Transformer in the right way? With RadioShack transformer - White wire from 220uF, Red to ground, Black to 100K, Blue and Green going to the diodes. What diodes are you using? Oh and check pin 2 make sure it's got a good ground it will sound very farty, buzzy and low volume if it's not. This pedal is VERY loud with every thing dimed out.

Here is a vero layout I cooked up today. (untested) It's to #$%@ big, fitting the transformer is a PITA. http://aronnelson.com/gallery/Bobtavia/Bobtavia_layout?full=1

Richard

[aut0tek]

Whoa whoa whoa... this schematic that is supposed to be good (I wouldn't know but everyone says that it works as advertised.)

http://www.generalguitargadgets.com/diagrams/bobtavia.jpg

Says that the Red/Whites are opposites of what you said... Which one is it?  I am such a n00b. lol. I wish I had enough theory to know which one would work and which one wouldn't.

-aut0

[guitar_199]

Given that.......   I'd be checking the transformer for correct direction and correct use of windings....   and I'd be checking the connection and polarity of each and every diode carefully.   That is a full wave bridge rectifier and the frequency "doubling" occurs because of correct bridge operation.  If anything is wrong in there you wont be rectifying correctly......and no double tone will be present.

[aut0tek]

Diodes are good... The transformer however, I guess I'll wait on clarification as to which way is correct... right now according to my schem, it's right, but RLBJR begs to differ. ;-)

-aut0

[jmusser]

I drew the EZ Build up a long time ago, and the Revision #1 should be correct. When I initially drew it, I had the output going to the ring, which has to be to the tip lug of the jack. The Radio Shack transformer red lead goes to the 220uf negative side of the electrolytic. The White goes to gound, Blue and Green to the diodes, and the Black lead to the 100K. If I remember right, this is a step down transformer turned around backwards to be a step up. You can get by with it in this case, because of the very low current. P.M. me if I can help. This was the first effect I ever made, and I'm kind of partial to it. I'd like to see it work right for you. The EZ Build was made for newbies, so that they could actually follow the picture, instead of having to actually know schematic symbols. Then you could use it as a reference against a standard schematic to help you learn to make the connection between the graphic component, and it's symbol.

[RLBJR65]

I could have wrote them down wrong and I'm traveling until late Friday so I can't check right now

That schematic is definitely good and if you layed the transformer out as its drawn on the EZ schem. (2 wires to the left, 3 to the right) you should be OK. Transformers are basically two (or more) coils wrapped around the same core, each winding is separate from the other, ct stands for center tap which means that somewhere in the winding you have another connection. Therefor it does not really matter if you had the red and white wires switched because they are at opposite ends of the same coil. Likewise it does not matter if you have the green and blue wires switched, but the ct or black wire would matter because it is connected somewhere in the middle of the coil.

If you want to learn how transformers do what they do I can't properly explain it to you but there are tons of good articles on the web. Check Wikipedia out. http://en.wikipedia.org/wiki/Transformer
Here is a little about octave effects http://en.wikipedia.org/wiki/Octave_Effect

What diodes did you use? It calls for 1N914 but 1N4148's should be OK too. It could also be that you have 1 or all the diodes in backwards the ring on them should all be facing away from the transformer. Also 1 or more diode could be bad or very badly out of spec.

Can you measure and post the voltages at each IC pin and maybe take pics of the board?

Check your volume pot since it does not appear to be working right. You could even take it out and just connect the .047 cap directly to the diodes and the tip of you jack.

Are you powering it with a battery or power supply? A bad power supply could make it hum or buzz and possibly overwhelm the octave.

Stick with it you will figure it out. I'll keep checking for your posts.
Richard

[aut0tek]

Well as far as schematic symbols I'm good with that... I had a couple classes back in Tech School for the AF in electronic principles. (with a LITTLE trbouleshooting but obviously not enough to figure this out)  The diodes I have layed out are like the E.Z.... Basically 2 sets of 2, facing each other... meaning diodes 1 and 2 are facing away from transformer, while diodes 3 and 4 are facing diodes 1 and 2 with the connection to the level pot ine between diodes 2 and 3... and yes, they're 1N914's. I checked them with a meter and they seem to work properly... current flows through one way but not the other... That's correct, is it not?

-aut0

[petemoore]

  IF you have a DMM with 'diode' mode, set it, put the leads across the diode, reverse the leads across the diode..one way'll make a number appear [like .7 or something like that] and the other way'll won't do anything, and the 'standby' '1' will just sit there on the screen.
  From this testing you can read the foreward threshold of that diode, [whether it's good], and which way around it is, I've never seen a mismarked diode ring though.

[RLBJR65]

Radio Shack must have a new supplier or multiple suppliers because I have 2 of those transformers 1 is in the Bob I built around 2 years ago the red and white wires are just like in the EZ schem, the other one that I bought just before the holidays has the red and white wire the way I drew it up. Good thing is that it still does not matter!

Arrrgh, I'm out of vero so I couldn't even confirm my layout! But I perfed one up since my original did not have the .0022 cap and used a pair germanium diodes instead of the silicon mod.
Definatly different than my old one. It's much brighter sounding to me.

With a fresh battery at 9.35 volts these are the pin voltages I got.
Note that even though pins 1, 8 and 7 are not used here there is still some internal stuff going on.
pin 1 - 1.36 v
pin 2 - 0 v
pin 3 - 0.01 v
pin 4 - 0 v
pin 5 - 4.8 v
pin 6 - 9.28 v
pin 7 - 4.69 v
pin 8 - 1.36 v

I played around with the diodes some, shorted across 1 still had some octave though not as pronounced

[aut0tek]

Using my BOSS power supplyat a blistering 10.72 vdc:

pin 1: 1.4
pin 2: 0
pin 3: 0.01
pin 4: 0
pin 5: 5.7
pin 6: 10.4
pin 7: 5.4
pin 8: 1.24

[soggybag]

I'm not really sure [please correct me if I am wrong, but! I think the Bobtavia is only a half wave rectifier? The diodes are two pair in series. Wouldn't it be full wave if the signal came in between each diode pair?

I built this one a few years ago and from what I remember it was suggested that you could use only two diodes if you liked or four for more output (this part never made sense to me, I would have thought that two would have had a higher output?).

I built my first one with two Ge diodes and it sounds great. I built a second with four Si diodes, as described in the Ez build diagram, and I don't think that i sounded as good as the first.

On the second one I tried the Spinal tap mod also. Which sounded very strange. Kind of a "I'm not sure if it's supposed to sound like this" kinda sound. But I had nothing to compare it to so I will never be sure if it was working correctly.

[RLBJR65]

Sorry had some problems yesterday got cut of midstream I don't even know how half got posted.

Anyway what I proposed was that we go through the circuit one step at a time until you find the problem.
The power supply looks OK, since your gain pot appears to be working correctly lets assume it's OK too, the pin voltages you posted also look good, the transformer is in right and you are sure that you have the diodes in the right way. That doesn't leave us with much!

I think I would remove and test the volume pot first because you will have to pull the diodes out to get an accurate reading.

I played around a lot with the diodes mixed and matched 1N914's, 4148's, led's and germaniums. Every combination I tried produced at least some octave. If you put them all in backwards, no sound at all. Short across one, still have some octave. Even shorting between the pairs made a little octave. Putting one in backwards produced a nasty buzzy fuzz  for a 1/2 second or so that abruptly stops like you flipped a switch off then the signal drops to something like a mild OD with no octave. The only thing I couldn't fake was a leaky diode so maybe you over heated one.

!Just occurred to me that maybe tagging on a Ge backwards would sort of simulate a leaky SI, since the Ge's have a much lower forward voltage that could work!

I'm not really sure [please correct me if I am wrong, but! I think the Bobtavia is only a half wave rectifier? The diodes are two pair in series. Wouldn't it be full wave if the signal came in between each diode pair?

I built this one a few years ago and from what I remember it was suggested that you could use only two diodes if you liked or four for more output (this part never made sense to me, I would have thought that two would have had a higher output?).

I built my first one with two Ge diodes and it sounds great. I built a second with four Si diodes, as described in the Ez build diagram, and I don't think that i sounded as good as the first.

On the second one I tried the Spinal tap mod also. Which sounded very strange. Kind of a "I'm not sure if it's supposed to sound like this" kinda sound. But I had nothing to compare it to so I will never be sure if it was working correctly.

SB I agree, it sounds much better to me with Ge diodes. I guess I would compare it to the difference between an Si. FuzzFace and a Ge one. The Si. FF's always sound to harsh to me as does the new Bob, I very much prefer the big fat mushy Hendrix type fuzz you get with Ge's, sounds much smoother to my ears.

I'm going to try a pair of 1N5817 Schottky rectifiers in it sometimes you get more of a Ge sound with them and they are much easier and cheaper to get.

I haven't tried to spinal tap it yet. 

I think you actually have 2 half wave rectifiers here which does the same thing just not as efficiently as a full bridge.


Richard

[RLBJR65]

Ah yes here we go a full wave rectifier using a center tapped transform scroll down the page a bit. http://www.allaboutcircuits.com/vol_3/chpt_3/4.html I think I have posted a link to this page before there is some good info here. IMO it is also very easy to understand.

Richard

[RLBJR65]

I would still recomend testing the pot first because a bad pot can also do some funky things.
However my simulated leaky diode did make the circuit buzz, hiss and pretty much killed the octave. The Ge I used according to my meter had a little more than half the forward voltage of the Si. So a leaky diode may indeed be the problem.

I also accidentaly dicovered a new mod. Simply disconect the blue wire from the diodes and leave it hang!
You get a very warm sounding OD that way. I added a spdt to the schem to switch between OD and Octave.
http://aronnelson.com/gallery/Bobtavia/Bobtavia_OD

Richard

[aut0tek]

Maybe that's it... I'll rip into the parts bin and find an unused set of the same diodes... See... In the beginning I think I fried that LM386 because I used an aluminum box and a metal PS connector... not realizing that it was center negative, so all my grounds were going to the positive side of the PS. Maybe it's possible I popped a diode as well? or all of them? lol.

-aut0