Help debugging an Electro Harmonix Bad Stone phaser pedal LFO

[beatnik]

Hi everybody,

I'm in very need of help. I have this phaser on my workbench and I can't get it to working right.

For starters, here is the webpage where I've taken the project from:

http://topopiccione.atspace.com/PJ05EHBadStone.html

schematic:

http://topopiccione.atspace.com/pjimages/EHBadStone.sch.gif

layout:

http://topopiccione.atspace.com/pjimages/EHBadStone.lay.gif

alternative schematic:

http://www.digitaldestruct.com/BadStoneSchematic.bmp

I've used the components specified in the project: for the CD4049 I've used the unbuffered version (don't know if the buffered version would be better..??), for the opamps I used TL072s and for the LFO the LM324 and for the transistor I've used BC549C (turned around 180 to match the pinout).

The first error I did, and it took me a while to figure out, was a short between ground and Vref. After I noticed it, I replaced all the ICs to be sure and when fired up the pedal started phasing.

However, I have a problem around the LFO I guess, because it actually isn't a sine wave sweep but more of a stepped one. Here you can listen to a sound sample to have a clearer idea:

http://soundcloud.com/mxxp_labs/bad-stone-lfo-sample

Another strange thing, when I switch from Manual mode (the LFO is out of the circuit and the phasing sweep is determined by the manual pot or expression pedal) back to Auto mode, the LFO stops working definitely and I have to power cycle the pedal to get it working again.

I've already checked all the connections and components values, replaced all the semiconductors with fresh ones and the polarized caps and diodes too, but I can't figure out what's wrong in the circuit.

As I'm totally stuck with it I really hope for your help. Any additional information or measurements I can provide, just ask.

Thanks in advance

 

[CJ]

if i remember right, you need to be careful with the type of caps you use in the phase shifter part, all those .047uf or whatever they are,

so it works but sounds funny?

if an osc will not self start, you can try moving wires around until it self boots, or it may get a little better with some burn in,

i am not a big opamp guy so i do not know if you have the right chips in there,
why not use the original noisy 4558 or whatever from radio shack just for an experiment? 

 

[beatnik]

Hi CJ, thanks for answering

do you mean they need to be in a very very narrow tolerance range? I just used normal box film caps which I suppose have +/-10% tolerance range. Is that enough?

Are they possibly the only cause of the weird lfo shape? Or am I missing something else?

And the start problem is just some of the off board wiring interfering with the signal path?

What do you mean with some burn in?

I used TL072s as opamps since the project reported them as ok.

 

[okgb]

I'd try some of the gtr pedal newsgroups , better chance someone has built one there and
ran into similar problems , Is the pcb associated with a particular grp that give some support ?

 

[CJ]

i think you need ceramic caps in there,

transfer function is -2arctan(wRC) so to get 90 degrees exact phase shift, you want zero inductance in the equation.

minus sign just means you are lagging the signal by 90, not leading it,

have you heard a real small stone?

they sound kind of weird to begin with,

like this guy says>

"I prefer ths EH phaser over the MXR types which are its nearest in class competitor. Both units have a classic phase sound but the Small Stone has a particularly weird warble sweep that is unique to its tone. No other phaser I have used has the same tone. I cannot say either the MXR or Small Stone is "better" it's just a matter of the tone you like. Jimmy Page used the Small Stone live to add a sweep warble to some of the tunes he had sometimes used a flanger or manually flanged. If you have seen/heard the last Zep concert before Bonham passed Knebworth '79 you will hear this unique tonal phaser on Nobody's Fault But Mine, Achilles Last Stand and some on In The Evening. The Color switch reduces the full lushness of the sweep but adds a really unique compounded sweep to the sound which sounds mutlt-timberal. That is where Page used it. Color switch down and it is more a normal full sweep phase. Seems to reach its best tones with the speed from 10-1:00. All the way down it is very stubtle and can be used to add a little background to your tone without really whipping it out of control. I do not really use the phase at full speed because I do not find the sound very musical. You would do well to get a Leslie tyoe unit for that if that is your object.

 

[beatnik]

i didn't think about capacitors type. i've found some pictures of an original bad stone and shows it's using greenies capacitors (mylar film??)

http://www.home-wrecker.com/BScomp.jpg

anyway, I'll try to replace mine with ceramics matched for tolerance, and i'll report back to you.

i know the sound of the bad stone since i've listened a lot to sound samples over the web before starting the project.

it definitely has a particular sound, i guess the q of the filter is kinda narrow so you get that almost wah-like sound, but i'm sure mine has something wrong because of the different lfo shape.

here is a sound sample from an original unit, and the one from my pedal.

http://www.home-wrecker.com/bstone1.mp3

http://soundcloud.com/mxxp_labs/bad-stone-lfo-sample

please keep helping, i dont know how to figure it out by myself. i tried to post on specific pedal building forums, but nobody came up with a solution yet.

 

[beatnik]

bump  :-X

 

[CJ]

ok, maybe not the caps,  :-X

those look like some type of poly material,

you might need to put a scope on those opamps to see what is going on with the LFO,