DOD FX69b Grunge - modifications  [schematic]

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DOD FX69b Grunge - modifications  [schematic]

Postby pinkjimiphoton » 05 Aug 2011, 03:28

hi guys,

was out earlier buying parts (what else) and stopped at a pawn shop, lookin' around...found a really cool old silvertone lapsteel, almost jumped on it...150 bux i didn't have.

anyways, as always, i asked if they had effects...

and they had one...a dod fx69b grunge.

don't close the window yet, tho. ;) it gets better.

i ask 'em what they want for it, they say what will ya give me?

so i, of course, being ever cheap, say 5 bucks.

he says eight. i say sold. i mean, for 8 bucks, ANY pedal is worth it.

so i bring it home, expecting it to suck heinously.

but believe it or not, it sounded pretty good. so i figured, after chilling with fine folks like you, to molest it badly and see if i could take it from ok to HOLY SHEET.

i actually had some success. here's a schematic...no idea if it's right. i don't know much at this point, still a newb.

Image

so i did a search around for mods, and found this:

http://www.thestompbox.net/forum/showth ... IY-Upgrade

this was the suggestions made:

"Pedal Hacker's Grunge Pedal DIY Upgrade

I finished it and it sounds amazing, way better than the OD-2 kit!! 2 resistors, 2 caps, and an LED diode! What a difference a few little components can make!! Here are the basic instructions if anyone is interested:

DOD GRUNGE MOD

REMOVE C19. YOU DON’T NEED TO REPLACE IT WITH ANOTHER COMPONENT OR ADD A JUMPER.
THIS ADDS MIDS AND GIVES A FULLER TONE.
CHANGE R16 TO A 1K RESISTOR. THIS LETS THE PEDAL CLEAN UP WHEN THE GAIN IS TURNED DOWN.
CHANGE R19 TO A 22K RESISTOR.
THIS MAKES THE DISTORTION A BIT RICHER WHEN CRANKED.
NOW CHANGE C24 TO A .0047CAPACITOR. THIS MAKES THE ‘HIGH’ KNOB AS MORE LIKE A HIGH KNOB SHOULD.
THIS CAP IS POLYSTYRENE, WHICH IS THE ‘HOLY GRAIL’ OF FILM CAPACITORS. EXTRA CARE SHOULD BE TAKEN WHEN SOLDERING AS THESE ARE MORE HEAT SENSITIVE THAN OTHERS.
CHANGE D2 TO THE SMALL YELLOW LED.
THIS GIVES A MORE COMPLEX, ENHANCED DISTORTION.
DON’T CHANGE C6, EVEN THOUGH IT’S MARKED IN THE PIC."


of course, here's the pic:

Image

i did some of the stuff,...changed r 16 to 800 ohm (it was handy) and had an instant difference in the response of the pedal.

also changed r 19 to a 24k resistor (i bought a huge grab bag of resistors earlier, and that was close enough to 22k for rocknroll)

took out c 19. it really DID help the sound of the pedal.

but what i did differently was remove d2 completely. get it the f*** outta there!! was like turning the suck knob way down.

so then i took out d3 (just below d2 in the pic) and replaced it ultimately with a ge 1n34a.

that did it. pedal sounded a bit nasally (which i like) and cleaned up pretty well with my strat's volume knob. not completely, but enough to play a relatively cleanish rocknroll rhythm.

huge difference...i mean i couldn't believe it.

but wait. i got creative. here's where stuff started to get strange.

i read all over the place about how peeps hate the 4558 chips in these things. they were actually marked something else, but i didn't bother writing it down.

i had read about stacking chips, and had an NTE889M "dual low power jfet opamp", so i tried stuffing it on top of the '58s on the board.
there's 3 of them, the left and right ones (looking at the component side of the board) didn't do a whole lot, but the one in the middle was hip.

i mean REAL hip...it's subtle, but seemed to add in noticeable headroom and sparkle (which, with my luck, probably has nothing to do with it being placed there.),

so i soldered that puppy's 8 legs to it's arachnid brethren on the board, and fired it up.

i was like..."yeah!!". it sounds like it probably should have in the first place now, to my ear, much better, louder and with more overdrive and slightly less unusable distortion.

(you can turn the guitar off even with everything pegged, and the pedal won't feedback by itself like it did. ;) )

but...i screwed something up. oooooopsies....

now, even tho the switch bypasses the effect, the bloody led won't go out.

i tried un-doing the mods i did, but to no avail...and am stumped. i mean, i can LIVE with it being messed up and always being on, but i'd really like to fix it if anyone has any ideas as to what the heck i did.

so ...long story short...

found a cool mod for a pos pedal.

and broke it at the same time.

can somebody send me a clue as to what to look for, or what i did, or how to fix it?

thanks in advance!!

jimi
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby JiM » 07 Aug 2011, 23:15

Hey, that's a nice story !
Getting an old crappy pedal for cheap, and pimpin' it to something unique, is the true spirit of this subforum. :applause:

Here are some details (and suppositions) about the purpose of the mods you did :
  • Removing C19 (the only 15nF cap in the schematic) takes out of the circuit one of the transistor-based gyrators. Probably a boost at a fixed frequency that turned out to be harsh or muddy.
  • R16 is the 10k resistor next to the gain pot, it sets a minimum gain. Lowering its value allows the control to reach milder overdrives, or even crunch tones.
  • R19 seems to be the series resistor after the clipping diodes. Reducing its value lets more current through.
  • The clipping diodes are the heart of the distortion. A pair of silicon diodes clip at about +/- 0.7V. The suggestion to replace one of them with a LED is to get asymmetrical clipping, and a moderate boost in volume. This is due to a higher threshold of the LED, above 1V and depending on color.
But what you did was more radical : by removing the diodes completely, you let the whole amplitude of the preamplified signal to attack the tone-shaping stages, and drive them into non-linear regions. No more a textbook distortion, but a heck of an overdrive. Now the volume pot deserves its name.
Adding back a low-threshold germanium diode (about 0.3V) clips the signal heavily again, but still asymmetrically : you get a lot of even-order harmonics. A nice addition would be a on-on-on switch to select various combinations of clipping diodes (or absence thereof).

The opamp that you enhanced by piggybacking another opamp seems to be the first tone-shaping stage. The one with a gyrator removed, and also the one that sees the increased voltage after the clipping diode(s), and increased current after R19. It seems logical that an increase in current handling capability is beneficial there. But it also seems that half of this opamp is unused ...

Except the opamp stacking, it reminds me a lot of the mods i did on a Danelectro FabTone. I still have to document that. :oops:

Now about your question :
The bypass stuff in a DOD pedal is handled by the 4007 CMOS chip (the 14-legged guy), wired as a flip-flop. This is well documented elsewhere in the forum. But as the actual bypass seems to work fine, i suppose that the problem lies next to the LED itself. By the way, it seems to be next to the clipping diodes (on the picture, i can just see the pins of something that is on the solder side).
Could there be a solder bridge that permanently allows a current through the LED ?
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby pinkjimiphoton » 07 Aug 2011, 23:40

wow, jim (fellow jim!!)
thanks for the explanation...it all makes sense now pretty much of why it worked the way it does. it's a really nice nasal warm overdrive with a smooth sustain now, kinda like a big muff notched back to about 9:30 on the tone control. it really nails that woman tone too when ya roll down the tone pot on the guitar, but that zappa-ish midrange honk (but smoother and browner) is pretty hip compared to what it sounded like originally.

so...are you saying i can add in another gain stage as well by using the unused half of the chip? that may be cool, too, to add a nice footswitchable boost...i am thinking of re-housing the pedal anyways, so another switch would be hip.

good call on the solder bridge, i will look at that later...thanks so much! ;)

peace!
jimi
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby HydrozeenElectronics » 08 Aug 2011, 04:33

How about some sound clips :applause:
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby pinkjimiphoton » 08 Aug 2011, 04:54

i can try, still haven't hooked up my daw yet since i moved a year ago. maybe some video...let me see if i can do it tomorrow!
wish i had a before/after so ya could hear the difference tho. ;)
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby Dirt Rich » 31 Dec 2011, 03:27

Any audio clips for this? I've always liked the dod grunge, so I'd love to hear it modded out.
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby pinkjimiphoton » 31 Dec 2011, 17:04

i'll try and up one, if i remember when i get a chance bro.

it sounds kinda like a cross between a big muff and a klon...something in the midrange..
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby cassette_tape » 15 Jan 2012, 06:39

Hey there. I found this thread after deciding to cut my teeth with pedal mod-ing on my Grudge pedal.

I removed the C19, changed the R16 to a 1k resistor, changed the R19 to a 22k resistor, changed the C24 to a .0047 capacitor, and removed the D2. I believe I did everything as dictated on this thread (at least choosing all of the mods that are available to me) and did a pretty thorough soldering job, but now when the pedal is engaged the volume is about 1/3 of what is used to be. What kind of newbie mistake am I making?
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby pinkjimiphoton » 15 Jan 2012, 14:46

did ya try stacking another opamp in there?
wonder if maybe there's more than one version of the circuit?
mine's crazy loud still...
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby cassette_tape » 16 Jan 2012, 00:51

Newbie mistake found. I replaced C24 with a .047 cap not a .0047 cap.

Thanks for the post, this cheap pedal I inherited from my stoner drummer is now a vicious!
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby pinkjimiphoton » 16 Jan 2012, 01:33

kewl!!! glad it worked for ya bro!!!!

really huge difference i thought..i got the pedal for 10 bux at a pawn shop cuz, well, it was a 10 dollar pedal...but man, did it SUCK stock,

:applause: :applause: :applause: :mrgreen:
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby TheShadowKnows » 05 Feb 2012, 09:57

I did a mod on my DOD Grunge but not so extensive as the above mentioned. I'm very green at this and only did a diode swap, but still got excellent results from it.

I put in a pair of Radio Shack 276-0304 Green/White LED"s 5mm 2.1VDC. This brought the screaming gain down to a reasonable level. Unmodded the gain starts at 10 and goes up from there. With just adding those LED's it starts at about 6 and goes up. The pedal still has that slight nasal tone to it. Since reading about the other mods done I might try them out. But the voice of that pedal is that nasal tone so who knows. I might keep it.

With the above LED mod the gain starts at 'above crunch' but 'below distortion' which is my preference. But you still have tons of gain on tap so choose cruch, heavy crunch / lite distortion or full blast distortion. That's what I came up with.

Can't wait to try out the other mods above. Should be fun. Thanks for all the info. The pic below is of the 2 Green/White LED's installed.
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby pinkjimiphoton » 05 Feb 2012, 16:51

cool, you gave it more headroom, AND more crunch!!

well done!! ;)
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby TheShadowKnows » 06 Feb 2012, 01:32

I just tried the C19 cap pull and it did really tame the edgyness to the pedal.
It made it more of a manageable distortion. That's good.
I already have 2 manageable distortion pedals, one being my Dan Electro FAB TONE.
Did a few mods on it that I read about here and WOW! This place ROCKS!
-
So I put back C19 and the pedal became more like a Dual Rec that plays nice with others.
So I'm keeping it that way. It's a matter of what's in my collection and my only other metal
pedal is a modded Dan Electro FAB Metal. So now I've got 2. Lucky me. Another mouth to feed.
-
Great mod tips in this post.
Thanks guys. Now all I need to figure out is everything else.
Guess I'll have to buy another junker stomp box and learn another lesson.
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby DrNomis » 06 Feb 2012, 11:36

Here's my contribution to this thread... :D

Although I didn't break this pedal, I still made some mods to it and a couple of them are purely aesthetic, anyway, a good friend of my younger brothers's gave me some pedals and one of them was a Dod FX58 Metal Maniac, it needed a new 9V battery clip and a replacement footswitch, so after I got it working properly I decided to make some mods to it, here's what I did:

* Replaced the three knobs with three new black and yellow ones.

* Replaced the stock 5mm Red Led with a 5mm Blue Led.... (yeah I admit it, I have a penchant for these).

* Replaced the three stock NE5532 Op-Amp ICs with TLO72s (made the distortion sound a bit less brittle).

* Added a spare plastic battery cover that originally came from a Dod Grunge pedal.


Here's a couple of pics of the pedal:
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby pinkjimiphoton » 06 Feb 2012, 15:47

looks great simon!! ;)
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby DrNomis » 06 Feb 2012, 16:04

It uses a mixture of Silicon and Germanium diodes to do the clipping, I've been toying with the idea of desoldering all the Silicon diodes and replacing them with Germaniums, by that I'm hoping to soften the clipping a bit, do you think it's worth trying?.... :hmmm:
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby TheShadowKnows » 06 Feb 2012, 16:45

I've been experimenting with LED's for diodes and have been having some good results.
You could try out different pairs and see how they sound.
Just another option.
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby pinkjimiphoton » 06 Feb 2012, 18:14

leds can sound great....more buzz/crunch. my personal fav is a diode clipper made like this...

ge 1n34a to 1n914 to ground on one side, 1n34a to 1n914 to 3mm GREEN led to ground. really tube like, a real cool combo...you get the warmth of the germanium, the crunch from the silicons, and the volume boost/buzz from the led. can really bring a dirt pedal to life...try it! ;)
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby TheShadowKnows » 06 Feb 2012, 22:50

:popcorn:
that recipie sounds yummy. Off to Radio Shack I go.
Another project needs updating again.
My tinkering is never done!
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