DOD FX69b Grunge - modifications  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.

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

Postby astrobass » 18 Apr 2013, 03:04

Yeah, it's definitely not going to resemble a fuzz face because the way it's clipping the signal is completely different, this is just an amplified signal running past diodes to ground. Glad to hear it worked out, though! Now I know if I see one of those pedals being sold used locally on the cheap because it's just a bunch of hissy noise I'll be able to do something cool with it ;)
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby astrobass » 15 May 2013, 05:44

Warning to anyone who finds the Digitech version of this pedal and thinks they'll do these mods: The board layout is different, it uses a 3 layer PCB, and most of the components are now tiny SMD components. From what I can tell the labelling is still 1:1 for the DOD schematic, though some components have seen subtle changes. D2 and D3 are now both SMD transistors that are wired as diodes, for instance.

I can't confirm by staring at the board if D3 is still D3, but I can tell that D2 is still D2, and I've experimented with replacing that with both a yellow LED and a 1N270 and it acts as one expects.

C6 is also still C6. Swapping it for 1uF still does what you'd expect (ie makes this pedal fucking rad).

C19 is still C19 and is easily removed, but it's now 0.015 instead of 0.013 uF. C20 is now a tiny SMD cap and there's no value indicated on it. It's also clustered far too tightly with other SMD components for me to remove it and test it at my skill level, so I'm not going to.

It's a bit of a shame because the Digitech equivalent comes in a MUCH nicer, much sturdier enclosure, and has two outputs, a mixer output and an amplifier output. Also the DC power jack is now the standard 2.1mm negative tip that everyone's accustomed to. If you're good with a soldering iron and you've got an exceptionally long, fine solder tip, it's the one to go for, but if your soldering iron is even a tiny bit bulky or your hands aren't steady, the way the components are nested will present a significant challenge.
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby skylark44 » 31 May 2013, 04:46

I've resoldered all the mods back where they belong, but I'm still having the same issue...as soon as I plug into the pedal, I get a weak sound, but it fades out almost immediately...then, nothing :roll: :? ...I'm stumped :scratch: :hmmm: . Any help would be appreciated...thanx. :mrgreen:
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby pinkjimiphoton » 31 May 2013, 16:53

sounds like a cooked electro coupling cap
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby skylark44 » 01 Jun 2013, 04:50

So, how do I fix that? :hmmm: (what's it look like & where is it? :? :scratch: ) :mrgreen:
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby pinkjimiphoton » 01 Jun 2013, 06:04

look for dc voltage on both sides of whatever caps, if ya see it on both and it's the same, that's probably the culprit.... maybe :scratch:
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby skylark44 » 01 Jun 2013, 23:53

Thanx Jimi...I'll check it out (again :roll: :? ). :mrgreen:
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby skylark44 » 01 Jun 2013, 23:55

pinkjimiphoton wrote:sounds like a cooked electro coupling cap

You mean a capacitor...right? :| :mrgreen:
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby pinkjimiphoton » 02 Jun 2013, 00:42

yah, a capacitor.

we used to call 'em coupling/blocking caps in the old tube days, cuz either ya used 'em to couple one part of a circuit together or to block dc from one stage to the next.

so... if ya see dc on both sides of a cap, especially the same amount of dc, odds are the cap's bad.
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby flowmastergfunk » 04 Jun 2014, 15:30

Just read through this whole thread. The early fx69 has been one of my favorite dirt pedals, disgusting as it may be. At the moment, I have two. I did a dual DOD rehouse with a grunge and BASS grunge combo, that can be seen here viewtopic.php?f=10&t=24265

At first, I made an attempt to replace the OP amps with sockets, so I could experiment with changing them out. The only changed I made to the pedal were the aforementioned sockets, and the removal of the level potentiometer, so it could be repositioned. After getting it all installed, I realized I messed something up...so I just ordered another fx69 and put the unmodified version in my dual pedal.

I have been trying to diagnose the pedal off and on, and I feel like I am getting close. I have spent hours probing, and managed to find a two shorts, fixed them, but it still doesn't work. No output when the effect is on. Happy to learn the trick for checking for DC on both sides of the caps! Hopefully I can narrow something down today and get it solved! I am looking forward to listening to the difference in changing the OP amps to 3 TI rc4558p chips. It will be nice to have two working pedals, so I can compare one mod at a time.

Long live the Grunge!
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby astrobass » 05 Jun 2014, 00:52

I could've sworn the 4558 was the chip already in there. Are you going to all this effort just to try different 4558s, or am I remembering wrong?

EDIT:
I was wrong. Remembered I had the schematics, the BOM lists NJM4560 x 3 for the dual op amps. I think there IS some rail clipping in this one, so it'll be interesting to learn whether the 4558 changes the sound for the better. I recall reading that the 4558 does in fact rail clip nicely.
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby flowmastergfunk » 17 Jun 2014, 01:04

In the version that I had, it did have a form of 4558 chips in it. I honestly don't know much about what the heck I was doing, other than experimenting. I don't know about rail clipping and such...In the world of being a pedal mechanic, I am pretty much a parts changer, not a true diagnostician.

The only reason I got the idea was from reading discussions of ts-808 pedals and how varieties of the same chip could change the sound. The tube screamer I have is equipped with a socket to facilitate chip changes, which gave me the idea.

I did pretty much give up on the broken grunge that I had, but ended up pulling c19 in my rehoused version. I hooked up two on/off/on toggles with different cap values and really enjoy the different tones I found! The lack of c19 in itself makes for a great and simple mod, and I am happy I went there.

I am going to spy on ebay eventually and get another fx69 on the cheap, but right now, my main interest has shifted back to the bass grunge. They are fairly rare and the prices are skyrocketing on ebay these days. I would really like to map out the board and make a clone, minus the DOD switching. Maybe even make it able to support 18v. I love my fx92 and I am terrified of not having one on my board :/ I need a backup!
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby flowmastergfunk » 27 Jun 2014, 19:50

NOW I am getting pissed! I got a broken fx86b death metal off flea bay, fixed it, modded it one piece at a time while testing it, and STILL ended up with a broken pedal, the same way I killed my fx69 that I abandoned!!!

It came in with a burnt trace. Fixed it, worked perfect. Took out c19 since it's similar to the grunge, GOLD! Put it on an on-off-on toggle with two different values, and it is an epic mod in itself.

Replaced the two jrc4560 chips with sockets and rc4558p, tested after each one, worked fine. Replaced the volume pot, worked fine. Mounted it in the case, and it started PULSING! I resoldered all the points to make sure there was no weak connections, and a few times it was cured. Tighten down the board or twerk the twist a knob, or wiggle the socket a bit, and the issue would re emerge. I put it back together several times only for it to start freaking out on me.

I ended up taking the sockets out and hard soldering the chips in the board, to delete possible weak contacts. Now I plug it in and it moo's like a damn cow.

I started figuring that it was a nearby component that was heat sensative. I took the time to make sure there were no shorts. I used a 40w iron with a needle tip, but worked quick and would turn the iron on and off as not to get too hot.

C27 and c28 are the only things near the volume pot, but I have not had the means of replacing them yet. I don't even know what the symptoms suggest!

Now, it has directly come to effect the bypass. The LED came on when I first plugged it in, then faded away. The switch was having an effect, although both distorted and "bypassed" tones sounded abnormal. After playing around with it for a minute, the bypassed signal came through, and the on position gave no output signal whatsoever.

Frustrated as I may be, I have NO idea where the problem is. I think it has traveled and turned into multiple problems.

This pedal may end up being another lost soul, but I am not completely over my attempt.

My biggest goal now is to completely delete any component related to the bypass circuit! I want every bypass component completely off the board, and would like to analyze the circuit from here on out as the effect only. If I can bring life to it, I have no problem with rehousing it and true bypassing it. Any help with removing the bypass circuit and adding jumpers would be appreciated! I will paypal someone a beer!

BTW, the pedal sounded great before it crapped out! It was no longer an uncontrollable feedback unit!
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby pinkjimiphoton » 29 Jun 2014, 15:14

how big was the cap you replaced?
sounds like it may be motorboating. too much bass in a high gain circuit can get real unstable and start to self-oscillate.

mine, i ended up having an issue with the bypass too... the led is all fucked up. sounds good, but in bypass the led still glows enough to be really fucking annoying.

it also sounds like the jfet that does the switching may be damaged and letting dry and wet signal combine.

no idea what to do bro, sorry good luck :scratch:
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby astrobass » 11 Jul 2014, 02:03

The schematic on page three of this thread is accurate and makes this obvious. The useful part of the circuit really begins with C4 and ends with C26. That drops the transistor buffers but I don't think it'll have any impact on tone whatsoever. The circuit doesn't end with the tone stack, there's an op amp right before C26 and C4 connects directly to an op amp as well.
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby Lostboy » 09 Jul 2015, 17:05

I know this is old, but I wanted to comment. I am a COMPLETE beginner and have never taken a soldering iron to a stompbox before. After reading through this thread I felt confident enough and decided to try. I acquired two old Grunge pedals over the years. Neither ever gets used. After completing pinkjimiphoton's mods, I have to say... this pedal sounds GREAT. The only problem I have is that to get the really crunchy drive I have to have the volume knob all the way up. This is only a problem because when bypassed the volume is significantly lower. I'm sure there is some sort of fix for this, but again I know very little about these types of electronics. I just thought it was worth signing up and telling everyone how great this mod really is. On lower settings (with my setup) it even has a real bluesy sound. Very cool.
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby Lostboy » 09 Jul 2015, 17:07

pinkjimiphoton wrote:how big was the cap you replaced?
sounds like it may be motorboating. too much bass in a high gain circuit can get real unstable and start to self-oscillate.

mine, i ended up having an issue with the bypass too... the led is all fucked up. sounds good, but in bypass the led still glows enough to be really fucking annoying.

it also sounds like the jfet that does the switching may be damaged and letting dry and wet signal combine.

no idea what to do bro, sorry good luck :scratch:


Mine does do the LED thing. Its almost like the Seeing Eye mod because it gets brighter as I play. It didn't do it until piggybacking the opamps.
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Re: so, i broke a new dod pedal tonite..but found cool mod..

Postby pinkjimiphoton » 10 Jul 2015, 05:28

Lostboy wrote:
pinkjimiphoton wrote:how big was the cap you replaced?
sounds like it may be motorboating. too much bass in a high gain circuit can get real unstable and start to self-oscillate.

mine, i ended up having an issue with the bypass too... the led is all fucked up. sounds good, but in bypass the led still glows enough to be really fucking annoying.

it also sounds like the jfet that does the switching may be damaged and letting dry and wet signal combine.

no idea what to do bro, sorry good luck :scratch:


Mine does do the LED thing. Its almost like the Seeing Eye mod because it gets brighter as I play. It didn't do it until piggybacking the opamps.


yep, exactly what happened to me. i dunno if i even still have it, i've gotten rid of a lot of pedals i never use anymore.
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FX69B help

Postby fortj3 » 28 Oct 2017, 17:26

I have a DOD FX69B Grunge pedal that I got for free with some other things I bought.
As received, it did not work. I disassembled it and found the D7 diode shorted and the trace between D7 and J1 was fried.
So, I soldered in a new 1N4001 rectifier diode and point-to-point soldered in a wire between D7 and J1.

The good news is that it now works, and it provides massive distortion. (more death metal than grunge, tbh).

The bad news is that it is a noisy pedal. I get a hum/hiss/noise through my amp when the pedal is plugged in and turned on.
Kind of like, if you unplug your cable from the guitar and grab the cable plug so that you're touching the shaft and tip of the plug at the same time.
Is this normal for this cheap pedal, or is there something more that I need to replace/repair?

Or, is it possible that one of my solder joints on D7 is not as good as it could be?

The schematic for it is here on the FSB forum, but I don't know how to post it.

TIA
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Re: FX69B help

Postby fortj3 » 28 Oct 2017, 17:29

BTW, I'm running it off a 9 volt battery, because the 9 volt DC adapter that came with it is putting out 12.25VDC.
I don't know if the extra 3.5 volt output of the adapter was the reason the trace was fried, but I didn't want to risk it without knowing more.
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