DOD - FX75B Stereo Flanger  [schematic]

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DOD - FX75B Stereo Flanger

Postby digitalzombie » 28 Aug 2012, 16:58

You can really get some whacky sounds out of it just by tweaking the internal trim pots. What I want to do to it though is just a little beyond me, but probably easy concept to more experienced tweakers. First off, I want to add some sort of wet/ dry blend control. The volume goes up noticeably when I've got the controls all whacky, not to mention the delayed signal can make the original note indistinguishable.

And Second, I want to mount this blend control, plus controls for the internal trim pots, externally. I saw a video on youtube when the guy had a plate over the battery compartment with the knobs for the now-externalized trim pots. It also sounds like he's getting much more of a blend than I am, so maybe he's already done this mod.


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Re: So I bought a DOD FX75-B Flanger and it's wild!...

Postby skylark44 » 07 Mar 2013, 04:25

I've got this pedal and I want to know what those trim pots control ...anyone know?
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Re: So I bought a DOD FX75-B Flanger and it's wild!...

Postby lolbou » 07 Mar 2013, 10:55

Here's the schem.

P2 (50k) appears to be the BBD bias.

P3 is for the clock frequency setting (MN3101).

Schem numbering goes up to P7, but along with the 4 control pots, I only see two trimpots, and, well, no P1. Or did I just miss it completely? :scratch:
- Are you a mod or a rocker?
- Uh, no, I'm a mocker.
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Re: So I bought a DOD FX75-B Flanger and it's wild!...

Postby Dirk_Hendrik » 07 Mar 2013, 11:56

lolbou wrote:P2 (50k) appears to be the BBD bias.


Uhuh... it's a wet level control.

The more interesting question is: How is this BBD biased? :scratch:
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Re: So I bought a DOD FX75-B Flanger and it's wild!...

Postby lolbou » 07 Mar 2013, 14:17

Dirk_Hendrik wrote:Uhuh... it's a wet level control.
Makes sense - obviously - ... :slap: :wink:

Dirk_Hendrik wrote:The more interesting question is: How is this BBD biased? :scratch:
Might be that missing P1, which has not been drawn? I didn't find guts to count trimpots...
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- Uh, no, I'm a mocker.
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Re: So I bought a DOD FX75-B Flanger and it's wild!...

Postby tube-exorcist » 07 Mar 2013, 14:27

lolbou wrote:
Dirk_Hendrik wrote:Uhuh... it's a wet level control.
Makes sense - obviously - ... :slap: :wink:

Dirk_Hendrik wrote:The more interesting question is: How is this BBD biased? :scratch:
Might be that missing P1, which has not been drawn? I didn't find guts to count trimpots...


exactly.... P1 ..... a 50k trimmer with a 470k resistor from the slider to the input of the BBD.

That schematic (like some others) from the web is NOT a factory schematic, it was traced by somebody not so skilled several years ago and all of these schematics contain mistakes (e.g. the DOD FX25B schematic contains at least six mistakes).
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Re: So I bought a DOD FX75-B Flanger and it's wild!...

Postby Dirk_Hendrik » 07 Mar 2013, 22:52

This one? And how many to be found without the real unit at hand?
dodfx25b_envelope_filter.pdf
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Re: So I bought a DOD FX75-B Flanger and it's wild!...

Postby tube-exorcist » 08 Mar 2013, 00:38

Dirk_Hendrik wrote:This one?
Yep
Dirk_Hendrik wrote:And how many to be found without the real unit at hand?

Lets see how many you will find by analyzing this schematic logically. Here is a start:
Looking at the inverting input of the OTA U2B, is there a (1k) resistor (R 11) to bias to be expected or not ?
Looking at pin 5 + 7 of OTA U2B, is there a (10n) capacitor (C6) to ground to be expected or not ?

Now it is your take.... maybe this could be of use:
http://www.americaspedal.net/fx25b/
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DOD FX75 - B STEREO FLANGER  [schematic]

Postby robrhy » 06 Aug 2013, 21:53

Just bought one of these on Ebay and as it's a bit beat up i'm thinking about re-housing it,so i was wondering if anyone knows if it's true bypass ?
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Re: DOD FX75 - B STEREO FLANGER

Postby iq01221 » 06 Aug 2013, 21:59

It's not true bypass, but U can mod it easly :thumbsup
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Re: DOD FX75 - B STEREO FLANGER

Postby robrhy » 06 Aug 2013, 22:07

iq01221 wrote:It's not true bypass, but U can mod it easly :thumbsup

Ok Thanks.I'd opened it up and looked inside and i did'nt think it was.
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Re: DOD FX75 - B STEREO FLANGER

Postby astrobass » 08 Aug 2013, 04:21

Play with the trim pots. This is what I managed to get it doing with very careful tuning: https://soundcloud.com/ryan-mcpherson-7 ... nger-fx75b

EDIT:
Also, this is an awesome pedal. Completely different circuit from the FX75-C, which is also very excellent but a little too hifi sounding for me. I prefer the colder sound of the 75-B. The casing is decent, reasonably durable, but has a stupid 3.5mm TRS style DC jack. I haven't tried removing it, drilling a new hole and upgrading to a 2.1mm negative pin yet. If you do, let me know how it goes. Very interested.
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Re: DOD FX75 - B STEREO FLANGER

Postby robrhy » 11 Aug 2013, 13:32

astrobass wrote:Play with the trim pots. This is what I managed to get it doing with very careful tuning: https://soundcloud.com/ryan-mcpherson-7 ... nger-fx75b

EDIT:
Also, this is an awesome pedal. Completely different circuit from the FX75-C, which is also very excellent but a little too hifi sounding for me. I prefer the colder sound of the 75-B. The casing is decent, reasonably durable, but has a stupid 3.5mm TRS style DC jack. I haven't tried removing it, drilling a new hole and upgrading to a 2.1mm negative pin yet. If you do, let me know how it goes. Very interested.


While i'm waiting for the new enclosure to arrive this is what i've done.Adjusted the trim pots to give me a clean flange,and as i'm not going to be running it in stereo removed the stereo socket and replaced it with a 2.1 power jack but it just would'nt work with that.Re-connected the battery snap and it was fine.After that i removed it from the box connected it to a 2.1 power jack,2 mono sockets,a footswitch wired for true bypass + led,tested it and it works fine.To do that you don't need to use the wires for stereo out,switch or led.
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Re: DOD FX75 - B STEREO FLANGER

Postby deadcopdisco » 29 Jan 2015, 03:08

I am also working on re housing my fx75B. I already adjusted the trim pots to my liking. I had all the guts out and was playing my guitar through it but I had to use a battery because I couldnt get the power supply to power it even though it worked before removing from case. Does the PS need to be grounded to the case? Can i ditch the battery snap when i figure the PS out? Any pics on how to true bypass? Im a total noob :D
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Re: DOD - FX75B Stereo Flanger

Postby guitarist » 26 Feb 2018, 05:48

Hey, just checking in on this. So P2 is a blend or mix pot?!
I upgraded some caps but it's a wee dry so I'll give this a shot. I also noticed that the buffer sure brightens and nets a wee boost too though its way down the board - prior to reverb and delay.
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Re: DOD - FX75B Stereo Flanger

Postby lolbou » 27 Feb 2018, 10:10

guitarist wrote:So P2 is a blend or mix pot?!

Looks like the dry signal level is fixed and alway on at the input of the mixer stage (U5A).

What P2 does is just adjusting the wet level (as per Dirk_Hendrik's remark) to be mixed.

The wet is cut by the FET (Q7) when the effect is switched off.
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