DOD - FX75B Stereo Flanger  [schematic]

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

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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skylark44
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Post by skylark44 »

I've got this pedal and I want to know what those trim pots control ...anyone know?

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lolbou
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Post by lolbou »

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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Post by Dirk_Hendrik »

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:
Sorry. Plain out of planes.

http://www.dirk-hendrik.com

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lolbou
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Post by lolbou »

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...
- Are you a mod or a rocker?
- Uh, no, I'm a mocker.

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tube-exorcist
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Post by tube-exorcist »

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).
"I've noticed there's an inverse relationship between cost of gear and talent. If you need the most expensive gear to get decent tones, then you suck as a player."

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Post by Dirk_Hendrik »

This one? And how many to be found without the real unit at hand?
dodfx25b_envelope_filter.pdf
(207.56 KiB) Downloaded 280 times
Sorry. Plain out of planes.

http://www.dirk-hendrik.com

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tube-exorcist
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Post by tube-exorcist »

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/
"I've noticed there's an inverse relationship between cost of gear and talent. If you need the most expensive gear to get decent tones, then you suck as a player."

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Post by robrhy »

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 ?
Attachments
DOD_FX75.pdf
DOD FX75 - B STEREO FLANGER factory schematic
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iq01221
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Post by iq01221 »

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

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Post by robrhy »

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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Post by astrobass »

Play with the trim pots. This is what I managed to get it doing with very careful tuning:


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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robrhy
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Post by robrhy »

astrobass wrote:Play with the trim pots. This is what I managed to get it doing with very careful tuning:

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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deadcopdisco
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Post by deadcopdisco »

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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Post by guitarist »

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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Post by lolbou »

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.
- Are you a mod or a rocker?
- Uh, no, I'm a mocker.

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Post by charonodaemon »

New to the board and have been trying to resurrect my old FX-72 Bass Flanger.

This unit is not sweeping, but I can tweak and hear shifting with the delay adjustment.

Doing some research and the build is about the same as the 75B.

The schematics provided on some of the older posts have been a great help.

There is a lot of talk on P1, P2 and P3 and my curiosity is what they control exactly?

Looks like biasing voltages for IC sets and wondering what the factory settings/voltages on those ICs are supposed to be.

Any help would be greatly appreciated!

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Post by ZonkerRoberts »

charonodaemon wrote: 27 Apr 2021, 16:34 New to the board and have been trying to resurrect my old FX-72 Bass Flanger.
This unit is not sweeping, but I can tweak and hear shifting with the delay adjustment.
Doing some research and the build is about the same as the 75B.
The schematics provided on some of the older posts have been a great help.
I had an FX72 with the same problem and it turned out to be a bad capacitor, C11 in the low frequency oscillator (15µF nonpolarized). I'd have a look at C26 and C27 in the LFO also (I replaced all 3, just to be safe).

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Post by slammer88 »

I'll buy this pedal tomorrow for a real cheap price and have intentions to retrace everything and create a new board for it.

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Post by slammer88 »

This pedal seems to suck my tone a bit too much from treble side even when bypassed. Can anyone tell me what can I do to fix that?

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