DOD FX50B, FX52, FX53, FX55B Switching/Volume fix

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

I posted this on FB already, posting it here too for safekeeping.

I bought a couple of DOD FX50B's, and I remember this pedal having an issue with being able to hear the overdriven sound in bypass mode. Sure enough I remembered correctly. I came up with this quick fix for it, it only takes 1 J113 FET and 1 1N4148 Diode to fix it. I also was unhappy with the amount of volume, especially when the gain control is minimum the unit can't even achieve unity gain with the bypassed signal. This is a 1 resistor fix, I swapped R24 which is 220K for a 10K resistor. Problem solved, it now has plenty of volume for me, since it IS supposed to be an overdrive pedal.

Additionally this pedal, the FX55B Super/Supra Distortion, FX52 Classic Fuzz, and FX53 Classic Tube all use this same circuit board and have the same issue with the bypass signal so the fix works there too. The changes on the schematic are marked with an asterix. The Classic Tube and Classic Fuzz also have the volume drop issue. The Super/Supra Distortion do not have that issue.

Interesting as well is the level control, it changes VBIAS and lowers it to 3.1 volts or so instead of 4.5ish, this creates asymmetrical clipping in the distortion stage of the circuit when the gain control is maxed out. Note that the FX55B does not have this issue due to the 22K resistors in the VREF divider.

Lastly, the taper on the tone and volume controls are W taper log/antilog (weird for the volume) and C taper reverse log on the gain control. This has been misstated in other threads on these pedals so correcting it here.
FX50B_Schematic.PNG
FX50BMod.jpg

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CheapPedalCollector
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Post by CheapPedalCollector »

I have a few more mods for this, I don't feel like altering this schematic or redrawing it though.

1) Disconnect the two hard clippers and cap on one end and hook them to a SPDT switch to enable/disable the hard clipping.
2) Add a DPDT switch. Between the volume wiper and U1B pin 5 on one side, and between the output of R8 and U1B pin 5 on the other side, on opposite poles. This will add a tone bypass for FX50 operation.
3) Lift a pin on one of the soft clipping diodes, and stick another 1n4148 in line with it, then add a SPDT switch to bypass the extra diode. This will give you a symmetrical/asymmetrical switch. You can also use an on/off/on for a different (and much louder) asymmetrical mode.
4) The above mod can also be done to the hard clippers.
5) Add a SPDT with a 1uf cap parallel to C5 for an OD250 mode.

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steakfinger
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Post by steakfinger »

I have an old FX55-B that I love dearly aside from one issue: it's too damned noisy. I'm 100% new to messing with pedals and I know it could be any number of things in a pedal this old, but generally speaking what would one do to lower the noise floor of a distortion pedal other than lower the amount of distortion? Maybe it cannot be done, but I like the character of this thing, it's just too noisy. Something like replacing the resistors with metal film resistors, perhaps? Apologies if this is a dumb question.

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Post by CheapPedalCollector »

Replacing the resistors and pots probably won't have much effect unless they have absorbed a lot of moisture, which can happen, particularly with the pots. Replacing the two op amps with something lower noise will help a lot, and doing further mods to fix the uneven voltage division going into the tone control will probably help a little bit too, that will require another cap on the volume output and a resistor to the voltage divider. In the distortion stage you can increase C3 in the feedback to cut the very highest high frequencies to further reduce noise but it won't have a whole lot of effect other than dulling the tone.

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Clavo
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Post by Clavo »

Hi all.

I've had an FX50-B for many, many years and navaer liked it until trying it on the saturated channel of an amp but then I wanted it to raise the volume to boost the solos, which is something this pedal does not.
Searching on the net found this thread which has the solution, but... I really caní tell which component is the R24.
Could someone please point at me which one of the two highlighted portions of the pic above is that resistance?.
I guess is the one on the right...

thanks in advance!.

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ppluis0
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Post by ppluis0 »

Yes, it's the upper right highlighted resistor.
Cheers,
Jose

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Clavo
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Post by Clavo »

ppluis0 wrote: 06 Mar 2022, 01:35 Yes, it's the upper right highlighted resistor.
Cheers,
Jose
Gracias, José.

Acabo de darle a ese pedal con mi begginer's Fender Frontman 15 and, thruth being said... it delivers some gritty sounds over the "gain" channel.

I have a Marshall 6100, the one with three channels and love the B and C modes on the 2nd channel but the LEAD channel is not that cool for solos.
This mod could solve the issue.

I use that Marshall with a 7 band Eq, since at low volumes I found some indiserable sounds, so the Eq is always on.

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Clavo
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Post by Clavo »

ppluis0 wrote: 06 Mar 2022, 01:35 Yes, it's the upper right highlighted resistor.
Cheers,
Jose
Che, boludo, ¿y dónde compro una sola de esas resistencias?.
Aguante El Diego y viva la Patria.

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Post by ppluis0 »

Hi Clavo,

I sent you private message

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