Dod foot-switching

[Yazoo]

I'm repairing a Dod FX57 hard rock distortion pedal - interestingly it has an MN3007/MN3101 delay.

I think I've got the circuit sorted out but I am having problems with the footswitch. The momentary switch does work but I can't get the footswitch mechanism going. After Googling the pedal, I've found pictures but the pedal I've got is set up differently.



There is what looks like a simple spring not in the pictures I've found online. If you know how to set this up properly, I would be grateful for your advice.

[Rob Strand]

Well from what I can work out that version of the circuit with a grounded switch requires a push on/push off footswitch.  The other footswitch circuit DOD uses has a floating switch and a momentary push button.

Schematic, grounded switch, probably push on/push off
https://3.bp.blogspot.com/-HcOTOMJVUuY/T-_Giv5nexI/AAAAAAAABag/gpxfIUm4czI/s1600/DOD%2BFX57.gif

I haven't traced out the details of the circuit but it just looks like it can't work with a momentary footswitch.


If the majority of the circuit is working, you should at least be able to hold the button down and change between effect/dry, it just won't stay in that mode when you lift the switch.

This is a (probably) later version (rev D, 1987 chip dates).  The circuit around the 4007 chip uses 1M resistors and caps.  That's more like the floating footswitch + momentary push button circuit.

https://mirosol.kapsi.fi/2013/09/dod-fx57-hard-rock-distortion/
http://mirosol.kapsi.fi/wp-content/uploads/2013/08/DOD-FX57-HardRockDist-Guts.jpg

[Yazoo]

Just to clarify, if I press and release the switch on the miniature pcb, the effect does engage and the led lights and stays on, so it must be soft switching. I understand I will need to adjust the height of the main enclosure foot switch to press the switch. Is that wire held down by the screw necessary and in the right position?

[Rob Strand]

If you know that switch is momentary and the LED is going off and on correctly then it is very likely your circuit is the later model with the momentary switch.   So you don't want the ground wire on the switch as the switch should float. --> not-necessarily the case, see next post.

If the effect isn't switching then the fault is probably elsewhere.

[Rob Strand]

Hmm,  I found this variant which seem to be a momentary switch and also connected to ground,
https://www.hobby-hour.com/electronics/s/schematics/dod-fx60-schematic.webp

The key feature of this circuit is the switch is wired to a JFET.  The JFET is emulating the floating switch.

whereas this one, which could be later, drops the JFET,
https://www.hobby-hour.com/electronics/s/schematics/dod-fx60-schematic2.png

Interestingly it's got a 1M and a cap.  Still looks momentary.

Yes, it's a different pedal but DOD copy the circuits for the switching across pedals.

Can your post a pic of your PCB?

After analyzing the circuit, it looks like the footswitch should be grounded.

What DOD have done on this version is replace the JFET with one of the MOSFETs inside of the CD4007 gate.
Also the FX57 schematic I posted earlier probably uses the same switching circuit as the later chorus unit.  What's happened is the FX57 schematic has an error, there's a cap missing - pretty normal for DOD schematics.

Edited.

FX57 Hard Rock Distortion (1987-93)
FX60 Stereo Chorus (1982-93)

[CheapPedalCollector]

You may need to check that the plastic part that depresses the switch hasn't broken off or gotten crushed, unscrew the switch PCB and look at it. If it's pushed in a bit you can start dabbing some cyanoacryalte glue on the end and let it dry to build it back up to the point of being usable again.

Also make sure the PCB is screwed down all the way.

[Yazoo]

Thanks for the advice. I did get this sorted out in the end. I took out all the circuit board (again!), took off the pedal part and isolated the microswitch. I found the switch needed to fit exactly into the hole in the enclosure - I couldn't see to work this out before. Once I had the switch in exactly the right place, I checked continuity while pressing it and once I could see/hear this, I put it all back together again. When I googled the Dod pedals I came across a warning about wires coming off - they were right! I put on new wires and put it all back together again and the foot switch does now work.

As you said in your last post Rob, the momentary switch is earthed. I now need to recreate the battery holder. There is a 3D printer file for the battery cover which I printed off today and it fits nicely.