DOD FX72 - MN3007 problems?

[bobober]

Hey there,

I've got a naughty DOD Bass Stereo Flanger FX72 that is misbehaving, little video link for what it currently sounds like. It's basically not flanging right.
https://youtu.be/ihYYAtO9b-k

I suspect the BBD chip. I haven't opened it up yet but apparently the circuit is very similar to the DOD FX75B
http://files.effectsdatabase.com/docs/schematics/dod_fx75b.gif

I haven't opened it up yet, but apparently the BBD chip is the same, an MN3007.

I just wondered if any of you had many come across this sort of problem before and maybe recognised it from the video? Do you think if I just replace the MN3007 chip it will start working again?

 

[JohnRoberts]

I have too much experience working with BBDs..  The original MN300x series was licensed from Phillips by Matsushita (AKA Panasonic). I'm not sure who makes them now.

The Panasonic parts were pretty robust, I actually used a MN3001 in my first delay line kit project published in 1976.

Before you replace the BBD, check for proper operating bias. There is often a trimpot tweaking the DC bias of the AC signal feeding the input of the BBD. If this voltage is too high or too low it may not pass audio at all. If borderline high/low it will saturate and distort prematurely.

JR 

 

[Bo Deadly]

If it's flanging at all, I doubt it's the BBD. It's much more likely to be a bad connection, a broken solder joint, bad footswitch, bad transistor, a dirty potentiometer, etc. Open it up, clean it thoroughly and look for obvious problems like broken wires, cracked solder joints, etc. If you have a DMM, sanity check voltages. If that doesn't help you identify the problem, explain your level of experience with E and what test gear you have and then maybe we can figure it out further.

Update: In particular, if the FX72 uses the same footswitch IC as the one in your schem, there is a JFET switch that is biased on with only the reverse bias current of a diode. If that diode or the input of the JFET is dirty, it can drain away that tiny current and cause it to not switch on or just act really flaky. So use some isopropyl to clean that area really thoroughly. Maybe even try to polish the leads carefully with a lens wipe gripped with some tweezers. Any little bit of grime on the leads between D4 and Q7 can stop it from switching properly.

 

[Whoops]

I've got a naughty DOD Bass Stereo Flanger FX72 that is misbehaving, little video link for what it currently sounds like. It's basically not flanging right.

Also do a search around these 2 forums as they are specific to guitar/bass stompboxes, and you might find someone there that had the same problem or info about any known issue with that unit:

https://www.diystompboxes.com/smfforum/index.php

https://www.freestompboxes.org/