DOD Echo FX96 distortion problem .. (calling Mark Hammer!!!)

[george]

Hi

I recently bought a brand new FX96 off ebay and the delay sound is DISTORTED! The delay sound distorts, even if I turn down the volume on my strat which has low output pickups to start with.

I checked out the schematic,

http://www.diystompboxes.com/pedals/schemsc/fx96schem.pdf

Because the delay sound distorts even at low input levels, I figured the distortion can't be happening in any of the op amp stages.  From what I know about the NE571, this compresses the signal before it goes to the MN3005.

I also note that the bias on the MN3005 input is hardwired to to 1/2 the supply voltage, rather than set with a trimmer as on other analog delays such as the Boss DM-2.

My guess is that the bias is set so low that it is on the edge of the MN3005 being able to pass a signal, ie not all the signal is being passed and is being clipped, thus causing the distortion.

My question is, will the following allow me to get a clean delay?

- Cut the trace to Vbias going to R51 (resistor just before BBD input)
- Hook up a trim pot (say 100K) to ground and Vcc
- connect the wiper to R51
- adjust trimmer until delay sound cleans up

I know this will stuff the warranty but the thing sounds like POO the way it is!

Thanks in Advance!

[Mark Hammer]

That sounds about right (you've learned well, grasshopper!).  I suppose the recommendation one might make is to reduce the value of R51 to 100k or so.  I can't recall ever seeing resistors as big as 150k as current limiting resistors, but certainly try out 150k for starters.

Also, stick a 10uf cap from Vbias to ground for decoupling, and consider sticking fixed resistors on each leg of the trimpot so that the trimpot is less "twitchy".  For instance, if you use a 10k lin trimpot, stick a 4k7 on each side  and you'll have more degrees of rotation to work with.  That mimics a 20k trimpot whose middle 50% of rotation is redistributed across the entire trimpot.

[george]

(you've learned well, grasshopper!)

Thanks! (still haven't quite managed to grasp the pebble from your hand, still too fast for me master! :lol: )

I suppose the recommendation one might make is to reduce the value of R51 to 100k or so.  

good call - nice 'n' simple to start with.  It's occurred to me looking at other delay schemos that the MN3005 input acts as a current source

[george]

mind you DOD don't make it easy by using crappy little 0.1 watt surface mount resistors!!!

but it works out as fairly simple to do once you figure out:

- where R51 is
- a convenient place to hook up a 10K trimpot (the cathode of D4 and the anode of C23, just make sure it points towards the battery compartment, or else the lid might not go back on! )

After the connection of a 100K resistor and 10uf cap to the wiper and negative lug, then connection of the 100k to the pad left by the removal of R51, appropriate appropriate continuity checks and tweaking of the trimmer,  it sounds like an analog delay SHOULD...

... which is like my AD-3208 ...
... which sounds so much like a DM-2 I bought that I sold the DM-2 again.

Of course it HAS voided my warranty, but I don't care.