DOD FX75B Stereo Flanger: Clipping

[Gringo]

I have this old beaten flanger i got second hand a long time ago. At first sight it had a busted output jack(fixed), but after some playing, i noticed it clipped in certain frequencies or when i strummed strings hard.

This happens when i plug it in front of the amp, if i run it through the fx loop, the clipping is not noticeable. I've tried it all by itself, in the middle of the chain, at begining/end, so i'm quite sure the problem is withing the flanger.

I've been looking for a schem but all i could find is this:

http://www.harpamps.com/schematics/dodfx75.pdf

The main difference are the opamps, in mine they are not tl022, but  2 lm1458 and 1 lm358. Also, the input oa buffer/thing is gone, replaced by a single 2n5088.

Any suggestions? Where can i start looking? Are there any "usual suspects"? Any parts i could replace to improve sound quality?

I have my audio probe and datasheets for most ic's. I know the standard procedure is to follow the signal path (this is proving to be quite a task)until i find the problem, but i'd like some advice nonetheless.

Since this is the only flanger i'll be able to lay my hands on in a looong time (bbd's are rare and expensive), i'd like to see if it can be restored to an acceptable working condition

Thanks in advance.

[smoguzbenjamin]

Hmm it looks like there's 2 diodes (1n4148) there back to back, could they be doing the clipping?

[Gringo]

I don't think so ben...

Anyone else?

[Mark Hammer]

From what I can see, the only thing the FX75 has to keep the signal level under control is the diode pair Ben referred to.  There is no companding, and no input level control, just diodes to keep the signal level (recirculation+input) from getting too large.  On the other hand, that is an FX75 and from your description, it is somewhat different from a 75B.  For instance the FX75 schematic I have shows no input buffer other than the TL022.

The TL022 is a lowpower op-amp but I don't know if it has better transient-handling capabilities than a 358 or 1458.  of course, a 1458 is not exactly your top pick for dual op-amp.

The FX loop should have an impact on the loading and bandwidth but not necessarily on the clipping.  Unless the send on your loop pads down the signal to a reasonable level and the return part of the loop gooses the signal to make up for that.  Is it possible to run the pedal to another entirely different amp so that you can tell if it is receiving a hotter feed from the pre-amplifier path than from the amp's loop? So, plug the pedal in your signal path but run it to another amp for listening, then plug your guitar into your main amp with controls set as typical, and run the pedal in the FX loop but feed its output to another amp instead of the return loop.

[Gringo]

I'll give that a shot, Mark, and come back with more questions.

Thanks!

[Gringo]

After some testing:

- Using guitar -> flanger -> amp (i tried with other amps) it clips as noted before (specially on chords, and more noticeable in some parts of the fretboard/strings on single notes).

- Sticking it in the fx loop -> no clipping, no matter how hard i strum, chords or single notes, up or down the fretboard.

- Using my amp (marshall valvestate), pedal in fx loop, but with output to other amp -> no clipping (same as with output to fx loop return of my amp).

I don't know what could be malfunctioning, maybe it's just the way this pedal works, so i thought maybe replacing the opamps, and all the electrolythic caps could do the trick  :?

I'm all eyes

[Gringo]

bump

[Aharon]

Some op amps are Jfet and others are not.
I think the standard TL022 replacemente is TL062,at least that's what I see for example with the 301 flanger.
Aharon

[smoguzbenjamin]

Maybe your pickups are too hot, or maybe not hot enough. If the effect isn't clipping after the amp's preamp stage, then I think the problem is your guitar's interaction with the pedal. Have you tried a booster/attenuator inbetween your guitar and flanger?

[Gringo]

I have hot pickups, but i also have a unusable local tele copy from a friend (has only 2 strings, neck is about to fall, a piece of crap with strings and one pu) with very low output, and if i strum really hard, i can make the flanger clip.

I've tried the flanger with a buffer in front of it, same results.

[smoguzbenjamin]

Have you tried turning your guitar volume knob down. It might just be thatn you're overloading the buffer in the pedal or something.

[Gringo]

I've tried that too, but doesn't help much, and takes away some of the "tightness".

[smoguzbenjamin]

el-Wierdo... I don't know what can be wrong with it.... but it works in the FX loop?

[Gringo]

I'm bringing this one back from the dead, since i tried the flanger with an orange squeezer i just finished in front of it, and voila!, no clipping. Even with the squeezer at full blast.

Now, someone could give an explanation of this behaviour?

[StephenGiles]

Ummmmm....could it be that the BBD bias pot needs adjusting? - first place I'd look.

[Gringo]

Well, that one didn't even cross my mind  

I'll try to spot the bias trimpot and see what happens.

Thanks!

[StephenGiles]

Hey Gringo - when you get the bias trim in the right place, set it with a blob of Dulce de Leche!

[Gringo]

And i might do so in your honor!! (maybe it'll attract ants, which will feed the spiders )

After some tweaking (there were three trimpots) i could eliminate most, if not all, the clipping  :twisted:

I owe you a big one!

[StephenGiles]

No problem Gringo. We are visiting Equador in September, have you ever been there? We spend just under 2 weeks there and as we fly KLM the plane makes a stop at Bonaire in the Dutch Carribean so we have a further 4 days in the sun. I don't think it will be very warm in Equador.

[Gringo]

Well, i'm sure it'll be quite warm in ecuador, even though it's winter.

I've never been out of Argentina, but i've been to the south lakes (my father is crazy about fly fishing). Very nice place to go, but 16hs of non stop driving is quite cumbersome!