Help for old DOD FX60!

[Myampgoesto12]

Hello all! I got ahold of an old FX60 chorus pedal last week and I love the tone on cleans. Nice smooth sweep that doesn't get sharp OR flubby. Not bad for a two knob pedal. But with either of my ODs on, the sweep sounds sort of sputtery and weak like halfway through the sweep range. Also on my high E played on the 4th string 14th fret it has a massive drop in volume and that sputter sound remains through the whole sweep. This is my first chorus and I may have a bit of learning to do for it hah.

My chain is guitar>crybaby>mxr modified od>mxr distortion 3>dod fx60>mxr 10 band>boilermaker boost>amp

The issue isn't at all present on clean. Any ideas of what may cause this?

Thanks

[thermionix]

Maybe chorus before dirt?

[Myampgoesto12]

I'll give that a shot, it'll have to be tomorrow though. Thanks for the tip.

Even though this is modulation, the way its interaction with my OD seems sorta pheser like to my ears. Sorta like Rush tone-ish. So I probably won't use it with OD much other than a few songs, but I'd like it to keep that smooth character it has with the clean setting.

Thanks again, I'll post the results of messing with its placement later.

[Myampgoesto12]

Well I managed to fool with it a little bit. Placement made no difference in the issues mentioned, however I did find that changing the depth setting changes what note ends up being the dead sounding one.

This is a stereo out pedal, but you have to use a stereo cable instead of two monos to utilize it. So I pulled the output cable out enough for the tip to make contact with the other output and the dead sound disappears, but the chorus effect is much less present.

Might this be a bad buffer or poor bias type of issue?

Thanks again.

[Myampgoesto12]

I may have found the problem!  There's a 2N4125 near the clock generator. I'm not experienced enough to know what its doing or even what its for there exactly, but when I touch the square hole for the 2N4125 AND the closest sides of the two resistors next to it the sputter dead sound goes away. The tone of the pedal changes.. But for all I know that may be what its supposed to sound like hah.

Finally figured out how to get images up-that blue text was elusive...., heres the schem for this revision.



And here's my board



And a close up of my suspect



I'm limited on knowledge about FETs and so on.. Maybe swap it out?

Sorry for the longer post.. Just kind of excited

Thanks

[duck_arse]

how does your stereo out work? if your cable is just connecting the two outputs together, they are out of phase and will cancel, if not all over, at least somewhere. it might be this somewhere that you can (not) hear?

and that jfet you are prodding will mute the effected signal from the two mixer/outputs when in bypass mode. it is possible you are switching it (or modulating it) with finger hum, and hearing the result.

[Myampgoesto12]

I play it mono. My thought is that the stereo out shorts one side to the ground on a mono cable since the ring part of a stereo male 1/4 plug isn't there,  for mono operation. Much like older stereo ins and outs on older mixers. Certainly this was designed for mono use leaving the unshorted output on the tip to go on out of the pedal? Its a single stereo jack wired in stereo (each side from the one of the two buffers that mixes with the buffered and split dry signal)

In this original schem from the first line of the FX60 there is a different loop between pin 5 and 7 of the clock generator, where in the later revision this little loop seems to be one of the few differences made to the entire circuit.

In this there's a diode in that little loop with the same value cap as the revision...but no fet



The trace from the originals little loop (and the newer revision loop) also goes over to the depth and speed controls. I'm not sure if there's a malfunction here or what. I did finally (after playing reasonably lout that this is happening on clean as well as OD. Same issues with the depth setting changing what notes are dead and sputtery

[Myampgoesto12]

Sorry to double post but I don't see how that fet mutes anything since its not connected to the bypass.. The fet on the right side of the newer revision is though.

[duck_arse]

in the second circuit you posted, the 4007 is the bypass flip flopper, controls the fet gate thru the same parts as you'd see in a boss circuit.

[Myampgoesto12]

That I recognize now that I look at it. But that's not the one I touched in my previous post.. The first schem has the 2n4125 looped from two of the OSC pins, that's the one I touched. The original schem doesn't have a fet in that OSC loop

From an earlier post of mine about the two outputs sounding different, in the both the original and the revised schems, both sides of the output stage have different feedback loops. The "right (-->)" side has fewer components. I think that's the side (the side i pulled my plug out enough to make contact with while disconnecting from the tip(left) prong in the jack) that doesn't have the sputtery dead notes since in every scenario I've read and worked the "right" side is the ring, not the tip.  Perhaps my issue isn't with that 2n4125 at all..

[duck_arse]

in this dia?



I can only see the fet far right, not connected to the lfo, but between the bypass and left/right outs, and the one left of that, in the supply protection/battery switch [?] part. and looking at those extra parts, there is an R//C in one feedback loop mirrored by an R//C to ground in the others output. they look suspiciously like de-emp to compensate for the boosting emphasis filter at the input opamp.

so is your machine have a single, stereo output jack, with left/right connections to the tip/ring lugs?

[Myampgoesto12]

It is a single jack with tip and ring connections yes.

In that diagram, under the clock generator, connected to pins 5 and 7 is the device I touched. After looking it up by its model number 2n4125, I found FETs with that model number. If that's the wrong term please let me know lol.

But now I'm totally lost because I now after having some time to spend with it and really listen, when I play the note that's weak and sputtery, the rate also SEEMS to double,  when I go to any other note the rate returns to normal. Its like this thing has some frequency response issue.

I really appreciate everyone trying to help out, it means a lot.

[duck_arse]

looks like a npn GP transistor from this datasheet.

http://pdf.datasheetcatalog.com/datasheet/fairchild/2N4125.pdf

I think that is the modulating element between the lfo and the clock oscillator. you'd need an expert to explain its working - except your finger will represent more than the 25pF in that circuit, and will affect the timing cycle of the clock gen.

[Myampgoesto12]

That makes sense.. Given where its located.. Should a thought of that.

Looking at this thing again I started to wonder why adjusting the depth would change what notes er..frequencies come out of the pedal dead, and why that would be more obvious with OD in the signal (before and after the chorus pedal).

Since this thing is older.. More than 15 at the least is it possible a cap is failing somewhere like maybe the input? On ODs, and wahs, which I have more experience with modding and a little troubleshooting, caps at the input whether series with the rest of the circuit or going to ground change what frequencies the pedal can accept, and/or will put out at the end result.. But that doesn't tell me how adjusting depth would change which frequency is dead.

Its a really strange problem.. I haven't found anyone else with it hah.

[bloxstompboxes]

I have one of these without the in and out buffers. It failed to produce any chorus at the start. But it is more than 20 years old so I changed out every electrolytic and now she sounds like a CE2 almost, but a little different. Wonderful chorus if you get it working.

[duck_arse]

like blox says - caps. I can only see three electros in the audio path, C7, 24, 26, you could just yank them and replace w/ poly, then forget them. it is interesting that they couldn't see their way clear to add a bypass cap on the Vbias line, too (?VBadj?).

[Myampgoesto12]

Thanks for the input! I though about swapping components out for newer ones. Caps would be first on my list. Do you guys think the electrolytic caps could be causing this? Also Poly's aren't polarized.. Would that be an issue here?

At first I thought that the depth pot could be bad, but there aren't any scratch sounds coming from it when rotated at all. I've seen videos of this pedal with super scratchy pots.

Also, on a barely related note, I had to swap out the input jack. The stupid clip for connecting the ring (even though this is a mono input) bent and got stuck on my (cheap) jumper cable. Ripped the tip off of it hah.

[bloxstompboxes]

You can check the pot by connecting your meter and turning the knob. It should show the proper resistance range however may change more rapidly at some points in the rotation than others due to the taper. I wouldn't worry about the poly's; they don't degrade the same as electros. I haven't the time to check the schematic to make sure, but changing the electros won't hurt anything and will have to be done eventually anyway.

[Myampgoesto12]

Ok, going for swapping out electrolytics first then. While I'm at it I got an mxr d3 I'm going to mod as well. Might do some of that tonight!

Thanks again. I'll hit this up with some results

[Myampgoesto12]

No luck with C7,24, or 26. I'm happy there are newer caps in it at least hah.

While I had it open(still do right now) I went ahead and added the delay pot that the fx65 has just to see how it sounds( might do it again if or when I get this fixed) its an interesting control but it didnt help anything. Like the depth control the delay mod/addition just changes what notes/frequencies get weak/low volume and sputter.

Gunna crack open the d3 for a few to vent on it hah.