DOD fx75b input buffer

[Baran Ismen]

I finally had chance to try my dod fx75b flanger with my setup and saw that it sucks the tone pretty much resulting with a treble loss in bypass mode. I know its not True bypass. I found the schematics and double checked from the board and it seems correct.

I suspect of the initial 10k resistor and maybe the following cap being too low. There is also not any input resistor to ground.

Is there anything to be done here?

Heres the schematics.

https://www.synthxl.com/wp-content/uploads/2020/04/DOD-FX-75B-Schematic.pdf

[antonis]

Make R2 1k (or so) and hope for Q5 buffer "oscillation free"..

(or move it right on Q5 Base..)

[Baran Ismen]

Make R2 1k (or so) and hope for Q5 buffer "oscillation free"..

(or move it right on Q5 Base..)

Made it 5.6k, hadnt no oscillation, will try that.

I wonder why theres no input resistor to ground, though.

[Baran Ismen]

Make R2 1k (or so) and hope for Q5 buffer "oscillation free"..

(or move it right on Q5 Base..)

Tried even 1r, no change whatsoever.

Opamps inside are 2x 1458 and jrc2904 on mine

[antonis]

I wonder why theres no input resistor to ground, though.

Look at R4 going to AC ground (C25)..

P.S. What exactly is "your setup"..??

[Baran Ismen]

I wonder why theres no input resistor to ground, though.

Look at R4 going to AC ground (C25)..

P.S. What exactly is "your setup"..??

The whole setup you mean? It exactly goes like this from guitar to amp:

Donner DT-1 Tuner (T.B)
Mooer MPS-1 pitch shifter (T.B)
Vivlex acoustic sim (T.B)
Mooer mod factory MK2 (T.B)
Digitech xp100 (buffered)
Diy Ts808 (T.B)
Mooer x2 preamp (T.B)
İrin 10 band eq (switched sliders with w ones, much better now) (T.B)
Mooer Mod Factory MK1 (T.B)
Diy ce-2 (T.B)
Behringer dw400 (buffered) + stomplfo to its EXP input(fortune wheel project i shared)
Diy small stone (T.B)
İbanez LF-7 (T.B)
Donner echo square (T.B)
M wave mini universe reverb (T.B)
Boss RC-2 (buffered I suppose)

Apart from these that are not in active duty, I got diy big muff, diy bassballs, diy hm2 (needs some fixes), and dod fx75b that has this tone-sucking.

Seems I love Chinese pedals lol, yet they work pretty well for their price. But it's confirmed that sucking is caused by this very pedal despite all the others in the chain, I checked one by one each of them in the chain in combinations.

[Clint Eastwood]

Replace the input transistor (2n5088? hard to read) with a jfet.

[Baran Ismen]

Replace the input transistor (2n5088? hard to read) with a jfet.

It's a 5088, yes. Any jfet you would suggest, or any will work? And what about the pinouts? Would it matter? I think I got one 2n5485 at hand, and 1 or 2 2sk30a

Revised the link with a better resolution.

[antonis]

Although I see no reason for such a replacement (470k in parallel with h_FEX 10k should be considered OK from impedance viewpoint), you should need to alter bias configuration..

[ElectricDruid]

When we talk about "tone suck" usually people are talking about a lack of high frequencies. Since the input impedance here is pretty good (it's going to be not far off that 470K value), that seems unlikely as a cause. The input cap is a fairly small value, but that would rob you of *bass* not treble. The following 1u cap is definitely large enough to pass anything audio.

What troubles me more is the following stages - the op-amps. There's a pre-emphasis/de-emphasis on the signal (6n8 and 10K in parallel with 47K). That's quite a lot of mucking with your tone for a "bypassed" signal, although it wouldn't be the only pedal from this era to do that. The CE-2 is another offender, iirc.

So...are we sure the tone suck is coming from that input buffer? Could it be from the subsequent stages? Baran, have you got an audio probe and probed the signal at different points to find out where the problem begins, or are you only listening to the pedal's output? It makes a big difference, because this circuit leaves a lot of stuff in the "bypassed" path.

[Baran Ismen]

When we talk about "tone suck" usually people are talking about a lack of high frequencies. Since the input impedance here is pretty good (it's going to be not far off that 470K value), that seems unlikely as a cause. The input cap is a fairly small value, but that would rob you of *bass* not treble. The following 1u cap is definitely large enough to pass anything audio.

What troubles me more is the following stages - the op-amps. There's a pre-emphasis/de-emphasis on the signal (6n8 and 10K in parallel with 47K). That's quite a lot of mucking with your tone for a "bypassed" signal, although it wouldn't be the only pedal from this era to do that. The CE-2 is another offender, iirc.

So...are we sure the tone suck is coming from that input buffer? Could it be from the subsequent stages? Baran, have you got an audio probe and probed the signal at different points to find out where the problem begins, or are you only listening to the pedal's output? It makes a big difference, because this circuit leaves a lot of stuff in the "bypassed" path.

Hey Tom,

Yes, im %100 sure there's a fair amount of treble loss with this pedal and its especially obvious when bypassed.

I actually didnt notice it before, because i had just directly connected it to my pc and played, but now when i place a preamp behind that or any other pedal, its quite clear that theres a low pass, or high filter going on.

And no, i dont have any probe. Of course it can root anywhere else, it was just my initial guess for a modulation pedal.

[ElectricDruid]

Ok, here's instructions for an audio probe - dead simple!

http://diy-fever.com/misc/audio-probe/

Once you've got that up and running, you can test various points in the circuit to see where the tone changes. Starting at the beginning, we've got:

1) C2, transistor buffer output
2) U1A LM1458 op-amp pin1, pre-emphasis output
3) U5A LM1458 op-amp pin1, de-emphasis output

The output at (2) should be brighter than normal because of the pre-emphasis. The de-emphasis is *supposed* to just return the tone to normal, but we'll see! Maybe it gives the high frequencies a haircut!

[Baran Ismen]

Ok, here's instructions for an audio probe - dead simple!

http://diy-fever.com/misc/audio-probe/

Once you've got that up and running, you can test various points in the circuit to see where the tone changes. Starting at the beginning, we've got:

1) C2, transistor buffer output
2) U1A LM1458 op-amp pin1, pre-emphasis output
3) U5A LM1458 op-amp pin1, de-emphasis output

The output at (2) should be brighter than normal because of the pre-emphasis. The de-emphasis is *supposed* to just return the tone to normal, but we'll see! Maybe it gives the high frequencies a haircut!

So, I'll be doing this when the pedal is on, the input jack going to the guitar but I'll get the output from the probe, right?

[antonis]

Right..

[Baran Ismen]

On a second and careful check, i saw that 4 or 5 caps have cold solders. I dont know how, because board seems it hadnt been touched before and dates back to 1995 according to its serial- number, but when i slowly push the caps, the joints / solders on the solder side were peeled up and moving with the parts. Renewed them all, and its all good now!

[R.G.]

Good work finding that!
It's nearly always useful to re-flow solder joints when you're seeing something puzzling on a PCB.

I ran into a really funny one. It was a thermal issue - the board worked when cold, quit when it got hot. I looked over the entire board under magnification and all the joints looked good. I only found it by pushing on parts while the PCB was hot enough to cause the problem. One lead of one cap could be pushed and it started working. The joint looked perfect - the lead standing up out of the pad, and a shiny round "tent" of solder all the way around it. Only when I scraped the joint open did I find that the tent of solder was just that - a tent. When the board was cold, the solder contracted and the top of the solder tent touched the cut end of the component lead. When it heated up, the solder expanded and the top of the tent quit touching the end of the lead. Apparently the sides of the lead didn't get cleaned or fluxed correctly and only the cut end of the lead accepted the wave solder. Worked fine when tested... until it got warm. Reflowing it with fresh flux was an instant fix.

[PRR]

the solder expanded and the top of the tent quit touching

Good description. I had that on a CRT monitor's main H-sweep lead. Worked when new, when it was older (tarnished) it needed frequent beatings to work.

Which I why I mis-trust "re-flow". If it wasn't fluxed, or tarnish has set in, casual "re-flow" isn't going to do the job. Scrub back to fresh bare metal and do it right.

[ElectricDruid]

On a second and careful check, i saw that 4 or 5 caps have cold solders. I dont know how, because board seems it hadnt been touched before and dates back to 1995 according to its serial- number, but when i slowly push the caps, the joints / solders on the solder side were peeled up and moving with the parts. Renewed them all, and its all good now!

Good work!

So were some of these caps in the pre-emphasis op-amp, perhaps? No caps there would mean no pre-emphasis, which would make for a *very* dull tone if the caps in the de-emphasis section were ok.

[R.G.]

If it wasn't fluxed, or tarnish has set in, casual "re-flow" isn't going to do the job. Scrub back to fresh bare metal and do it right.

Yeah - what this here joint needs is a liberal application of some chlorine tri-fluoride or difluorine dioxide. (search "derek lowe pipeline FOOF").

[amptramp]

If it wasn't fluxed, or tarnish has set in, casual "re-flow" isn't going to do the job. Scrub back to fresh bare metal and do it right.

Yeah - what this here joint needs is a liberal application of some chlorine tri-fluoride or difluorine dioxide. (search "derek lowe pipeline FOOF").

If it cleans my pipes out, maybe I should drink some.