Whining Sound DF2, Does Anyone Have Suggestion Which Capacitor Could be the Culprit ?

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visserman
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Whining Sound DF2, Does Anyone Have Suggestion Which Capacitor Could be the Culprit ?

Post by visserman » Thu Jan 20, 2022 10:44 pm

HI Everyone,

I have some issues with the DF2, the main issue is a whining sound underneath the feedback sound.
Distortion sound is fine, it is the Feedback function which is at fault.
Suspect it is a capacitor which is about to fail.

When you check the pedal there are lots of capacitors inside the pedal, I am hoping some of you may have experience with working on this pedal with this particular problem.

Any pointers?

Cheers!
Eddie

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Pepe
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Re: Whining Sound DF2, Does Anyone Have Suggestion Which Capacitor Could be the Culprit ?

Post by Pepe » Thu Jan 20, 2022 11:32 pm

I would swap them all. Someday the next one will fail. In my experience especially the 10µF mini electrolytic capacitors are prone to failure after over 35 years (DD-2/DD-3!). If it has some of these, swap them. But somehow I don't really believe that it has mini caps on the board.
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visserman
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Re: Whining Sound DF2, Does Anyone Have Suggestion Which Capacitor Could be the Culprit ?

Post by visserman » Fri Jan 21, 2022 4:28 pm

I agree but changing them all is a lot of time and monies. I rely on a engineer to do this for me.
Was hoping some of you may have experience with this pedal and be able to identify a few caps. which would need replacing.

Let me be more specific about the problem since I have got the pedal back today:

Basically the overtone sound is there all the time underneath the normal distorted sound. It is weak and faint, and when you put up the level of the overtone it becomes more pronounced, but nowhere near as strong as when you engage the overtone properly by stepping on the pedal.

The functionality of the pedal has not been affected, the only matter is: There is a overtone sound all the time, even at times you may not want it!

The fact that the problem sound is part of the overtone may give a clue to some of you as where to look.
So far I have been given the numbers of about 12 different caps. which may cause this issue, but since so much of the circuit is integrated it will be hard to tell as to which cap. is responsible for this problem.

Swapping all caps. is a option, as what Pepe mentioned, what about another process?

So far I am thinking of changing the 12 caps. I have been given.

Does someone else have a suggestion about what other process to use. The main issue is: Changing all caps is time consuming and will cost more.
I understand there are no guarantees to this whole process.

I have had issues with a pedal before where we swapped caps. without any result, want to avoid spending too much without getting any results.

Cheers!

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visserman
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Re: Whining Sound DF2, Does Anyone Have Suggestion Which Capacitor Could be the Culprit ?

Post by visserman » Mon Mar 21, 2022 10:21 pm

Okay, could anyone give some suggestions as in where to look or what to do?
Here a update:
All Caps have been replaced with new ones, the warble sound is still there, the feedback does now not work at all times: Sometimes it works and sometimes not. Seems to have gotten worse!

What would be the next step in replacing parts, which ones would you take out and swap for new ones?
Change all components maybe?

Looking at other forums I did see one post about this issue, but no one could come up with any useful suggestions.

The sound can at best be described as a broken circuit: There is a warble underneath the regular dist. sound, it is there all the time. Kind of cool but annoying at the same time. Certainly not a sound I enjoy using from this pedal.

Anyway, really hope someone can give some clues as in where to look for the next move.
Cheers!

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Re: Whining Sound DF2, Does Anyone Have Suggestion Which Capacitor Could be the Culprit ?

Post by Old_Iron » Mon Mar 21, 2022 10:49 pm

Not an easy circuit to guess a malfunction...

Laurie is our most capable technician, but I think you're going to need somebody to have the pedal in hands and probe it with an oscilloscope, feed it with a sine wave and follow the circuit path with the scope to find the culprit.

Here is the service notes and the schematic, it is scary:

https://www.electronica-pt.com/esquema/ ... nfo/72259/

Image
Last edited by Old_Iron on Tue Mar 22, 2022 10:21 am, edited 2 times in total.

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laurie
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Re: Whining Sound DF2, Does Anyone Have Suggestion Which Capacitor Could be the Culprit ?

Post by laurie » Tue Mar 22, 2022 2:13 am

If you don't mind the postage... send it to Canada for a holiday and I'll sort it out for you.

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visserman
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Re: Whining Sound DF2, Does Anyone Have Suggestion Which Capacitor Could be the Culprit ?

Post by visserman » Tue Mar 22, 2022 10:47 pm

Thanks Old_Iron, that is what the engineer will do.

Laurie, thanks and your suggestion may be the best option, I will keep you informed.
Cheers!

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Re: Whining Sound DF2, Does Anyone Have Suggestion Which Capacitor Could be the Culprit ?

Post by visserman » Fri Mar 25, 2022 6:53 pm

laurie wrote:
Tue Mar 22, 2022 2:13 am
If you don't mind the postage... send it to Canada for a holiday and I'll sort it out for you.

Laurie I would like to take up the offer for the holiday for the pedal.
Will sent you a PM with questions about detail.
Cheers!

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Re: Whining Sound DF2, Does Anyone Have Suggestion Which Capacitor Could be the Culprit ?

Post by laurie » Thu Jun 16, 2022 12:11 pm

An update. visserman's DF-2 has been on vacation here in Canada for a couple of months.

This was the strangest problem... the distortion sounded "off", and the switching wasn't working properly.

The circuit uses the 4066 CMOS analog switch chip, and a 4001 quad NOR gate for the flip-flop and to manage the intricate switching. There are also a number of discrete components as part of the "switching matrix" to manage the timing of the switching (e.g. press the pedal for 0.3 seconds or less to turn the distortion on or off).

Because all the components of the switching matrix are so tightly coupled it has taken ages to track down the problem. At first it appeared that the analog switch (the 4066) had failed - because the symptoms changed when the chip was replaced. After much head-scratching and many hours of carefully deconstructing the switching matrix component-by-component it became apparent that something was unbalancing the footswitch signal "inverter" (Q14).

The culprit is a failed Q10. Replaced it and the pedal works perfectly.

The reason changing the 4066 changed the behaviour is that each chip has slightly different manufacturing tolerances and the modern replacement 4066 was sufficiently different to the vintage one in the pedal to subtly change the failure-mode.

I will revert everything in the pedal to original including the capacitors that were changed before I got it, then run it on the bench for a few days to ensure all-is-well.

This is a good example about how to approach electronic repairs - assumptions are rarely correct. The initial assumption (by others) was that the capacitors had failed. It turned out to be completely unrelated - a switching transistor.
Last edited by laurie on Thu Jun 16, 2022 12:18 pm, edited 1 time in total.

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Re: Whining Sound DF2, Does Anyone Have Suggestion Which Capacitor Could be the Culprit ?

Post by laurie » Thu Jun 16, 2022 12:17 pm

This is the failed transistor. The gain (hfe) should be around 500.
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DF-2.jpg
DF-2.jpg (358.33 KiB) Viewed 468 times

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