DOD FX90  [SOLVED]

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ericnickel
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Post by ericnickel »

Greetings all. I hv a FX90 Delay that is misbehaving. I only get dry signal. LED switching off and on.
I hv a scope, sig generator and audio trace at my disposal. I'm trying to understand how the pedal switches off and on. From the 4007ic it goes to a diode then to the mosfet which actualy switches the pedal off and on. I can hear a signal from the 4007 into the diode but nothing out. I replaced the diode but still nothing. I should hear my generated tone on front and back of diode right?
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20220216_212439~3.jpg

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ppluis0
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Post by ppluis0 »

Hi Eric and welcome to this forum.

The 4007 circled in green does not carry audio signal. Simply this IC is arranged to convert a single pulse from the foot switch to a steady latching state.
The Jfet Q7 inside the red circle short the mix of dry and wet signals to avoid changes in level.
From the same pins of 4007 that controls the Jfet, the transistor Q6 is also driven on and off to short the indicator led.
From the same pins of 4007 the transistor Q8 short pin 16 of U2B to ground to mute their output (at the same time that led is off.)
Reading your description the 4007 seems to work OK and also the dual operational IC as dry signal is alive.
As you have an oscilloscope and signal generator please trace audio signal in the following order:
pin 6 of NE570 = input of compressor
pin 2 or 3 of NE570 = output of compressor
pin 3 or 4 of MN3005 = output of delayed and unfiltered signal
pin 14 or 15 of NE570 = input to expander. The signal will be filtered at this point.
pin 10 or 11 of NE570 = output of expander (will be muted and enabled pressing the foot switch)
Tell us if you lost signal at any of those test points and we will think how to proceed from you measurements.

Cheers,
Jose

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ericnickel
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Post by ericnickel »

Thanks so much for the help( I didn't realize you replied. Need to check my settings)
I will check wht you suggested. FYI I hv a working DOD chorus that I use to see how the 4007 works. I see the 4007 provides a high or low sig to the diode turning j113 on or off.
I'll let you know.

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ericnickel
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Post by ericnickel »

So here is what I found.
I can hear my sig generator on all of your above mentioned pins.
Pin 6 on 4007 goes high and low According to button. Pin 6 go through resistor and diode. I hear my tone at front of diode but not back. I can hear my tone on source and drain on jfet. Voltage is low on gate (which is connected to back side of diode)
Red circle there is "Mix" which is external pox but there is also a
"V Bias" at bottom of that internal pot. I don't know history of this pedal so someone could hv been turning those pots like a steering wheel. Lol. Would that hv any effect on circuit path?

I don't understand the signal flow through the Q7 but for what ever reason it is always "on" right?
FYI Diode, Q7 and 4007 are new.
Thanks.

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ericnickel
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Post by ericnickel »

Another thing I noticed is that sometimes the LED is VERY bright and am unable to turn off. I can never figure out if I've touched something but for no reason it starts to turn on/off proper.

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Post by ppluis0 »

Hi Eric,

Perhaps the fault is merely an adjust. :roll:

The MN3005 input pin needs to be biased to a correct level to process signal through its output and that bias voltage vary a little from chip to chip even if there were manufactured at the same time from the same vendor.

For this reason the boards that employ BBD lines as delays, chorus or flangers also have mounted trimpots to find the operating point at factory, board to board.

In extreme cases the bias voltage is misadjusted to the point that the delay chip don't pass signal.

Try to clip oscilloscope probe at emmiter of Q4 or Q5 and adjust slowly the preset of 50K (P3 in your diagram) to see the output waveform at this point.

With little levels of signal you can view a sinewave.
Increasing the level and the output signal (blue) can be clipping unsymmetrical.
Increasing generator level even more (yellow trace at the right image) you will be able to adjust bias trimpot until the clipping will appear symmetrical.

Image

That point is the optimum bias point.

Cheers,
Jose

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mediy
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Post by mediy »

I would try shorting the source and drain of Q7. If you then get wet signal at the output then replace Q7.

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ericnickel
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Post by ericnickel »

Hopefully I'm doing something wrong bcs there is no change to the signal at Q4 or Q5 while turning P3.
FYI I can see the clock change on pin 2&6 on mn3005 when I turn the "delay" know on front of pedal.

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ericnickel
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Post by ericnickel »

mediy wrote: 11 Mar 2022, 23:25 I would try shorting the source and drain of Q7. If you then get wet signal at the output then replace Q7.
I actually removed Q7 and Q8 from circuit to eliminate that possibility.

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ericnickel
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Post by ericnickel »

Just thinking out loud here. If my P3 pot was shorted to ground that would kill the input to pin 7. Correct? I can't remember if I hear a sig w my probe on that.
Will need to try that.

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ericnickel
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Post by ericnickel »

Getting pretty frustrated. Not getting anywhere. I see inputs and outputs on ic and circuit but not sure what things are supposed to look like. For a delay I'd immagine that output trace would lag behind input? I hv a sig generator at 400Hz and using scope and audio tracer.

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CheapPedalCollector
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Post by CheapPedalCollector »

Does anyone have this schematic in the OP? I need this one for this revision, but part of the one shown is cut off.

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Axldeziak
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Post by Axldeziak »

CheapPedalCollector wrote: 29 Jan 2023, 18:33 Does anyone have this schematic in the OP? I need this one for this revision, but part of the one shown is cut off.
Not much got cut off.
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DODFX90(1987) revH.pdf
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CheapPedalCollector
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Post by CheapPedalCollector »

Thank you! I didn't want to pay $8+shipping for it.

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