Flanger pcb /Pavs debug thread

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Pav
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Post by Pav » Mon May 27, 2019 11:48 am

What's your lowest voltage on pin 7 U10 (delay pot at min and trimmer down as low as it can go)? It should be around -6.5V. If not that negative then check circuitry around U10..
-2V around u10 p7...we replaced zeners and R39 (again) but they did not help. Checked CV circuit upstream with 5v triangles into CV1 CV2 sockets and saw correct frequency with Cv2 pot centered. This suggested U10 ok
as are resistors including r34 from delay pot which works.

We were curious how moving CV2 pot can change frequency (and get closer to -6v without CV2 and CV1 having anything plugged in ?
If it shouldnt, the we suspect one half of U17 might be injecting a voltage.
It appears pin1 of U17 has between 13.8 and 14.2Volts depending on the position of CV2 pot . Is this ok ?
I need a few days for replacement tl072s to be delivered before i can replace.

With the FBK trim we are looking to set the maximum amount of feedback. We're not really looking at distortion but the level of self-oscillation. There should be no input signal used in this part of the calibration.
Ok retried with no input, drive minimum, fbk maximum I get to 9am position and it oscillates with feedback. The trimmer only has an effect if i back off Drive to say 12 pm. Im rechecking BBD signal ins and outs.. I have a 142Hz in and a 19.2Khz out - all pots minimum.
Have you checked that the LEDs are the right way around? Check the voltage at pin 5 of U13. You should see a positive voltage at this point when you have a signal present at the input of the module. The voltage should be proportional to the signal level.
yes ..proved by accidentally shorting pin7 to pin8 ..both light up. Pin5 has no voltage. The voltage from the signal appears at U11 pin1 and p2 D14 but not p1 D14 . Ill be replacing this next. EDIT Replaced D14 and I now have 6.x volts on pin5, 1.2V on pin 1 U13. Both LEDs alight.
..Now how do I turn the red one off .. I reduce frequency of input signal to lfo level and still both. Removing input completely works. It seems a 4.x V signal at U11 pin1 is now uplifted to 6.x V signal appearing at p2 of new D14 :hmm: If cv2 pot is centrered and i move freq pot from minimum i can find a tiny sweetspot where red goes off...so external cause assumed for now.

enjoy rest of bank holiday.
Last edited by Pav on Mon May 27, 2019 1:45 pm, edited 3 times in total.
rgds Pav

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Pav
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Post by Pav » Mon May 27, 2019 12:18 pm

I forgot to mention i had some help testing from elJay today.:yay:
I listened to his flanger with all minimums .it is quiet .and it has a tremelo like flange "beating! i havent achieved yet.
rgds Pav

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Post by Synthbuilder » Wed May 29, 2019 3:15 am

Pav wrote:It appears pin1 of U17 has between 13.8 and 14.2Volts depending on the position of CV2 pot .
Pin 1 should be a copy of what is appearing on the CV2 input. So with nothing connected to CV2 pin 1 should be 0V. If CV2 were +2V, then pin 1 would also be +2V.

The CV2 pot should have no effect on the voltage of the op-amp outputs at pin 1 and pin 7 of U17.

Whenever any op-amp pin is close the supply rails suspect a fault. There are exceptions to this, eg. comparators, but for the most part the output should always be between +/-10V.
Ok retried with no input, drive minimum, fbk maximum I get to 9am position and it oscillates with feedback.
That suggests too much gain somewhere in the system. If the feedback phase switch makes a difference suspect the resistors R72 and R73 are different.
The trimmer only has an effect if i back off Drive to say 12 pm.
Although that suggests a problem with the connections around U16 pins 1, 2 & 3. Maybe even the drive pot itself.
Im rechecking BBD signal ins and outs.. I have a 142Hz in and a 19.2Khz out - all pots minimum.
The 19.2kHz could be the clock frequency. It's probably best to measure any audio through the BBD at the outputs of U8 - which will filter the worst of the clock from the true audio signal.

Tony

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Pav
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Post by Pav » Wed May 29, 2019 9:04 am

Thanks for sticking with me on this
some progress to report:

excess feedback fixed fbk trimmer does its job.
I have it just on the cusp with Feedback pot full CW and backed off.
(reflowed R72,R73,and R14 C58)

Calibration of clock minimum to 33Khz now completed.
CV2 Pot now has no effect. (until you plug something in to cv2 socket of course)
Red LED no longer stuck on.
(replaced U17).

The next and hopefully the last thing to achieve is "the auto flange" .
or maybe its the cyclically varying phase shift ? ( ack wikipedia )
I dont have it. Eljay does. Both on Mix Out and Delay Out.

and Delay pot is acting like as knotch filter as i turn it which i expect.
Green LED flickers on and off as I do this. Its reacting to "peaks" ..
not sure if it should.

Ill continue checking post BBds in the circuit but a pointer may help.
rgds Pav

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Post by Synthbuilder » Thu May 30, 2019 3:07 am

Pav wrote:The next and hopefully the last thing to achieve is "the auto flange" . or maybe its the cyclically varying phase shift ? ( ack wikipedia )
I dont have it. Eljay does. Both on Mix Out and Delay Out.
Actually, I'm not sure what you are referring to here. The delay time should not change much with no CV input, so the sound should remain mostly static unless the unit is self oscillating. There is a slight wibble each time the delay is changed - this is because the high frequency VCO takes a short time to settle. I quite like this effect so didn't make a big deal of 'fixing' it. The ADR30 does it too and it's more noticeable (and nice sounding) on that one because the delay times are longer.
and Delay pot is acting like as knotch filter as i turn it which i expect. Green LED flickers on and off as I do this. Its reacting to "peaks" ... not sure if it should.
The LEDs monitor the signal going into the BBDs. If you have feedback applied then that will affect the signal going into the BBDs and thus affect the LED status. Without feedback the LED status will only be affected by the drive and input signal level.

Tony

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Pav
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Post by Pav » Thu May 30, 2019 6:49 am

I expected it be static too without cv ins. In that case, i think i am done (or close to) with debugging yay :yay: I will just repeat calibration tests .

curiously, I now have the tremolo effect ... but its not the flanger, its the dixii II oscillator beating when midi-to-cv is connected. :hmm: I had knudged the mod wheel on my trusty Korg Prophecy.

Im taking feed from my eurorack as its more convenient for testing..nothing else connected.

Previously we had mididac plugged in.

ill be off to attempt recreate your sample 1 if i can. Particularly like the first .
I also like to recreate an old classic lead line "Telstar".
rgds Pav

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