High LDR Calibration Issue

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  • #41209 Reply


    I am having some trouble calibrating the Quaverato. Power, Tap and Bypass lights all operate as they should. Gain and Low trim are set as per the manual. However, I can not get High trim to go below 5.7k. It will go higher, but not lower. With power on, and a 1/4 plug in the input jack I’ve shined a light all around on the Optocoupler with no change. I’ve checked the LED and it has 1.8 ish voltage drop. I’ve reflowed the trim pot, the led and the LDR all with no effect. One thing that is sticking out to me is that when I probe TP1 and 2 with no power to the pedal, I’m getting about 28M Ohms and it rises. TP3 and 4 give an open reading. Is it possible the LDR is bad? Is there a way to test it?

    Thank you for your time and attention.

    #41210 Reply

    Update: I shined light around Opto-1 when there was no power to the pedal and I got a change in resistance when probing TP1 and 2. So, I reheated the heat shrink and used tweezers to clamp down on the shrink and was able to seal it enough so that when there is no power to the pedal and I probe TP1 and 2 I get an open reading. I also re-flowed Q1 and tested R8 and R7. R8 and R7 are the correct values and both have 4.98v. I also tested the trim pot and it seems to be operating correctly. Next I am going to measure the resistance between R15 (the pin of R15 closest to pin 1 of U2) and pin 3 on U2, with the power off and the voltages on the pins of U2A, pins 1,2, and 3.

    Thanks for your time and attention.

    #41211 Reply

    It seems like it’s all working correctly as you described. Is the pedal not sounding correct to you?
    Remember the calibration values given in the manual are arbitrary values to just ensure the circuit is working more or less correctly, and by your measurements, it is. Keep the high trim pot set to whatever gives you the lowest value and then adjust the low trim pot until you like way it sounds. Make up for any low signal levels with the gain trim pot.
    Also remember when you measure the resistance of the test points, you are measuring the LDR resistance in the optoccoupler while the LED light is shining on it, which the trim pot is controlling the brightness of (see the schematic). That is why the resistance is going up to megaohms/open when the light/power is off.

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