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  1. Audioguru

    Bench power supply upgrade

    When OP3 was changed to a 741 opamp then you still have the problem of the led lighting when it shouldn't and no output voltage control after power-up when the voltage pot isn't turned all the way down at power-up? The output of OP3 should always be at +10.2V and the output of OP2 controls the...
  2. Audioguru

    Increase the amperage fo PNP transistor

    Current gain is not used when a transistor is used as a switch. For 350mA output then the datasheet says the base current must be 35mA or more. The LM393 has a minimum output current of only 6mA and the 1k base resistor limits the output current even more.
  3. Audioguru

    Bench power supply upgrade

    The very old original circuit never had a not needed resistor that the Chinese used for R22. It is simply parallel to the voltage setting pot R13. You can replace the opamps with 741 opamps but the 741 uses 10k for R1 and a piece of wire for R8 connected to the negative supply.
  4. Audioguru

    Bench power supply upgrade

    I forgot about cheap Chinese fake parts. I never buy Chinese junk online. OP3 has no negative supply so you should measure its output voltage from its output to the 0V output. What was the meter wrongly set to measure when you switched it to measure voltage? I am sorry that I do not have time to...
  5. Audioguru

    How do i repair a noise sayonapps 1192bt subwoofer?

    It is normal for a cheap and poor quality speaker system to have your hiss problem. Replace it with a Name Brand higher cost, higher quality system.
  6. Audioguru

    How do i repair a noise sayonapps 1192bt subwoofer?

    A cheap and poor quality amplifier or amplifier IC produces hiss. The sales sheet from the amplified speaker system manufacturer says 18,000W (!) for the subwoofer parts then also says 100W but with no audio spec's like what kind of Watts, with how much distortion and at what frequency. They say...
  7. Audioguru

    How do i repair a noise sayonapps 1192bt subwoofer?

    Noise can be high frequency hiss, low frequency hum, a continuous or pulsing tone, clicks and pops when an electrical device turns on or off nearby, or radio interference. Please tell us which kind of these noises.
  8. Audioguru

    Bench power supply upgrade

    It is OP2 that senses current in R6 and if the current is higher than the setting of the current pot then the output of OP2 goes negative which turns on VT4 and the red warning LED, and turns down the output voltage through VD2 and OP1. Your problem occurs when the voltage setting of R13 is...
  9. Audioguru

    Bench power supply upgrade

    When you have the problem of no variable output voltage and the output of OP1 is -4.1V, is the red LED turned on (when it should be turned off)? The correct R8 allows you to to turn down the voltage to minimum and adjust R7 for an output of 0.00V.
  10. Audioguru

    Bench power supply upgrade

    The output voltage of opamp OP1 is always -4.1V probably because diode VD2 is shorted or it has its polarity backwards and the red light is turned on showing that the current control has a problem. Intermittent problems are caused by an intermittent part or a poor solder joint.
  11. Audioguru

    Bench power supply upgrade

    You should make the R8 resistance 1.5k and connect it to its negative supply voltage as shown on its datasheet. Like I said, when the output voltage is wrong, measure the output voltage of OP1 and all the other parts said.
  12. Audioguru

    Bench power supply upgrade

    In post #73 you said the output is 0V but now you say the output is -0.659V. R7 sets input offset voltage of opamp OP1 so that the output of the project is 0.0V if R8 has the correct value and is connected to -10.2V. R20 allows the current regulating opamp OP2 to have its +input at a tiny...
  13. Audioguru

    Bench power supply upgrade

    The current LED is supposed to light only when the output current (measured in R6) exceeds the setting of the current pot R21. Yours has a problem. If the output current is zero then R6 has 0V across it which feeds R17 and the - input of opamp OP2. The pot R21 feeds the + input of OP2. When the...
  14. Audioguru

    Bench power supply upgrade

    I do not know the part number of your 5.1V zener diode. It might have its 5.1 voltage when it has 5mA or have 20mA of current. Its tolerance is probably 5% so its rated voltage is from 4.85V to 5.36V. Your zener diode is 5.35V so it is close to 5% high. The voltage of a zener diode rises a...
  15. Audioguru

    Bench power supply upgrade

    I am sorry to be late with this reply. The test report of the power supply kit did not mention: 1) The lack of adjusting a 0V output with R7 that is connected much differently than the TL081 datasheet. 2) The complete loss of voltage regulation when the voltage setting pot R13 is set too high...
  16. Audioguru

    Bench power supply upgrade

    Aren't you using old TL081 opamps? They need R8 to be 1.5k which is connected to your -5.1V supply.
  17. Audioguru

    Bench power supply upgrade

    My resistor connects to ground because I used a modern TLE2141 opamp and not the antique TL081 that was used in the original European power supply circuit. Your kit uses the old TL081 opamp and its datasheet shows the resistor connected to the negative supply voltage.
  18. Audioguru

    Bench power supply upgrade

    The maximum regulated output is about 6V less than the input voltage because the opamps and transistors are not rail-to-rail. The circuit I fixed uses a 28VAC transformer producing rectified 38VDC at full load. It produces about 40VDC with a low load current. Then I used a 1.4V negative supply...
  19. Audioguru

    Bench power supply upgrade

    PS2 must be 30VDC for a 24V at 2.4A output.
  20. Audioguru

    Bench power supply upgrade

    No. The emitter of VT3 connects to 0V, not to -5.1V. The original European circuit got it wrong and the Chinese copied it.
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