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  1. Cirkit

    3.5 digit LCD IC - faulty multimeter (NJU9203)

    Hello all, I am trying to repair a faulty multimeter which is based around an NJU9203 3.5 digit LCD IC. When the multimeter is powered on, there are no segments displayed on the LCD. How would I check if the IC is faulty or other external components? I have established that the battery is...
  2. Cirkit

    Cable voltage drop - capacitor reservoir

    If you have a scenario where there is a cable (which cannot be shortened) between a battery and a circuit, is it better to have electrolytic capacitors or a super capacitor on the circuit input to help with transient demands from the circuit?
  3. Cirkit

    DAC vs digital potentiometer

    What is the advantage of using a DAC over a digital potentiometer? If the digital potentiometer is supplied from a stable regulated supply, is it not serving the same purpose as a DAC? I understand that a DAC may be faster however, for applications where a fixed voltage is set at power on...
  4. Cirkit

    Microwave Inverter PCB (F6645BA01GP) - Shorted Bridge Rectifier

    Hello all, I am attempting to repair a F6645BA01GP microwave inverter PCB (photos attached). The shorted bridge rectifier tripped the circuit breaker. Before anyone mentions it, I did check that all the high voltage capacitors were discharged before testing the components on the PCB. The IGBT...
  5. Cirkit

    GFG TVS Diode

    Is anyone able to identify this TVS diode marked GFG? I don't recognise the manufacturer's logo? It's from a switching regulator circuit.
  6. Cirkit

    Wireless doorbell chime - no sound

    I am trying to troubleshoot a wireless doorbell chime. It started to fail with a short chime and then no chime, just a flashing LED. The two ICs I cannot identify are: F0R1958 FLC-3A-58 Both are SOIC-8 packages.
  7. Cirkit

    Faulty LED driver (BREL-2D-36T)

    I am trying to repair this constant current LED driver. The output diode (MUR460) is reading a forward voltage of 0.071V and a reverse voltage of 0.087V. The forward resistance is 197Ω and the reverse is 218Ω. So clearly faulty. I have also measured the 330μF output smoothing capacitor and it...
  8. Cirkit

    Replacing surge varistor

    I need to replace a shorted surge varistor which has taken out the incoming fuse in a power supply. How does one determine the surge voltage rating based on a 240V AC line?
  9. Cirkit

    Dual battery powered tube light - blown transistors

    I have a battery powered dual tube light and one side fails with the transistors blowing. Attached is a photo of the PCB. The two D882 transistors on the left keep blowing. What would be causing this?
  10. Cirkit

    Resistor to discharge capacitor

    What is a suitable resistor to safely discharge a large main filter capacitor ~400V found in SMPSs? Would a 220Ω 2W resistor be too small in value?
  11. Cirkit

    Buzzing Makita DC1414 F charger

    I have the above Makita charger which is buzzing with a variable frequency when the green LED flashes. I have tried prodding the transformer and various components but the buzzing doesn't stop. I don't want to start replacing capacitors on the chance that may cure the buzzing. Attached are...
  12. Cirkit

    Fan making humming sound but not turning - CBB61?

    I am trying to repair an air filter. The fan just makes a humming sound on any of the three speed settings but does not rotate. I've measured the 1.5µF motor capacitor (CBB61) and it's around 1.2µF. The tolerance of the capacitor is 5%. Would this be enough to prevent the motor from rotating...
  13. Cirkit

    Low noise power supply - linear?

    Is it possible to design a low noise, low ripple power supply using a switching design with an LDO? One typical application is for use with sensitive pre-amp circuits typically measuring pA/nA currents? The preference by many is to just use a linear power supply.
  14. Cirkit

    IR2153D

    I am trying to test a SMPS controller daughterboard from a compact PA system. The daughterboard is based around an IR2153D IC. I have removed the daughterboard from the main PCB and am attempting to test it in isolation. I have applied between 10-20V between Vcc and COM but do not see any...
  15. Cirkit

    Ceramic capacitors substitute

    I have been reading about the global shortage of MLCCs and also how the properties of ceramic capacitors can vary if the PCB is flexed. Some manufacturers are recommended the use of newer polymer (electrolytic) capacitors as an alternative. Can polymer capacitors be substituted where a ceramic...
  16. Cirkit

    Non-inverting audio Op-Amp

    I am trying the reduce the gain of a non-inverting Op-Amp circuit used as an audio pre-Amp. I have measured the output and need to reduce the audio level by about 20dB. How do I calculate the correct resistor value change? Is it a linear relationship or do I need to convert dB gain to voltage...
  17. Cirkit

    433MHz door bell - Tx antenna feed

    I am trying to work out whether the component indicated by an arrow in the photo in the antenna feed, is a capacitor and what its value is likely to be? I have tried measuring it with a few different meters and it reads open circuit in resistance and capacitance mode which makes me think it...
  18. Cirkit

    Constant current LED drivers - faulty, flashing panels

    I have come across a few faulty LED drivers which cause the LED panels to flash on and off. Often this occurs after the driver/panel has been left switched on for a while. I've opened the drivers up and noticed some swelled capacitors - usually on the primary side i.e. the main 400V capacitor...
  19. Cirkit

    Blown multimeter fuse again - Doh!

    Is it just me or do others infrequently blow multimeter fuses when measuring current? :( I use the lower range current terminals when I think the current consumption is going to be lower than the fuse rating but the circuit being tested often doesn't behave as expected and tries to draw the...
  20. Cirkit

    Touch activated torch - increase sensitivity

    I've been given the attached circuit to improve. The LED should light when a finger is placed across two contacts spaced around 1cm apart. However, the LED doesn't light with everyone's skin. I have reduced the value of R2 from 150 Ohms to 47 Ohms which has slightly improved the sensitivity...
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