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  1. duke37

    Video signal amplifier.

    Coax can be of many different impedances, from 25 ohm up. High quality cables will have the type written on the side. In the UK, amateur radio operators usually use 50 ohm but TV and satellite dishes use 75 ohm. I also use 300 ohm twin feeder to connect to a folded dipole. Measuring the...
  2. duke37

    replacing 4 wire dc motor with 2 wire - need help

    I would not think that brushes could be very noisy but bearings can be. Try turning the motor by hand and feel if the bearings are smooth. Organ blower motors and Hi-Fi turntables use plain bearings because they are quieter. Bearing life can be limited by fatigue of the rolling surfaces giving...
  3. duke37

    Strange voltage variation in circuit

    Is this how it operates as you say or how it should work? Sound is a collection of AC signals, I think you will need a diode detector to give a voltage envelope when sound is detected.These envelopes can then be amplified by the 741 to trigger the 4027.
  4. duke37

    dc measuring

    To quote the voltage to such precision is absurd since the voltage drop of the diode is unknown and not taken into account. The two lower diagrams are similar. The only difference is that the diode is placed at a different place in the same wire. Have you considered polarity?
  5. duke37

    Need help with design

    At 1A you will be dissipating a lot of heat even with a small voltage drop. Consider using an appropriate transformer tap to minimise the drop. Chose a lowish voltage supply and then use a switch mode boost circuit to get the higher voltages.
  6. duke37

    5 Transistors Linear Triangle Wave Generator

    You should not be amazed at my ignorance. I assume that the triangle wave is across C1. But where is the square wave taken from the multivibrator? Is there any way of showing the nodes in LT spice?
  7. duke37

    Using a radial cap instead of an axial one - shielding needed?

    Depending on the circuit, I would connect the long lead to the earthy potential. The picture shows the long lead connected to + which may have AC on it.
  8. duke37

    Please advise a good substitute potentiometer

    I guess that the trailer brake power is needed to be adjusted to give appropriate retardation with a certain load. If a variable resistance cannot be found, then five 1 ohm resisters could be connected in series and a six pole switch could be used to select the required resistance.
  9. duke37

    How to: power this old ringer?

    The ringer signal is AC on a phone. There is thus no negative. Your picture blanks out after a second or so thus I cannot see the circuit. Are there pionts to actuate the coil?
  10. duke37

    HV Rat Killer

    Oh dear:) Your 110V primary transformer will be no good to be fed from 230V, it will go pop. Electric fences use a very high voltage, several kV but the energy in the discharging capacitor capacitor is limited to about 1J or in some countries 10J. The high voltage is necessary to spark through...
  11. duke37

    Solder sucker

    I have had a simple solder sucker for many years but more recently purchased a soldering iron with a hollow bit and a sucker pump. I think this came from CPC as a special offer. Works well.
  12. duke37

    Control alternator voltage back to 6vlt battery?

    Have you got the correct size of bulb?
  13. duke37

    Phone call causes LOUD screeching noise from unknown location near my apartment building

    It would be a help if you said which country you are in and the name of the electricity supply company. It appears that the noise level is well above what a mobile phone can deliver.
  14. duke37

    Making a FM transmitter

    I do not see any components that can be eliminated. Split the circuit into an audio amplifier and an RF oscillator. Audio. The three resistors set the operating voltages and the gain The base current will be low so the gain will be approx R4/R3 or 100. The collector voltage should be about half...
  15. duke37

    Class AB Power Amplifier Design

    Q3 and Q4 are emitter followers and the emitters should be close to half the supply voltage so that clipping will be symetrical for maximum power output. RB3b and RB3a will therfore need to be the same. The resistors should be low enough to give sufficient current to drive Q3 and Q4. RB2...
  16. duke37

    Please explain this resistance reading

    You have good pictures so draw a schematic and take a photo of it. Mark on it where you are taking voltage measurements. Do NOT use the meter for resistance measurement when power is applied.
  17. duke37

    DC 9v replacement

    There will be plenty of room inside the battery compartment to fit a 9V or 10V regulator. I was asked to repair a US 120V ghetto blaster which had been run on 240V. I fitted a 9V regulator in the battery compartment. This was a 7805 with the reference pin raised with a red led. It worked but...
  18. duke37

    DC 9v replacement

    You can make a couple of wooden batteries and fix some thin tinplate at the end to make a connection. Solder a wire to the tinplate and feed with 9V of the correct polarity. Note that 8 batteries will give more like 12V.
  19. duke37

    Why relay contact stick even when power off?

    Relays can stay closed due to retained magnetism in the pole pieces. I used to repair electric fencers which would have this problem after many cycles. The relays had a brass screw which stopped the pole pieces closing fully. This would wear down after a few years and allow the relay to stick.
  20. duke37

    My homemade heating mantle doesn't work properly

    Asking a 4A device to supply 20A is a recipe for disaster. What is the resistance of the wire? Try the dimmer with a 100W bulb. Does it still work?
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