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  1. duke37

    Need with homemade inverter with tl494

    The dodes look to be correct. There is no information on the fets. 100V into 260Ω is about 400mA. Whic needs 4.4A input. Will the fets do this? Does the capacitor have a low ESR? Does the supply have a low impedance, try up to 0.1F close to the transformer. You could measure the voltage across...
  2. duke37

    Surround sound system repair

    The relays in an amplifier are most likely to be run on DC and will not hum. If the amplifier works, then the relays are working. Is the hum coming from the amplifier or a speaker? If the hum is coming from the amplifier then try to find the component which is vibrating. If the hum is comming...
  3. duke37

    unbiased portion of transistor

    #12 " the scr should continue to conduct. not so here." The scr will only continue to conduct if it is passing more than a minimal holding current.
  4. duke37

    Multiple batteries in a car.

    Some comments. 1. A car alternator will not have the output you want. 2. The alternator should not be harmed by charging a very big battery. 3. Big alternators are fitted to trucks, tractors and busses, often 24V. 4. Some tractors have mains alternators built into the engine intended for driving...
  5. duke37

    Need help with homework

    I prefer a common ground line across the bottom of the diagram rather than lumps of ground floating in the air, it is easier to see what is happening. There is a 6mH inductance connected to ground so the answer must be less than this with more inductance connected across it. Calculate the...
  6. duke37

    Help Identify Component

    1 is one 0 is zero 4 is 0000 so 100,000pF or 100nF or 0.1μF I think the K is the type of insulation and the 630 is probably the permitted voltage. There should be NO DC connection through it, it is a capacitor
  7. duke37

    DIY mesa boogie mark iic+

    As I said, I cannot read the digram properly but assuming an anode resistor of 100k and 100V across it, that is 1mA. Four valves equals 4mA. You can calculate better if you know the valve voltages. Make a supply capable of 20mA or so to give plenty of leeway.
  8. duke37

    Crate BT-100 Bass Amp Blowing Fuses

    Run the amp with a series bulb (say 100W) to stop the fuses blowing. Does the light glow? Disconnect C20 and C21. Are the tops flat or domed? Does the light glow? Note that these are shown as 0.1, this will be 0.1F i.e. 100,000μF.
  9. duke37

    Trouble understanding shunt resistance for short-circuit current measurement

    I went to get a slow puncture fixed yesterday and the mechanic said that he repaired valve televisions.:) He showed me a device for measuring current without connection in automobile circuits. This would work on AC or DC. It was very sensitive to orientation presumably due to the earth's...
  10. duke37

    need to make a circuit

    The minimum current is probably not a poblem, the maximum current is however important.
  11. duke37

    DIY mesa boogie mark iic+

    I cannot read the diagram properly but it looks as if the tubes are low power amplifiers such as ECC83. They all have cathode resistors which will set the current. You can look up the tube characteristics to calculate the current but if you assume say 100V on the anodes, it is easy to estimate...
  12. duke37

    Trouble understanding shunt resistance for short-circuit current measurement

    Lead resistance should be taken into account. Various load resistors could be used to extrapolate to zero resistance. A mosfet, though good will have appeciable resistance.
  13. duke37

    Christmas tree lights

    I have just managed to get the christmas tree lights working from last christmas and now it is looming up again. The lights are small low voltage bulbs in series to get up to 230V but here the mains can be over 250V, hence the life of the bulbs can be quite short. I have placed a series diode...
  14. duke37

    Voltage Rectifier/Regulator Design for Magneto

    A car dynamo regulates its output by altering the field excitation. It can be damaged by taking too much current so a control box reduces the excitation at at certain current level. A car alternator is built with a leakage inductance so that the maximum current is safe even with the output...
  15. duke37

    Volume knob, Roberts radio, intermittent failure

    I would check if the rivets connect to the legs and solder if necessary.
  16. duke37

    Volume knob, Roberts radio, intermittent failure

    Obviously good connections are necessary for it to work. Try to find the problem area. A picture of the volume control would help, it may be soldered to a printed circuit track which is cracked. There will be three connections for the volume control and more for the on/off switch If the volume...
  17. duke37

    RF Choke Repair

    Looks as if you can repair by soldering the wire to the stub. If necessary you could take off one turn. If it is Litz wire (lots of very fine strands) then it will be difficult to get them all to wet. A solder pot can be used with copious amounts of flux.
  18. duke37

    Which relay should I use?

    Is the motor a rotating permanent magnet type or a squirrel cage. At very low speeds the phase voltages must be very low to limit the current to a reasonable value. I think these motors need a voltage proportional to frequency.
  19. duke37

    Home stereo compatible with 12v campervan?

    Make very sure that you connect with the correct polarity.
  20. duke37

    Da sander blowing fuses ?

    If it was bifilar wound, two wires side by side, then the winding direction does not matter, just keep them in the same direction beginning to end. Starting in the middle as edd says makes it much easier. It is easy to get in a mess particularly with thickish wire which will not go where you...
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