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  1. audioguru2

    fm tranismiter

    Hi Alun, Of course, I forgot, you can't tune or FM modulate the carrier, you modulate the ceramic filter. The schoolboys tuned the capacitance to ground on one side (the input) of the ceramic filter, like a crystal oscillator in a clock is tuned.
  2. audioguru2

    Electronic Stethoscope

    Hi Dazza, I can hear your heartbeat!  ;D It sounds pretty good at about 1/4 volume but the crackling with more volume seems to be the LM386 clipping at its max output. It also seems to be motorboating at about 8 or 10Hz. I wish I had 'scope software on my computer to see it. During my tests I...
  3. audioguru2

    fm tranismiter

    Hi Alun, Sure I noticed. There are simple FM transmitter colipitts oscillators on the web without any emitter resistor and their designer says they work. I doubt it.
  4. audioguru2

    Correct way to Measure Power Supply Amperage

    Hi Prattek, AC syncronous electric motors and switched-mode power supplies use more current when the supply voltage drops as they try to maintain their power output. The motor slips phase which increases the current, and the PWM of the switched-mode power supply increases the "on" time of its...
  5. audioguru2

    Electronic Stethoscope

    Hi Dazza, Your voltage measurements are perfect.
  6. audioguru2

    Electronic Stethoscope

    Hi Dazza, Sure I will help you on this project.  ;D Please don't give-up on the other member's pcb yet. I am only guessing that U5 is oscillating, maybe something else is wrong. 1) Measure the DC voltage at the microphone. With a 9V supply, it should be between +2V and +7V. 2) Measure the DC...
  7. audioguru2

    fm tranismiter

    Hi Alun, The capacitance change of a diode seems to be very unlinear with voltage change across it. On the graph you posted, the capacitance changes from 29pF at 0.1V reverse bias to 19pF at 1.0V reverse bias. But when you increase the bias voltage another 0.9V the cap changes by only a couple...
  8. audioguru2

    bidirectional re

    Kevin, Are you feeding a signal to the emitter of a transistor and expecting to get the signal amplified out of its base?  ??? ??? ??? ??? Transistors don't work like that. A common-base amplifier that is biased correctly has the emitter as a very low impedance input and the collector as a...
  9. audioguru2

    Correct way to Measure Power Supply Amperage

    Hi Mhz, Ohm's Law says that if a power supply is putting out the correct voltage, then it must[/] also be putting out enough current for the load. If it was unable to provide enough current, its voltage would drop.  ::)
  10. audioguru2

    fm tranismiter

    Hi Alun, 1) The graphs don't show the capacitance of a diode when it is slightly forward-biased. Is the capacitance change with voltage change still linear to avoid distortion? 2) How would your 1N4001 "varicap" work if it was 2 diodes back-to-back like your overmodulation limiter? Then the...
  11. audioguru2

    STK465 Stereo Amp

    Hi Roby, Build it as shown on the pcb layout and it might work OK. Notice that the outputs have many turns of wire wrapped around a 10 ohm resistor. For "the price of catering" or something!  ;D
  12. audioguru2

    LM317 Variable power supply

    Hi TechnoBoy, Welcome to our forum.  ;D Don't worry about inrush current. It is limited by the resistance of the transformer. Even a little 1N400x 1A rectifier has a surge current rating of 30A! To get a 12V output, the LM317 needs a minimum input voltage of 15V. A 12V transformer would just...
  13. audioguru2

    Electronic Stethoscope

    Hi Dazza, I'm sorry I didn't make a pcb for this project.  :'( It's too bad the pcb shorts flattened 2 batteries and I see where its R7 and R8 are swapped. I don't think the IC's are damaged by the batteries being shorted. I think that the LM386 as U5 is oscillating when you turn up the volume...
  14. audioguru2

    Electronic Eavesdropping Devices Detector

    Hi Duke, Welcome to our forum. I am sorry to hear that the project didn't work for you. Please tell us what errors you found on its pcb layout. 1) Did you substitute any parts? 2) When you turn down the sensistivity control, dose the pitch of the VCO reduce to a low-pitch growl or stop? 3) Do...
  15. audioguru2

    STK465 Stereo Amp

    Hi Roby, I think the parts list is for two separate amps but the pcb has them and their supply filters combined. Did you find the Sanyo STK 465 hybrid amp modules anywhere? On a Google search I found a Telefunken stereo repairman saying that they are rare since they were made only in 1981.
  16. audioguru2

    fm tranismiter

    Hi Alun, Excuses and more excuses: You couldn't even make a 100MHz oscillator on a circuit board so you had to use a tangle of heavy buss-wire. He, he. ;D Now you are blaming the varnish on the wire?
  17. audioguru2

    differentiator

    Hi Jit, Welcome to our forum. A differentiator is just a highpass filter. Its series capacitor has a high capacitive reactance to block low frequencies but is like a dead short to pass high frequencies. It can be used to pass a spike at the edges of a square wave.
  18. audioguru2

    fm tranismiter

    Hi Sasi, That's a neat circuit.  ;D It oscillates at 10.7MHz but you have its output tuned to the 9th harmonic at 96.3MHz. I didn't know you could do all that with only a single transistor.  :o ;D Watch Alun modulate it with a varicap for "pure" FM modulation. Our world is going to be crowded...
  19. audioguru2

    fm tranismiter

    Hi Alun, The ceramic filter is the crystal of their 10.7MHz oscillator. The note I've attached shows that a Cmos inverter has enough gain at 10.7MHz to oscillate. N3 squares it up and the paralleled inverters even more. I can't believe they have any output of the 9th harmonic at 96.3MHz. In the...
  20. audioguru2

    fm tranismiter

    Hi Alun, The BB109 varicap has been used in some FM transmitter projects on the web, for FM modulation and for channel tuning. I couldn't find one anywhere near me. It was made by Philips or Vishay-Siemens. Here is a very long essay by some University students for them to transmit data using...
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