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Wednesday, April 21, 2010

Bad Vibes (No Vibes) :-(

Thanks for all the advice. I especially liked the LTSpice model -- it was good to get back to that great program. I plugged in the values that I am using (from Hans' circuit) and the 2N2222 transistors -- Spice says it should work. But no joy in the real world. I tried it with .1 uF caps -- nothing. I tried it with two fairly big 15 uF electolytics (as always, with the positive terminals to the collectors). No luck. Both collectors are just sitting there at .015 volts, both of the bases are at .6 volts. Sounds like both transistors are constantly at saturation, right?
Any other ideas? (I know I could do this with an IC, but I want to try to make it work with the simple two transistor circuit.)

7 comments:

  1. As both transistors appear to be on, will it begin oscillating if you briefly short one of the transistor bases to ground.

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  2. Sounds dumb, but post a photo.

    Maybe you are overlooking something simple that you can't see because you've been looking at it too long (happens to me all the time).

    Paul - K0EET

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  3.    I tried just the multivibrator part of the circuit in LTspice. I placed a probe where the 1M resistor would have been. I wasn't including that or anything to the right of it (except +V and ground of course)

       I set the simulation to go from 0 to 120 seconds and it did not vibrate. I used 2n2222 models and tried various voltages from 1.5 to 12.

       Then I lowered the resistors to the values I got from Dale Wheat's 'Wee Blinky' schematic. The two 220k became 1k and the two 680k became 33k. It takes 37 seconds to start but it does vibrate. Pulses last about 1/2 second with 22uf Caps and those resistor values.

       I'm curious, with this circuit how do you id? Or is it such qrpp that it falls under the Italian equivalent of part 15?

    Good luck and 73!

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  4. Oh, I almost forgot, I only did the multivibrator part because I wasn't sure about the inductance of the transformer. Do you have those values?

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  5. Are you certain that the transistors are good? If FETs were involved, I'd check that first as they hate static and it doesn't take much to fry them. Since the 2N2222s are BJTs, static is much less a worry and employed as they are, overcurrent is not a problem. Is there a chance that either or both were DOA before you installed them?

    Keep us posted and good luck!

    73 - Scott

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  6. I suggest that you lift the two transistors's emitter and add a 1k resistor to ground. this ensure that on a cold startup the transistor that conducts heaviest first will cause the other transistor to stay off because of the voltage across the 1k resistor.
    wayne va7at

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  7. You got me hooked and I slapped together a similar circuit using two 2SC1815 hfe 114 general audio purpose transistors, 220k collector resistors, 660k (three 220k in series) base resistors and 47uF 35V capacitors.using 12 volt power supply I measured the collector on one side with my trusty 1980 MICRONA radio shack digital meter and its oscillating over 15 seconds. measuring the oscillation the wave voltage reads a flat line of 23mV and then ramps up slowly to a value about 8.9V then collapse to 23mV again.Bill, it does work first time slapped together. hope you didn't use a low impedance multimeter?

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