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Old 23rd May 2020, 4:48 pm   #1
eddie_ce
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Join Date: Aug 2010
Location: nr. Hannover, Germany
Posts: 372
Default Problems with MiniMod

Finally having got round to some homebrew work, I decided to build the MiniMod. Unfortunately without much success. The RF side of things is fine, the LC oscillator is stable, the output coil has a good defined peak.

However, the LM386 is extremely unstable and bursts into oscillation at the RF frequency. If I disconnect the 390R resistor to the BC109 the IC is somewhat more stable and does what it is supposed to do although it still does sporadically burst into oscillation. I have removed the capacitor between pins 1 and 8 with the aim to reduce gain, but the problem persists.

I have read elsewhere that the LM386 is prone to this sort of problem Does anyone have any tips on how to tame the beast. I have designed my own PCB, but can't see anything obvious, I have kept the input and output tracks as far apart as possible.

When in rogue oscillator mode the circuit draws 130mA and needless to say the IC gets hot after a short time. I am running it with a 9,5V supply. All decoupling is in place and as close to the IC as possible. I have soldered an extra 100nF C directly under the supply pins of the IC.
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