Quote:
Originally Posted by Aussie Bloke
G'day all. I found the exact problem. TR21 is the culprit as when I removed it from the circuit I got a perfect sawtooth waveform at the 4.7uF cap and at a much lower voltage. I was using the MPF102 transistor which according to my tech friend is close to equivalent to the 2N3819 though I wonder if the slight differences with them would play a big factor in causing the capacitor to charge to quick?
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This extreme charging had to be due to a gate-drain short in the defective TR21. Normally the gate current should be zero, and either of these parts should work.