Re: Funny things with the Diode LPF Switching
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I use almost the same circuit in my power switch control for the T41.? The difference is that I use a 150 ohm resistor in the Drain lead of what here is Q101… that keeps the current to a reasonable level for the 2N7002 yet still accomplishes the same goal by swamping the load side of the power switch to ground.? I can’t see how the existing circuit can survive long term because there are points at which Q101 and Q9 are both conducting. ? ? Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ VP2EHZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton – J68HZ Soufriere, St. Lucia W.I. Rent it: ? Moderator: North American QRO Group at Groups.IO. Moderator: Amateur Radio Builders Group at Groups.IO. ? email:? bill@... ? ?
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From: [email protected] < [email protected]> On Behalf Of Gordon Gibby Sent: Monday, January 1, 2024 9:29 AM To: [email protected]Subject: Re: [BITX20] Funny things with the Diode LPF Switching ? I found Allison's RC circuit: Suspecting that at some point Q9 IRF4905 and Q101 got both ON and the 4905 destroyed the 2N7002.....? ? ? On Mon, Jan 1, 2024 at 10:15?AM Gordon Gibby via <docvacuumtubes=[email protected]> wrote: OK, energized by your history, I started trying to measure the voltages surrounding Q9.
1.? ?The DC output going to R77 (and the other stages of the transmitter) is HIGH when the unit is turned on (and oscillates)? Verified with scope DC coupled probe.
2.? ?The TX output is LOW (verified with scope DC coupled probe & also by the fact I had consistently noted that my opto-isolator indicator is NOT on)
3.? ?The voltage on the gate of Q9 (one end of R6) is intermediate -- I would estimate 9 volts but I didn't measure it other than watching the trace on the scope at 5V per division.
So my understanding so far is that Q10 must be very slightly conducting, so that there is a voltage drop in R6? (estimate voltage drop of 4 volts from scope, so 0.4 mA flowing through R6).??
This is enough to turn Q9 ON and provide power to the transmitter stages.? ?
What I don't understand is what is happening with Q101:? ?If the voltage on its gate is +9V, it should be fully ON and carrying many amps of current -- enough to burn it out!? So has it already been destroyed?? ?Q101 looks very vulnerable to destruction if the circuit ever stays for more than a moment with an intermediate voltage on the gate of Q9 IRF4905.
In order to reduce the supply of current to the transmitter, it looks like I should add a lower resistance in parallel with R5 to further turn off Q10.
Any wisdom from the group on this???
Gordon KX4Z
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