Re: QCX 50W amp low power - dropping at LPF
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Charlie, the RF voltage measurement at the input of the filter is not necessarily being taken at a 50 ohm point so if the impedance is higher, then the voltage will read higher.?
You really need to set the bias as directed for the amp to provide expected power output. With no signal in, key the amp and look at the current drain. Slowly rotate the bias pot clockwise until the input current starts to go up, then carefully back it down to what it was before it started rising. You are correct it won't make any difference in the apparent measured loss across the LPF, but that voltage measurement is really meaningless except when taken right at the 50 ohm dummy load.?
Ron
On Sat, Oct 19, 2024 at 12:50?PM Charlie, N8DD via <instantcrow= [email protected]> wrote: I built the 50 W amp for 40m. In my testing I found that the amplified signal goes from 80 V P-P after T3 (just before LPF) and drops to 50V P-P after LPF (measured just after c14 before c15).?
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I'm running 12 V and I set the bias at R5 fully counterclockwise (note setting bias as per manual does not change the V drop over LPF). Drive is about 5w coming from QCX+.?
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I triple checked L4, L5 and L6 (all the windings are correct in # and continuity) as well as C14, 18, 11, and 12 (correct values).?
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The signal loses about 30Vp-p amplitude, though it's much cleaner after LPF.?
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I reviewed the N5NY troubleshooting video, and everything checks out. It's just that I'm losing signal at LPF.
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What might likely culprit be? Thinking it might be one of the capacitors in LPF. I'm not sure how to test them without removing from circuit.?
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Thanks for any feedback!
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Charlie
N8DD
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Ronald Taylor