I have a TinySA, NanoVNA, 50MHz scope, DVM's, Taidacent Pwr Mtr, KiwiSDR, audio generators, assorted dummy loads, attenuators, and couplers.
I used the NanoVNA to sweep the 20 wt Power Amp (I built 2 of them last year) and they were very flat and stable from 3-50 MHz at 12VDC and ~1.5ADC and ~5 wt output.
I have put 22 AWG wire over all the TR switch RF output tracks.? I have soldered the TR Switch into the circuit to rule out connector issues.? The amplifier works great with the home built TR switch bypass perf board (from 3-54MHz).? It oscillates with the TR switch installed on the 12, 10, and 6 meter bands at roughly 33MHz and drawing about 2ADC when the PTT is engaged.? In CW mode it has the desired frequency and the rogue 33 MHz. Tomorrow, I'll get the NanoVNA and sweep the TR Switch circuit from J23 (FROM LPF) on the LPF-Cntl board to ANT1 with the PTT engaged.? Maybe that will give me a clue.
I have 2 TR Switch boards - both exhibit this behavior.? The relays are working fine at 21MHz, so don't think they are the problem.? Thanks for your help.
dave
First off¡ what sort of test equipment do you have Dave? ? Just stuff like DVMs or do you have something like a VNA or a Spectrum Analyzer? And possibly a signal generator or another radio you can test this PA on? ? ? Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ VP2EHZ ? Owner - Operator Big Signal Ranch ¨C K9ZC Staunton, Illinois ? Owner ¨C Operator Villa Grand Piton ¨C 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@... ? ? ? I get back tonight.? Ill have some simple tests cooked up to get to the bottom of this.? ? Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch ¨C K9ZC Staunton, Illinois ? Owner ¨C Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it: ? email:??bill@... ?
On Mar 17, 2025, at 10:28?AM, D Solt via <davesolt@...> wrote:
? So, I am still tracking down the 20W amp oscillation, and making progress.? I have C24 in the amp, although it makes no difference to the oscillation in the high bands.? I also have the onboard attenuator in the PA.? Bypassing the BPF and the LPF made no difference to the oscillation.? Ferrites, RF chokes, and additional bypassing right on the center tap of the output transformer made no difference. The oscillation goes away when I take the output of the LPF directly to the dummy load bypassing the SWR circuit and the Transmitt Receive Switch.? I wonder if I have something wired up wrong in the transformer or the switch. Thanks again for all of your help. ? Sounds like the inpuu connection to the PA is high impedance (like something is not connected on the boads in front of the PA).. ?you migjt take the BpF put of the circuit temporarily.? Just jumper across it on the LPF board pr remove ?the LPF too just temporarily. ? ? Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch ¨C K9ZC Staunton, Illinois ? Owner ¨C Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it: ? email:??bill@... ? ?Thanks Jerry and Bill. I¡¯ll get back to this on Monday. 3 dB pad on input of PA reduces oscillation, 6 dB pad stops the oscillation? ?Remove the bpf and see what happens. ? ? Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch ¨C K9ZC Staunton, Illinois ? Owner ¨C Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it: ? email:??bill@... ? ?Yes, I agree. The amp is solid. Previously , I swept the amps and got solid performance. And, when I disconnect the drive to the amp the oscillation stops. It is being caused by something in the T41 interacting with the amp ?Something is wrong.? The amp will NOT oscillate by itself. ?? ? Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch ¨C K9ZC Staunton, Illinois ? Owner ¨C Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it: ? email:??bill@... ?
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