So, I swept the TR Switch from 5 to 50MHz using port J23 to ANT1 with PTT energized trying to find what is causing my amps to oscillate at ~30MHz.? Attached is a plot from the NanoVNA.? I tried shorting the relay on the TR Switch and putting in my cu perf board short across transmitter to antenna connectors (this stops oscillation).? At 24MHz the impedances are all within 1-2 ohms of each other.? I have tested with and without the LPF and BPF.? Nothing stops the oscillation.?
Bill suggested looking for feedback.? When I stop the receive signal from reaching the LPF-Cntl board, the oscillation stops.? See pix of TR Switch elevated so receive pins don't engage with connector. So, it is being caused by feedback.? Not sure what to do next.
dave
Ok you have a lot of good equipment to solve this. ? Yeah it would not hurt to try shorting over the relay on the daughter board. ? I would also try removing the BPF board and just use a jumper to reduce the number of things in line.? I would expect an oscillatory behavior if one of the TR switches either on the BPF or on the LPF was not switching correctly (so feeding back in TX).? Those things are really small and just a tiny drop of solder could cause it. ? ? 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 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. ? 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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