tag:blogger.com,1999:blog-7590176649168185428.post4104521741648779200..comments2024-03-29T07:34:28.586-04:00Comments on SolderSmoke Daily News: Woe is me! Why Podcast is Late: Parasitic Oscillations! Bill Mearahttp://www.blogger.com/profile/07662500663603350847noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-7590176649168185428.post-90337103131512410502013-01-22T20:34:18.397-05:002013-01-22T20:34:18.397-05:00Hi, I am having a identical problem with my Beach ...Hi, I am having a identical problem with my Beach 40 DSB rig. I also have bypassed all the power connections, inputs and outputs are as separated without completely rebuilding. Everything looks perfect on the scope until I couple the final into the circuit, then the parasitic oscillations start. I realize it is because there is more RF present with the final hooked up. It looks perfect on the scope while hooked to the dummy load, but when I hook up the dipole with a very good match at that frequency I'm testing, the oscillations start. I'm baffled, any ideas? Maybe it will be better when I enclose it in a box.<br /><br />Joel<br />KB6QVI caulktelhttps://www.blogger.com/profile/10779271046772673688noreply@blogger.comtag:blogger.com,1999:blog-7590176649168185428.post-69596076968126791202012-12-27T00:13:10.918-05:002012-12-27T00:13:10.918-05:00I concur with Bob KD4EBM, check the antenna.
Anoth...I concur with Bob KD4EBM, check the antenna.<br />Another idea is use the antenna tuner between the amp and the dummy load to add some mismatch and reactance. If you cannot get it oscillating with the mismatch then that will let the antenna off the hook.<br />I know you said that you cannot see the oscillations at the input but can you ground the amp input at the mixer. That will verify that unwanted RF is not getting into the mixer.<br />And try grounding the base of the next stage with a capacitor.<br />Can you see the oscillations at intermediate stages of the amp?<br />Does is start oscillating at power up or after the you put an audio signal in?<br /> Cheers<br />Mark<br />VK2WUAnonymoushttps://www.blogger.com/profile/05524301715471422375noreply@blogger.comtag:blogger.com,1999:blog-7590176649168185428.post-75480910261930932602012-12-24T01:38:32.887-05:002012-12-24T01:38:32.887-05:00looks like the resonant 250KHz created by the 100...looks like the resonant 250KHz created by the 1000uH choke and the total series 1000pF coupling cap and JBOT input capacitance . 1000uH choke and the 1000pf resonance is about 160kHz .add the input capacitance will raise it to 250kHz.<br /> I would try a 1k ohm resister instead of the choke since the impedance of the choke is about 820 ohms at 14.2MHz.<br /> good luck and merry Xmas<br /> wayne VA7ATAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-7590176649168185428.post-42938106999704408642012-12-23T23:45:06.837-05:002012-12-23T23:45:06.837-05:00Can you walk the circuit with your scope backwards...<br /> Can you walk the circuit with your scope backwards, from the antenna, to see where the LF oscillation is starting? Can you disconnect stages and drive the remaining ones with lab gear or other bench detritus to see if you isolate what stage the problem is starting in? (e.g. drive the PA with another transmitter via a pad?)<br /><br />Shots-in-the-dark: <br /> - check the antenna/lead with an analyzer or other transmitter to make sure they are OK.<br /> - Try it on a battery, just in case it's some power supply weirdness.<br /> - try an appropriate choke on the antenna lead. It may be radiating back into the final, due to some kind of mismatch. (Not a solution, but a clue if it helps.)<br /> <br /> My shaky hypothesis: It's starting when you switch to an antenna and the PA has the "worst view" of that switch. Unless it's caused by something radiating from the coax back into another stage, it's the most likely culprit.<br /><br />Good luck and 73, <br />Bob KD4EBMAnonymousnoreply@blogger.com