so ive had this 757 for a few years and it's been solid but lately when i key up on any band theres this low frequency buzz underneath everything, like a 60hz hum almost but it changes slightly with the power output setting which is throwing me off. recieve sounds totally clean, SWR is fine, but the signal report from guys on the other end is pretty rough apparently.

i swapped the mic thinking maybe a ground issue there but no change. the DC supply is a pretty old astron RS-35A that's been running since like 2003, so that's my first suspect honestly. but before i pull it apart i figured id ask here because last time i went digging around in an Astron i ended up with more questions than answers.

anyone dealt with this kind of thing on the 757 specifically or is this almost always a power supply cap problem? the rig itself is probably a mid-80s unit, no idea if the filter caps have ever been touched.

yeah that really does sound like supply ripple to me. the RS-35A uses a pretty straightforward linear design and those filter caps dry out over time, 20 years is a long time for electrolytics to stay in spec. i'd pop the lid on the Astron first before you go chasing anything in the rig itself. the main filter caps are usually 15,000uF or so at 50v and you can check them in-circuit with an ESR meter if you have one - if the ESR is high you've found your problem. also check the rectifier diodes while you're in there, cracked solder joints on those are pretty common on that vintage.

the thing about the buzz changing with power output is actually a decent clue - if it was something inside the rig like a bad bypass cap on the PA board you'd expect it to be more consistent. ripple from the supply getting worse under heavier current draw makes sense with what you're describing.

had almost the exact same thing with an old TS-430 and it turned out to be a cold solder joint on one of the driver board connectors, not the supply at all. not saying thats your problem but dont rule out the rig just yet. i spent like two weeks blaming my supply before someone talked me into just reflowing all the connectors and the buzz went away. those old Kenwoods and Yaesus from that era have board connectors that get flaky after enough heat cycles.