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duncanlowe · 30th Nov 2022, 8:05 pm

Older voltage regulators were notorious for being susceptible to external RF if they didn't have a fair capacitor on the input. So if it does have one but it's degraded that could easily be the fault.

Back in the nineties, I had one part of a complex system that kept failing when we subjected it to RF at a particular frequency. Display, LEDS and control of the outputs all went bezerk. It was on the third iteration of 'solution' where there was no improvement in performance, I looked at the circuit. Absolutely no capacitors on the 5v LM regulator. Went to the test facilities component drawers and borrowed an electrolytic, tagged across the input to the VReg. Instant cure. NEVER trust suppliers have a clue! They might but just as easily they may not. And this particular device, was developed and built a few miles from the Sutton Coldfield transmitter, and the suscebtibilty was, yes you guessed it around th 88MHz region!

30th Nov 2022, 8:05 pm	#11
duncanlowe Nonode Join Date: Jan 2016 Location: Stafford, Staffs. UK. Posts: 2,532	Re: Linear voltage reg drop out or failure? Older voltage regulators were notorious for being susceptible to external RF if they didn't have a fair capacitor on the input. So if it does have one but it's degraded that could easily be the fault. Back in the nineties, I had one part of a complex system that kept failing when we subjected it to RF at a particular frequency. Display, LEDS and control of the outputs all went bezerk. It was on the third iteration of 'solution' where there was no improvement in performance, I looked at the circuit. Absolutely no capacitors on the 5v LM regulator. Went to the test facilities component drawers and borrowed an electrolytic, tagged across the input to the VReg. Instant cure. NEVER trust suppliers have a clue! They might but just as easily they may not. And this particular device, was developed and built a few miles from the Sutton Coldfield transmitter, and the suscebtibilty was, yes you guessed it around th 88MHz region!