[R-390] Symptoms of a mechanical filter failing?

[Roger Ruszkowski]

John, You are asking about your 2Khz filter circuit going off its rails.If the problem was AGC we would expect some symptoms on allthe filter circuits as each was selected for use. Bad news from history.Inside the filters was a bit of spacing stuff. (plastic)That stuff is now known to be deteriorated.These are mechanical filters in real motion.Very small bits of sticky spacing stuff settle as dust on the mechanical vibratorthat has a very narrow point of frequency.Mechanical damping of a mechanical signal transfer. Do not smack your receiver.You can turn the IF deck with or with out receiver upside down and bump it a bit.Carefully returning the IF deck to the full upright position can leave the pesky loosestuff laying some where else inside the filter. It may take a couple timesto clear your current dust. You may have to repeat the process from timeto time as stuff is still deteriorating. There is a whole pearl of wisdom devoted to getting a filter off and then back onto the IF deck.A second pearl covers the process of opening and closing the solder packaging.We call it a filter. It is an assembly and soldered shut.A third pearl and some web pages describe that deteriorated stuff in the filters thatcause problems. If you find you can jiggle the deck and find it changes the receiver operation,you need to consider stuff is adrift inside the filter and their is a veryreal mechanical operation in progress inside the can.Stuff can catch like lint in a speaker cone and damp the circuit gain. Dropping your IF deck into oscillation as the mechanical operation node moves and changes the phasein feedback around the mechanical filter stack sounds like a different symptom.It has the same root cause. Old age inside the filter parts. Some bits fall as attenuators. Some bits falland the circuit goes into oscillation. Armstrong explained why and Murphy says it will do it every time.  Short of California we just do not shake our receivers enough.Go for the easy idea first.All the original equipment and parts have the same old age issue.Ceramic filters are available as a solution.The filters can be opened and rebuilt as micro bench maintenance operations. (coil winding). Now because of age every time we move one of these receivers we face the chance of gumming up a filter.Turn the beast over pat it gently on its bottom side and set it back up right.Check for acceptable operation and repeat until satisfied. The panel meters have the same issues with sticking after the receivers get transported and bits of dust lodgein the movements. John, It is a mechanical filterYou may have a mechanical problem. Respectfully, Roger AI4NI