I’m getting microphonics (audible ringing noise) that seems to resonated on voice or music around 1200 Hz. Tapping the tubes does not expose this culprit. Can you point me to any specific area to investigate?
Hi, considering that it is an aging portable radio (and quite tightly packed I may add) unit anything could be the culprit. Over the years of its use if the radio had some mechanical abuse such as being dropped on the floor or bashed against other hard objects it could be that some elements inside the tubes (some tubes or just 1) now have loose or bent elements. And even though It might not necessarily be the case there is still a high probability. Now, lightly taping each tube with a non-metallic object should (in theory) do the job but the best way is if you could temporarily have the tubes substituted one at the time with a known good set of tubes. The most sensitive one to any vibration would be the input area of the radio such as converter/mixer area. On the other hand if the radio is still in the original condition and it has never ever had any thorough testing, alignment or restoration done to it now is the moment (providing you’ve got the know-how and enough patience ofcourse). Loose and/or dirty/tarnished tube sockets, dry soldering joints, bent components touching each other (especially ceramic caps !) when they shouldn’t can also cause a similar effect. As this radio has its speaker’s body (the rear end of the speaker) protruding thru the chassis metal body this implies that the speakers moving part (the paper cone) is getting really close to some components behind it. Try to visually inspect that space (behind the speaker) and look for components or wires that are too far out and actually could touch that speaker paper cone. One thing to note (like Mr. Carlson had pointed it out many times before) is that all ceramic capacitors are micro-phonic (by nature) so i would say if at any point someone has repaired this radio’s maybe the audio stage or any other stage and placed some new ceramic caps but left their legs overly long and they are toughing some other components or lean against the chassis and the speaker while working will (inevitably) transmit its sound vibrations thru the metal chassis then that ceramic cap could (in theory) be peaking that vibration up acting as microphone and in turn that sound will then get amplified further by the audio stage etc. It is worth checking everything at this point. Good luck. PS: one more thing comes in my mind. I know for a fact that some old radios had some vibration dampeners (in the form of rubber washers) mounted on the bolts/screws that hold the speaker in place and is they are hard and crusty by now (or entirely missing from a previous and careless repair) then they will need to be replaced. it may not apply to your particular radio but it is worth checking if it has such things or not.