Ah, summer holidays - time for long walks with the dog, and catching up on this and that. I was given some shellac records (78s) a few months ago, and finally got some time to play them. When I put the first on one there was a weird squeal along with the music. The technical name for this condition is 'parasitic oscillation'. I thought about what might be causing it, then remembered that I had changed my amplifier since playing any 78s. The rather hot output of my 78 cartridge plus a higher gain input was the cause.
Now for the cure - I put an R substitution box in series and increased the resistance until the oscillation stopped. Then I read the value: 15KΩ. So that was the minimum series value to increase R by. I then got a 100KΩ pot and jumpered it up in an L-pad configuration, and wound the pot until the volume was similar from the 78 and radio inputs. Reading the pot values on the multimeter gave 66K in series and 33K for the shunt path. I selected the closest values (68K and 33K) from my 1% metal oxide resistor draw and soldered them in on the RCA plugs of the 78 record deck.
Job done - now back to listening to Bing, Elvis, and Doris Day.
Now for the cure - I put an R substitution box in series and increased the resistance until the oscillation stopped. Then I read the value: 15KΩ. So that was the minimum series value to increase R by. I then got a 100KΩ pot and jumpered it up in an L-pad configuration, and wound the pot until the volume was similar from the 78 and radio inputs. Reading the pot values on the multimeter gave 66K in series and 33K for the shunt path. I selected the closest values (68K and 33K) from my 1% metal oxide resistor draw and soldered them in on the RCA plugs of the 78 record deck.
Job done - now back to listening to Bing, Elvis, and Doris Day.