I have a Beomaster 1000 receiver that has been in my family since first purchased in 1968-1970.
Last year, I bought a Beogram 1000 turntable, and Beovox 1000 speakers, to complete the original system.
All have sounded good until recently. Now, when I start a record on the turntable, the first 20-30 mins. sound great. Then, the sound gets muddy in both channels, like there is big distortion. I do clean my records and stylus frequently, and have found that this does not make a difference to this change of behaviour. I get tired of the noise, so shut it all down; then it sounds good again, next time I get it going.