telefunken: Tone controls on the Telefunken Opus Stereo

Dear esteemed members,

I would appreciate your advice on how I might resolve a problem with my Opus Stereo.

The radio plays perfectly with the tone control selected as "Solo", but gives a fluttering sound when the other tone control modes are selected. While the sound in "Solo" mode is pristine and clear, the other modes present the radio programme with low level fluttering and the high frequencies (treble) have crackles. The artifacts are subtle but discernable. The EM84 eye tube shows a slight dimming during those modes with distorted sound, with the signal strength at about the same level compared to "Solo" mode.

To make the problem more interesting to troubleshoot, if the radio is powered up with the tone control in non-solo mode, the sound is OK, provided the tone control is not switched to other modes. Once the tone control is changed, the distorted sound returns. Sometimes the tuning ability is also lost; the radio is not able to tune in to any stations but keeps playing 2 stations faintly (these 2 stations are far away in frequencies from current dial position.)  However, tuning ability and pristine sound are restored when tone control goes back to "Solo".

The paper capacitors in the radio had all been changed, as well as the 6V 100uF & 160uF electrolytic capacitors near the speakers. The high voltage power supply caps are not replaced as they are OK (radio has no hum and plays perfectly in Solo mode.) Contact cleaner was used to clean the tone control switches.

Any advice to troubleshoot the above problem is much appreciated.

PS: I'm also interested to know the function of the special UKW tuning knob on this radio and how to use it. Thank you.

 

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