Radio Breakthrough on added 'aux-in' - how to cure this?

[Old Sparky]

To explain further - I have added a small box containing a 3.5mm stereo jack socket and a DPCO switch, connected via a 4-core screened lead to a Blaupunkt New York SQR 83(An early 80s car stereo). This all works fine on my bench, except that there is breakthrough from either the radio or cassette player audio at high(ish!) volume settings. The NY has a conventional dual-gang rotary pot. volume control coupled with the usual Balance, tone, & fader controls, and I have cut the very short lengths of printed circuit tracks between the detector/tape audio o/ps and the 'hot' sides of
the volume controls. What can I do to solve this problem - is it due to capacitive coupling? The PCB is double sided, with tracks on the underside carrying audio signals. Internal switching between radio & tape audio is carried out electronically in a purpose made hybrid circuit.
I've tried, without success, to upload a circuit diagram & PCB layout, but they have 'got lost' in my computer somewhere!!

[pwdrive]

Either capacitive coupling or insufficient decoupling maybe?

Lawrence.

[Eddystonepete]

Hi Sparky, what kind of 4 core screened cable have you used? Make sure it has 4 separately screened cores, not 4 cores with an overall screen. Peter

[Kalee20]

It's a common sort of problem - you really need to kill the signal sources as well as switching them out. It's almost certainly capacitive coupling. It could be in the switch itself.

If the coupling is in the switch itself, one way forward - which would require a more complicated switch - is to break the signal source not by a single contact, but by two in series. Then the mid-point connection (which 'floats' when the two contacts are open) is itself shorted to ground by a third contact which closes when the other two are open.

[Old Sparky]

Thanks, both, for your suggestions. I was hoping not to have to do the job a second time, but it seems that I'll have to! Whether to go down the 4-core individually screened cable route or the 4-pole c/o switch one I'm not sure. The latter would be easier if I can get a suitable toggle switch which will be a straight swap with the DPCO one I used.