Hi All,
I've got a perplexing issue here that am looking for some advice on.
For 15 years I have successfully ran a background music add on to my Rowe Tropicana. I invented this and it has worked perfectly and kicked in when the jukebox finished scanning, re opened the mute relay to play line input until the user made a jukebox selection which would over ride it.
The line in source was operated by switching in the signal at the volume pot. So essentially the power amp only is used to amplify the signal.
Phase inverted as it's a Stereo Round jukebox. All that taken care of with isolation transformer in line.
Anyhow, as I like to mess, I decided I wanted the input feed to go through the preamp to make use of the same tond controls and loudness contour of the jukebox. So I scrapped it all. I set up an RIAA inverter for the line in and wired the relay so both input feeds go to the preamp phono input.
Now, upon testing.
Jukebox audio from the cartridge is perfect.
Line in audio has hum that increases with volume.
If I bypass the relay and feed line in direct, through the RIAA inverter, it's perfect.
Anyhow I narrowed the source of the hum down to the 28v ice cube relay that I use to switch between the signals.
If I remove power to the relay by physically removing the 28v plus and minus wires, the hum disappears.
But what I don't get is that when the relay is powered with the 28v, there is no hum (relat activated by the scan out switch on the mech). The hum appears when either of the plus or minus leads is connected and there is NO power to the relay.
The relay is powered only when playing a record, hence it's not powered when the line in signal is present, but when there is no power, it hums.
The unpowered relay coil is in no way connected to the source signal switching contacts.
Is there anything I can do when the relay is not powered to, for want of a better word, break the circuit between it and it's power? Like adding a couple of diodes or something ?
I don't get why it works when powered and initiates hum when unpowered.
Any ideas please ?
Thank you,
Nigel, uk