Hi all,
I recently acquired an Onkyo TA-207 cassette deck which was in mint condition and worked flawlessly at first. I'm guessing since I've given it more use in the last 3 weeks than it's previous owner probably has in the last 9 years, I noticed that a strange harmonic-like electical hum had developed. So, after moving the unit to several locations finally setting it inside a farraday cage and was unable to remove the noise, it was apparent it was sourced inside the unit. After a few minutes I discovered the VFD was the culprit, creating this electronic hum when turned on (this unit has option of turning the VFD on and off and selecting brightness), and increasing when the brightness was turned on. I listened to this unit quite a bit when I got it and it did not have this problem. I've noticed it's gotten louder over maybe the last week. Now, a few things I've noticed about this. The noise is not present on tapes I've recorded when the VFD was on. I verified this in other tape decks. It's also getting in somewhere in the tape pre-amp circuit, becuase if I turn Dolby on, the noise is greatly reduced.
I'm not sure what exactly I can do aside from creating a shield around as much of the VFD as possible, but even then that might not work. I'm sure this thing was engineered so the VFD wouldn't interfere, and being made in 1991, it's completely possible that the sudden shock of use might of caused a componet or two to start breaking down, perhaps a resistor that's going to ground for shielding has failed...although it's hard to see a small resisitor like that fail.
I haven't yet obtained a service manual or schematic...so if anyone out there has one they might be able to dig up for me, i'd appreciate it, but any other advice would be helpful too.