Yamaha AX-900 right channel not working properly

[taivo]

I recently acquired a Yamaha AX-900 amplifier. At first, it appeared to be working fine. However, after some listening I noticed some noise coming from right channel. The noise comes from the tweeter, is synchronous with the music and gets louder together with the volume.

I've tried switching the speakers so I know it's the amplifier. I've also cleaned the potentiometers with contact cleaner. The capacitators appear fine on visual check.The noise also appears when I use the CD Direct or Tone Bypass mode. However, when I turn the amp off and on again, sometimes the noise disappears for a while. Few times, the right channel has also turned totally quiet after long listening.

If anyone could come up with suggestions what I could check next, it would be very much appriciated.
I don't have an oscilloscope, but I do have a multimeter. You can find the service manual for the amp in here: Dropbox - hfe_yamaha_ax-900_900u_service.pdf

 

[sgrossklass]

Issues like this usually point to bad contact in the input selector switch or protection relay. Try tapping either with a screwdriver handle to see whether you get some noise (maybe with a bass test tone running). The short-travel push switches of the lesser models in this series enjoy a reputation of being a real bear to service. Relays can generally be unsoldered, opened and contacts inspected - if no burned/charred spots are visible, pulling paper with contact cleaner through (followed by sealing afterwards if needed) may clean them up sufficiently, otherwise a replacement will have to be found. With many types you can find something that'll work with minor adaptations assuming the electrical side fits (voltage, coil impedance, amp rating).

In an amp of this vintage I'd also look out for the dreaded "killer glue" - fixing glue used on bigger parts that has a tendency to go brown to black and brittle while absorbing moisture (originally having been translucent and flexible) and then corroding parts' legs and wire jumpers. Suspect patches can be chipped off using a screwdriver or something.

 

[taivo]

Thank you. I found that there is a modern replacement available for this relay so I will replace it and see if it solves the problem.

However, I found something very odd with the existing relay. It appears that the previous owner has attached two little magnets on the sides of the relay with electrical tape.
What on earth could be the reason for this?

 

[huggygood]

that means that the previous owner had already located the problem on this relay, it is rather a good thing ... or not 🙂

 

[musicmerhant]

the relays in the AX-900 all had those two little magnets so not unsual

 

[musicmerhant]

separate preamp from the main amp ( pull connectors at back) and feed the signal source directly into amp i.e. "main in" (turn down the signal source to start with!) or try using a different preamp plugged into main in. This will at least isolate the problem to either the amp section or the preamp.