Increasing hum with Fisher 500-c

[pinkchry]

I just got a Fisher 500-c which sounds great! However while playing vinyl though the phono in, after about 20 minutes of play, a loud hum starts up, and refuses to go away even when I stop the turn table, The other inputs seems to work fine.

Any idea where the problem could be. I did mess around with the cartridge trying to figure out how it was attached :( The grounding wire was connected to the ground in the Fisher. The turn table is a Thorens 165.

Also there's a perceptible hum/noise when switching to phono input. This is different from the very loud hum I get now. Is that normal?

Thanks

[audio]

Has your Fisher been serviced? Have you tested all the tubes? Any vintage tube amp such as yours needs 40 year maintanance. These amps are not plug and play until after they have been checked out by a tech who knows what he/she is doing. You're in San Jose, right? That means you are near me. If you want to, send me a PM and I can give you some names of a couple of techs that are not all that expensive and do beautiful work.

[-=Rudy=-]

Originally posted by pinkchry
I just got a Fisher 500-c which sounds great! However while playing vinyl though the phono in, after about 20 minutes of play, a loud hum starts up, and refuses to go away even when I stop the turn table, The other inputs seems to work fine.

Quick test: while it's humming, disconnect the turntable from the amp. Completely...both the inputs and the ground wire. If the hum remains, it's the amp. If not, you may have a loose ground somewhere in the turntable.

Like prix says, it probably could use a tune-up...hum problem or no hum problem. :)

[pinkchry]

OK, I played a record through and got no hum. Amp was still on. After about 10 minutes, the really loud hum came back. When I removed the phono connection (just the left rca), the hum went away, and when I plugged it back no hum. What does that mean?

BTW, the seller I got it from said that the unit had been serviced by Soundwell in Berkeley, the unit looks pretty clean and good. prix, I've sent you pm anyways.

Thanks for the replies

[Ronflugelguy]

Phono section tubes??????????After they warm up failing????

[VU Master]

Speaking as a tech, my guess would be that the hum problem is not caused by the Fisher, but by some wiring issue. Messing around could have made things worse, sorry. Did the turntable work with other preamps? Could the time factor be related to the position of the tone arm as the disk plays? Is the ground wire on the turntable securely connected to the Fisher?

Just some ideas, good luck.

[pinkchry]

I always had a problem with the rca leads of my turntable. They seem to make a noise whenever connecting to the phono in (of the Fisher and al a Radioshack Little Rat I had before). Maybe I need to rewire them. I opened them up and one of them looks frayed.