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Fisher 400 Phono Hiss in one channel

S

streetwise

Active Member
My 400 has alot of hiss on the left channel and I can't figure it out. All pretty new caps resistors, new tubes. Ive attempted to switch tubes no luck. The hiss is the same hiss that is normal in the other channel but at half way on the volume its louder than the hiss on the opposite channel at full volume. Doesn't matter if a turntable is plugged in or not? any ideas?
 
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Make some shorting plugs (rca plugs with the inner and outer connections shorted together.) Install in the phono jacks and re-test. It should quiet down. If not then you have problems somewhere in the preamp sections.
 
larryderouin said:
Make some shorting plugs (rca plugs with the inner and outer connections shorted together.) Install in the phono jacks and re-test. It should quiet down. If not then you have problems somewhere in the preamp sections.
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Larry Thanks for the idea
already tried that definately somewhere in the preamp section? Hiss is with tape head and phono input in the left channel. doesn't distort audio or anything just that good old background hiss at a much higher level than the other channel. tried resoldering most of the connections in the phono section with no luck. and ive poked around a bit and havent heard anything change it.
 
I also might as well throw this out at another seperate issue, my input channels are wired backwards? And I can't figure out where, at the balance control left and right control the correct speaker, but the inputs are backwards. Il probably need to post some pictures for both issues. as far as the phono issue is concerned the hiss is on the left channel proper not my reversed left
 
If the shorting plug is in the phono jack when it's in phono, then the phono circuit should go silent if it's working correctly.
 
Serial # above or below 48001??? It makes a difference as far as the tube #'s. AND do you have any spare 12ax7's???
 
Flip open the unit and remove the bottom. Turn it on and short together the CENTER LUGs of the Volume control. If the individual channels even out then the problem is before the Volume control. If not, then after.
 
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I will try that when I get a chance larry. its a very late model 7xxxx serial, I do have replacement 12ax7s, all the 12ax7s other than p.i. rca's in my 400 are new and have not changed the noise. Sovteks in phono section tung sols in line amp and tone amp positions
 
You got a tube tester or acess to one? If so check each side of the 12ax7's (Voltage amp Tone amp, and P.I.) for imbalance's emissions wise. It may be a case of instead of one side being higher than the other, but one side lower due to a weak or dead section.
 
That's a weird one... hiss. If it were hum I'd most definitely think it was a faulty ground somewhere in that L ch circuit. I know you said you've resoldered all the connections, but I'd still go over all those ground/component connections and make sure you have good connectivity. Hope you find the problem whatever it is.
 
larryderouin said:
You got a tube tester or acess to one? If so check each side of the 12ax7's (Voltage amp Tone amp, and P.I.) for imbalance's emissions wise. It may be a case of instead of one side being higher than the other, but one side lower due to a weak or dead section.
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Larry I don't have a tube tester. I did find the issue though It was one of the new Sovtek reissue tubes. I didn't notice it was a tube because I replaced these tubes when I did my rebuild and thought it was something else do to them being "new". When I attempted to replace the tube before I think I replaced the wrong channel because they are reversed still.

My next thing to do after replacing that tube is to figure out this channel reversal. It has to be something I did backward when I did Dave's mods...

One other question I have is has anybody tried the vitamin Q & K40y-9 in the phono section? I currently have some film caps supplied by Jim Mcshane in there but ive read alot praising the vitaminQ russian cap deal in that section, yet I haven't seen anybody say whether or not it really does make a difference or not???

And thank you Larry your always a huge help!! and same to you Bruce
 
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