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Fisher 400 Phono imbalance issue

S

streetwise

Active Member
Hello all

My 400 has been acting up for a long while and I haven't had the time to fix it. Hopefully Dave or one of the other Fisher experts can point me in the right direction.

The left channel on Phono and tape is quieter than the right and also has slightly more hiss with no input signal. I have tried swapping tubes between channels in the phono stage switching rca's no luck. This isn't an issue on FM or Aux. to balance the two listening in stereo I have have to turn the ballance to 7o'clock. I have all of Daves mods including the phono riaa and have no other issues sound wise.
Any idea where to start poking?
 
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Had a similar problem yesterday with an Harman Kardon A500: it was the rca connectors in the back, I wiggled the plugs and it returned to normal behavior.

Could be the cartridge in the TT too. Try another turntable or reverse left and right connections maybe...?
 
First step is to prove that the phono preamps are in fact the problem, which is easy enough to do: with the phono playing and the imbalance apparent, turn the selector switch to Mono Phono. If the problem is in fact in the phono preamps (or TT), the imbalance will balance out and disappear. If the imbalance remains however, then other issues after the preamps are in play.

Now normally, if the imbalance disappeared in Mono Phono mode, then I'd suggest that you go back to Stereo Phono, and as Patrice suggested, swap the L and R phono interconnect leads. If the problem swaps between the two channels, then the cartridge and/or interconnects are at issue. If the problem remains in the same channel with the cables connected either way however, then the problem is confirmed to reside in the phono preamps -- but you say you've already swapped the cables, so that jumps us right to the phono preamps themselves.

Tubes are also ruled out since you've swapped them, and since you are invariably using the "Phono Low" input jacks, then the attenuation networks built in for the rarely used Phono High input jacks can also be eliminated as well unless you're using those jacks.

The next two checks then would be classic troubleshooting efforts: Make sure the selector switch is good and clean. The phono preamp signal is switch on BOTH the input and output side of the preamp circuit, so there are twice the possibilities for dirty contacts to enter the picture. If you've cleaned the switch to rule it out, then finally, take some voltage readings at both preamp tubes, so we can get a picture as to how the tubes are actually operating.

Let us know!

Dave
 
Follow Up
problem solved

after following dave's recommendations switched the tubes between the Left and Right phono channels and the imbalance is gone! My guess is there was maybe a dirty pin on one that cleaned up once i switched them. this is my second set of matched sovtek 12AX7LPS so im very happy it wasn't a failing tube; with the first pair I had one tube created a hiss and distortion after a couple months of use.
 
I love those LPS tubes. Somehow they produce clearer and punchier sound at least to my ears. I use them wherever I can. Thorne
 
Ok another question maybe I should start a new thread... Why does the phono output go way down at the speakers when I connect RCAs to record out. tuner sounds about the same but phono is way quieter?
 
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