• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Fisher 400c

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Hi all

I could use some help, so here we go.
I have bought a Fisher 400c and now im trying to make it sound good. I have changed most of the components by now but I am still getting a fair amount of noise. Please give some input on how to solve this.

I need to get a new volume pot, anyone know where to find one?
A set of original buttons would be nice too, but my hopes are low that I will find them.

One thing in paticular bothers me. I got hold of the attached schematics and they have been a great help so far. One thing im missing is information about the values that the RC packages consist of. Anyone?

Thanks in advance

Edit: file too big, get it here instead: http://rapidshare.com/files/88210932/Fisher400C_Sams.pdf.html
Hi Klaus,

I once owned a Fisher 400CX which is very similar to your preamplifier. It's a nice unit but it is old enough that the coupling capacitors will be leaky. When they leak they allow DC to come through. And it only takes a small amount of DC on the volume control to cause it to become noisy. If this is part of your problem, carefully check the capacitors going to the high side of the control. I think the on-off switch was also connected to the volume control. Hopefully the switch is still good because a replacement is impossible to find.

As far as the flat-pack multi-lead RC networks in this preamp go, I think they are part of the tone control circuits. These were, I think, standard Baxandall style circuits. If you google that name you should find some information. I can't remember if Fisher's were special (proprietary) or not. Most of them seemed to last a long time, but like anything else, could become defective. I do remember one old American company "Centralab" that sold them. But sorry, they are long gone.

You say that you have changed a lot of parts, so perhaps you've already changed the coupling capacitors. But here is something to keep in mind. Be very careful when replacing parts in the phono RIAA stage. These values were tuned with paper or ceramic capacitors and using modern plastic (polyester/polypropylene) parts can de-tune the RIAA network causing it to be unfaithful to the reproduce curve needed for proper record playback. Typically the high frequencies will be accentuated requiring re-tuning with a reverse RIAA network, signal generator and scope or dB meter.

One last thought is to contact the Fisher Doctor. http://fisherdoctor.com/

HollowState thank you very much for the advice. It is very usefull.

I did change every capacitor that wasn't a part of the riaa.
I also changed most of the resistors due to their values growing.
I have left the riaa alone for now, but it will need some work because a previous owner has made a few questionable changes in there.
A suspect for the noise issues is the volume control. It doesn't give the same volume on both channels at every setting (mostly the lower volumes). The noise is pretty much the same unless I turn the volume all the way up (always less noise in the left channel than the right). It has the on/off switch (which is fine) integrated, unfortunatly.

Well ... As I have replaced most everything else I am now thinking about replacing the controls and the RC networks.

Eli, thanks for the pointer, I'll go have a look :)
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