Fisher 400 - help with diagnosing phono hum

Jon Kasper

New Member
Hi all,

So the project of bring my Fisher 400 back to being able to perform to its full potential continues!

Would really appreciate your help with diagnosing some undesirable noise coming from my speakers when using the Fisher 400 as both phono stage as well as amp for my turntable.

I'm having some trouble with a hum (equally from both left and right speaker) & slight crackle (only from right speaker) when phono-stereo input is selected. Details as follows:
- Have my TT (Rega Planar 3) connected directly to phono-low input on the Fisher.
- Fisher 400 directly to Klipsch Cornwall II speakers.
- When phono-stereo is selected, a distinct hum can be detected once the volume knob reaches 50%.
- Further, when I turn the Fisher 400 on, it takes approx. 10 seconds for the hum to to begin.
- no such noise when phono-mono selected as input, and same when aux is selected.
- Also, slight crackle coming from right speaker, even when control knobs aren't being moved.

Here's a video:

Here's an audio click of noise coming from right speaker (volume knob turned to 75%):
https://drive.google.com/open?id=1b47U9LT2vzTSOJLA1a2NEdYWMqRZOGQE

Here's an audio clip of noise coming from left speaker(volume knob turned to 75%):
https://drive.google.com/open?id=1hiOUDfV_h8fcy5NkUPmCtqJ2Mxb7itPk

Any ideas where to start guys? Could it be the tubes? My tech has told me my output tubes are a bit weak and need to be replaced soon. The rest of the tubes were supposed to be ok. Output tubes in use are original "The Fisher", phono tubes are Toshiba (toyko shibaura electric), the tone control driver tubes are Sylvania and “ASB USA”, the phase inverters/driver tubes are "made in usa for PILOT radio".

Any help is greatly appreciated!
 
Ground loop maybe... Did you tried to reverse the prongs in the outlet? TT on the same outlet or powerbar? 75% is a lot of volume and it is not uncommon to hear tube rush. This sounds like some kind of buzz...

Also, you could swap other tubes in the phono position.
 
Is there a grounding wire from the REGA to the 400??? If not make one from a length of wire and attach to the chassis of both units. Turn on, check for hum using parameters above. Report findings.

Crackle could be a slightly loose socket leaf, which expand and contract as they heat up and cool down. Use a dental pick to tighten up the sockets (all of them) in the whole signal path. You didn't say if there was any crackling or hum in FM (off station). It could also be a DIRTY SLIDE SWITCH on the Front of the 400. DeOxit ALL switches and pots on the 400. If there are any pots or switches on the REGA, De-Oxit them all. REPORT FINDINGS.

Make some shorting plugs. Take an old RCA Cable and cut the ends off about 2" from the ends. Bare the center and outer leads and short them together. Insert into the phono plugs. Test again. REPORT FINDINGS.

If the RCA's from the REGA to the 400 are completely removable, swap in a new RCA cable and TEST. REPORT FINDINGS.
 
Really appreciate your help guys. I feel like a complete doofus!

Tried all the above suggestions without eliminating the source of the hum.. As well, discovered the Rega Planar 3 apparently achieves grounding to the Fisher by way of the RCA plug shielding. Not sure how that works, some suggest re-wiring the Rega Planar 3 tonearm to provide a seperate ground, but that's for another day.

The Fisher was on the shelf directly below the turntable. Discovered out of pure coincidence when I drop the Fisher one shelf down, the hum decreases by at least 80%, so think this was just a case of the Fisher being too close to the TT. Not sure how that works, but it appears to resolve the issue for the most part for my rookie ears anyways!

Appreciate the time!

Jon
 
Typically, turntable ground will not be via the audio cables. Cable grounds are isolated from everything else and usually, even the ground from each channel is isolated until connected together at the preamp.

How close was the cartridge to the Fisher power transformer before you moved the turntable? The cartridge is magnetic and will pick up the magnetic field of the power transformer.
 
There are actually some tables that do ground through the RCA. Stock Thorens TD-160 and the AR XA are both like that. Magnetic coupling from being near the transformer sounds likely though if moving the amp and turntable apart stopped it.
 
Are the tube shields on the 400's tubes, or are you running them naked???? If naked put the shields on(Especially the phono tubes) and test. Shield the phono tubes 1st. As they are the most susceptible to RF/IF interference, I would suspect they are picking up outside noise. If this corrects the problem I suspect you might be able to put the 400 back on the upper shelf. The shields are grounded to the chassis so any interference picked up will be immediately grounded to the chassis. The shields can be buffed up and won't hurt anything. Naked tubes look nice, but NO HUM because they shields are in place is even better.
 
I had the phono hum problem on my Fisher and a Technics turntable I restored. The hum was not as pronounced but was annoying. I had the turntable grounded with its ground wire and both items plugged into the same outlet. Just for S&G, I plugged the turntable into one of the outlets on the Fisher. Problem solved...
 
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