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Some Help But Not Everything You Need To Know About FP III Noise !

Posted by Grainger49 (A) on September 22, 2009 at 06:49:32

In Reply to: Hum on FPIII - A Definitive Troubleshooting Checklist posted by breakfastchef on September 21, 2009 at 14:21:28:

BTW, the difference between hum and buzz is high frequency content. Since you are not looking on an oscilloscope just listen. If it is all low frequency, you will hear something that really says “mmmmmmmm.” If there is a high frequency component it sounds nasal. You will probably hear something from your tweeter.

Ok, let’s see, these are mainly for hum. But can apply to buzz.

My first question, are your outlets grounded, did you check with a three prong tester?

Remove the interconnects and leave your components plugged in and turned on. Then take your meter and measure the AC voltage from each chassis to the screw on the outlet (the screw is grounded). Then turn them off and measure the resistance from the screw to the chassis of each. This last one tells you if the chassis is grounded.

If the voltage on the chassis (I was recently informed that this is correct for the plural) are different by as much as 20V AC then there will be noise introduced. Each chassis should be grounded to the ground lug of the power cord. The readings should be 0V AC.

That done, moving on. The FP III has many improvements that eliminate hum. And the fact that it didn’t hum before would indicate that they worked. The move must have changed the grounding scheme or interconnect integrity or broken a ground point in the preamp.

You say that it hums with all inputs, more with two of them. First, you should short each pair of inputs, at the RCA input jack from center to grounding tab. Try each individually. If this eliminates the hum it probably isn’t in the FP III. Even if this works, eliminates the hum, check if you have continuity between the hot and shield of each interconnect. Moves can be hard on them. Winding them up puts strain on the RCA connectors.

Since you say that the hum goes with volume I would guess that it is injected before the volume control. Good! There are few things to look at. You should touch up, resolder, each of the RCA jack hots and grounds. Do the same thing where the TSP enters the selector switch. And again where the signal leaves the selector switch and enters and leaves the volume controls.

That brings you to the voltage amplifier stage and it shouldn’t be involved since the noise follows the volume setting.

Hopefully this will find the problem. I’m not certain that an inline resistor will do more than pad the input. It is used in some really sensitive systems that are loud at low volume settings on the FP III volume controls.


Remember, YOU are the only one who needs to be happy with the sound of your system

Grainger Morrison
There Is Only One (Grainger Morrison, it seems)

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