I learned A lesson in Grounding today!

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A client dropped off an amplifier for me to fix for him. He reported that when the fan kicks on, there is a low level hum that kicks in with the fan and goes away as soon as the fan shuts down.

This is a High end Bass Guitar amp that has seen a lot of road duty so i naturally assumed the fan was bad. one look at all the gunk on it and if it wasn't bad now, it will be soon. So i ordered a replacement. It arrived today and i installed it. hooked it up and shorted across the thermal switch to get the fan to come on. sure enough, there is the hum even with the new fan. you have to turn the master on the amp up quite a ways to hear it but its there just enough to be annoying.

SO, i look into it a bit further. The fan circuit takes a tap from the transformer to an isolated lug on a terminal strip. One lug of this strip is also grounded to the chassis and has several ground wires from the pre-amp section attached to it as well as the ground side of the fan circuit.

The transformer tap continues on from the terminal strip to a PCB board that has a half bridge and a 1000uf cap and the thermal switch leads and fan leads attached to it. I checked all the diodes and the cap and everything checks out fine.

So now im curious. I have worked on many of these same amps and never noticed a problem such as this. However this is an earlier version of these amps so things may have changed on the later models??

So i rig up the equivalent circuit out of bench parts and alligator clips but instead of grounding the circuit in the same place. i attach the alligator clip to a small but long screw driver and touch it to the chassis where the lug is bolted to the chassis. HUMMMM as expected.

Thats when i noticed that the center tap of the transformer is connected to the power supply PCB and the ground from the output side of that PCB is tied to a spot on the chassis near the corner. some 6-7 inches away from where everything else is tied to ground. as i look around the chassis i see several other ground points on the chassis for various things.

So this time i take the screw driver ground lead arraignment and touch it to the chassis right where the output side of the PCB is attached to the chassis and NO hum! what do ya know about that.

I have always heard that all grounds within a chassis should be tied to once central place. usually the buss bar between the caps. In this amp the MFG was using the chassis as buss bar of sorts.

So now im even more curious. touching the screw driver in several chassis spots, even a few inches away and the hum comes back. I checked all the ground lug screws and they are all tight and clean. just chassis eddy currents i suspect.

I really wish i had a AP1 here to do some testing. I wonder if i extended all the grounds to that one point if and how much the noise floor would drop?

You really do learn something new every day i guess!

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