🚀 go-pugleaf

First off, is your scope well grounded such that you see no hum on the
ground
at the capacitors.

How much hum are you seeing.  Is the hum very loud in the speaker, or low
level?

What is the difference between hum on the input and output of the regulator.
What is the input and ouput regulator dc voltage?

Is it really 60 Hz, and not 120Hz? If it is really 60 Hz it could be a
failing diode in
the bridge. (full wave rectifiers will produce 120Hz when caps are bad).

Does the hum go away if you short the +15V supply to ground? (just kidding)

Tracking the hum levels through the components will give you a clue to its
source.

Bruce



"DaveC" <dave+usenet3016@mailblocks.net> wrote in message
news:0001HW.BAEDC6B30001868DF0386600@news.cis.dfn.de...
> I agreed to help my friend find out why his 5-channel surround system
> (Harmon-Kardon; don't remember model number) has 60hz hum in all speakers.
> Hum is uneffected by the volume control. It is not in the headphone
circuit.
>
> There is a small power supply circuit that feeds +15 and -15 vdc to some
of
> the ICs on the power amp PCB. This circuit consists of a small transformer
> with c.t. secondary feeding a single bridge rectifier. Tied to the plus
and
> minus side of the bridge are two 2200uF 35v electrolytic caps. Beyond each
of
> these caps is a (pi?) filter: 47uF elect. cap (to ground), a series
resistor,
> and another 47uF elect. cap (to ground).
>
> These two supply "rails" each feed 15v 3-terminal voltage regulators, one
> +15v, one -15v.
>
> The plus supply has ripple voltage on it everywhere in the circuit. I
checked
> the voltage at the 2200uF cap with a scope and saw the ripple, and said
"Bad
> filter cap" and replaced it. Ripple remains. I pulled each of the 47uF
caps
> and tested them with a Fluke cap tester. They are within 1uF of spec.
>
> I had to leave my friend's place before getting to diagnose it further,
but I
> can't think of what else could cause this ripple. I am aware of the
epidemic
> of failing electrolytics. Is a capacitance test sufficient to declare a
cap
> good? Or must I test resistance, also? Can a shorted bridge diode
introduce
> ripple? I haven't yet checked the series resistor in the filter. If this
> changed value, could this cause ripple?
>
> I've checked for bad solder joints. I can't imagine that the v.reg. can
> introduce ripple into the filter circuit. Can it?
>
> Any other suggestions would be greatly appreciated.
>
> Thanks,
> --
> Dave C
> dave+usenet3016@mailblocks.net
>