🚀 go-pugleaf

Your regulator should handle the periodic input voltage variations unless
it's being overtaxed by excessive load, input voltage is outside range it
was designed to reg or regulator could be defective.  What's the peak to
peak on the ripple?  Frequency should be 120Hz on a full wave bridge
rectifier.  Sounds like open diode in bridge.
"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
>