🚀 go-pugleaf

Look at the input voltage to the +15 regulator with the scope.  The lowest
point in the ripple must be over about 17 Volts or the regulator can't do
its job.  If the low points are above this, suspect the regulator.  If not,
since ripple is on the output of the regulator, I will guess that you have
massive ripple on the input to the regulator with the low points below 17 V.
You can check the bridge diodes or look at the ripple waveform on the plus
and minus caps.  If the plus side cap ripple is half the frequency (60 vs
120) you have a bad diode.  Another cause could be excessive loading on the
+15 rail.



"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
>