🚀 go-pugleaf

DaveC wrote:

> 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,

Maybe one of the diodes in the bridge is shorted (although that ought to
blow a fuse).  Even the biggest capacitor won't remove the ripple from
AC!