🚀 go-pugleaf

On Sun, 18 May 2003 23:24:35 -0700, DaveC
<dave+usenet3016@mailblocks.net> 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,
>-- 
>Dave C
>dave+usenet3016@mailblocks.net

Ripple can be perfectly normal for a diode rectifier/filter cap, there
will almost always be some. Unless the lowest peak level of the ripple
is less than about 2V above the regulator output voltage (+/-17V in
your case) then the regulators will handle it fine. Are you getting
ripple on the output of the regulators?, this wasn't entirely clear in
your post. If there is no ripple on the output of the regs then look
elsewhere for your problem.

Regards
Dave :)

---------------------------
(remove the "_" from my email address to reply)