"Asimov" writes:

"Sam Goldwasser" bravely wrote to "All" (11 Dec 05 19:56:01)
--- on the heady topic of "Hum on Optimus STA-20"

SG From: Sam Goldwasser
SG Xref: core-easynews sci.electronics.repair:351037

SG This is a nice little low power receiver.

SG The problem is an annoying low level hum present at all volume levels
SG and all sources. I tested the two main filter capacitors for uF and
SG ESR, and paralleled each one with a good capacitor with no no apparent
SG change. Are there other power supply filter caps?

SG The hum is still present with nothing attached to the receiver except
SG one speaker.

SG A schematic or suggestions appreciated.

SG Thanks.


Sam,

Since the main filters are good I would then suspect some dry preamp
bypass electros. If not this then a misplaced ground wire. There is
often a point near the middle of the main pcb with a H-shape (or
other) where all the grounds meet. Typically there will be one wire
comming from the supply, output, and pre-amp. Try using a short length
of thick wire bent into a U shape to jumper across various points of
this grounding location. Relocate the preamp ground to the spot with
the least hum noise. The power supply ground should not be ahead of
the preamp ground as this will amplify the ripple travelling in it.
i.e. The dreaded ground loop. Perhaps the problem is external to the
reciever, like sitting on top of a big transformer that induces hum?
Other outside chances are ultrasonic oscillation at the output or a
bad bypass in the FM pilot circuitry.

I did finally find another auxiliary power supply cap that was high ESR.
It was near the front of the cabinet, with only reasonable access (without
totally disassembling the thing including the dial cord) by
drilling a hole through the plastic bottom. That reduced the hum
to perhaps 1/10th its original level. Not perfect yet though.
There's probably some other high-ESR caps but with no labeling on
the bottom of the PCB, it's a royal pain to locate the correct pads,
especially for smaller components.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
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