Electronics

Sam Goldwasser · Guest

This is a nice little low power receiver.

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

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

A schematic or suggestions appreciated.

Thanks.

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Michael Ware · Guest

If you are talking about a Realistic STA-20, I might be able to hook you up
with a service manual.

Mike

dkuhajda@locl.net · Guest

It may not be the same one I remember, but I had two different hum
problems on some older Realistic receivers.

One had a bad ground connection to the frame from the board to the
screw. Seemed they were using the metal chassis to send the ground to
the different boards.

Another one had a problem if the wiring was not routed correctly.
Having it too close to the transformer would cause the hum.

David

Sam Goldwasser · Guest

"Michael Ware" <mware@insight.rr.com> writes:

Sam Goldwasser · Guest

Guest · Posted: Mon Dec 12, 2005 8:52 am Post subject: Re: Hum on Optimus STA-20

Can you crank up the gain on a scope and trace back from the audio
output stage? Or, short out/ ground the audio signal at the detector
and see if the hum persists or not. That would be a starting point

H. R.(Bob) Hofmann

Bob Parker · Guest

I remember seeing that problem a long time ago. If there are screws
connected the PCB grounds to the frame, make sure they're tight.

Bob

Mark D. Zacharias · Guest

Have you 'scoped the output of any voltage regulators?

Mark Z.

Asimov · Guest

"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 <sam@saul.cis.upenn.edu>
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.

A*s*i*m*o*v

.... A stereo system is the altar to the god of music.

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