Hum on Optimus STA-20

[Sam Goldwasser]

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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[[email protected]]

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

 

[Michael Ware]

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

Mike

 

[Sam Goldwasser]

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

I assume it's Radio Shack/Realistic.

Thanks.

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[Sam Goldwasser]

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.

I suppose this is a possibility.

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

There is very little wiring to route inside and no one's been in there
before me so I doubt that's it.

Thanks.

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[[email protected]]

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]

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]

I assume it's Radio Shack/Realistic.

Thanks.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
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Have you 'scoped the output of any voltage regulators?

Mark Z.

 

[Asimov]

"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 <[email protected]>
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.

 

[Luddite]

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?

is the hum 60hz or 120hz? big difference in the causes for each..

 

[Sam Goldwasser]

"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 <[email protected]>
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.

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[David Harmon]

On 15 Jan 2006 15:38:33 -0500 in sci.electronics.repair, Sam

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.

Sometimes helps to put a bright light on the top side & find 'em in
the shadows.

 

[Sam Goldwasser]

On 15 Jan 2006 15:38:33 -0500 in sci.electronics.repair, Sam


Sometimes helps to put a bright light on the top side & find 'em in
the shadows.

Good point - thanks. Someday if what's left of the hum really bothers me
or gets worse, that should make locating the remaining electrolytics and
testing with an ESR meter relatively painless.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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