Hum on Optimus STA-20

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

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

Mike

is

I assume it's Radio Shack/Realistic.

Thanks.

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I suppose this is a possibility.

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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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

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

Have you 'scoped the output of any voltage regulators?

Mark Z.

"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.

A*s*i*m*o*v

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

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

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

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:

Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.