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help with hiss/static

1iwilly

Well-Known Member
Dec 7, 2008
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so today i notice that when i have the volume off on my cobra 2000gtl i hear a hum/static that doesn't go away even if i turn the squelch up
when i switch over to power supply from 120volts it gets louder after searching if i disconnect the power plug to the frequency counter it goes away
Has anybody ever dealt with this kind of issue if so what was the fix
 
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nomadradio

Analog Retentive
Apr 3, 2005
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The electrolytic caps in the clock/counter module are failing. I'll guess this is a Taiwan radio. That makes it 33 or more years old. That module is full of digital chips constantly turning each other on and off. They will each draw a major surge of current during those on/off transitions, and the voltage drop from those surges will produce hum/buzz noises in the radio's audio circuits. There are filter caps in the module that provide the current for these surges and smooth them out. Until they go bad.

Electrolytic caps fail from age and use, both. A capacitor like the ones in that radio that was kept on a shelf in the proverbial "cool dry place" will check just fine as a rule. But the heat of the radio's operation will speed up the deterioration. The highest temperatures in that model are found inside the clock/counter module. We see the caps inside that metal shield tend to go bad a year or three before the caps elsewhere in the radio start causing trouble.

Quite frequently the "hum" will exhibit small changes in the sound as the segments in the digits turn on and off when the displayed frequency changes. Switching from clock to counter will generally affect the sound if it's being caused by failed caps on the clock/counter board.

The circuit board inside the clock/counter module is different from the single-sided main pc board for the radio. The counter board has foil traces on both sides, and it has plate-through holes. This means that a tiny copper sleeve in inserted in holes to bridge the top foil to the bottom foil through the hole. If that sleeve is damaged by removing a part, the circuit function it served will quit. Some of the 11 or 12 electrolytic caps you'll need to change have only a "doughnut" foil on the solder side. It will tend to slide off when removing the old capacitor at that spot. Somewhere around here I started to write up the "comb-over" trick we use to get around that. Laying the wire lead down so it reaches to a nearby pad connected by the top-side foil will make sure that cap gets connected securely.

I should find the pictures we shot for that and get it written up.

Makes me wonder if Klondike Mike sells a "counter only" kit of caps for the Cobra 2000. Would be handy.

I could be wrong about the root of your problem, but if unplugging the counter module's power plug shuts up the noise, this should clinch the question of where the noise is coming from.

73
 

loosecannon

Sr. Member
Mar 9, 2006
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you will notice this issue more if you have expanded the receive frequency response by changing the cap values.
LC
 
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1iwilly

Well-Known Member
Dec 7, 2008
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update change all the caps today the noise it's still there but it's more like 1 or 2 % there before it was very noticeable now I have to have nothing on to hear it
 

nomadradio

Analog Retentive
Apr 3, 2005
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Louisville, KY
www.nomadradio.com
There is a 10uf 50-Volt electrolytic on the tiny power-supply regulator circuit board. Pretty much located dead center. This one causes so many wacky headaches we started replacing it on every Cobra 2000 we see, no matter the customer complaint.

And then again, the big fat one on the main circuit board just inboard from the audio power chip is important for that kind of filtering.

73
 

Wire Weasel

Senior Moment
Dec 13, 2008
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Many newer rigs do this too - similar. Have witnessed several Galaxy Mobiles with Freq. Counters and Illuminated Faceplates emit an audible hum through the external speaker when you turn the volume down. With volume up, the regular received audio covers up the noise. Cheap circuit design - lack of sufficient filtering.
 
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