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 Post subject: Cobra Scanner SR900, weird problem.
PostPosted: Mar Thu 28, 2013 4:37 pm 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 1363
Location: Hutchinson KS
I'm not sure if a old scanner qualifies here or in the clubhouse, but a customer brings this thing to me with the complaint it works fine when cold, but as it warms up the scan speed starts to run slower and slower till it stops on a channel and stays there. You would think using a heat gun and a can of freeze spray it would be a piece of cake, right? Nope, you heat anywhere on the board and the scan slows down to a stop, you spray anywhere on the board and it starts to scan faster! I don't have a schematic for this thing to see exactly what determines the scan speed. And why temperature has an effect. I thought maybe all the amber colored glue they used at the factory to hold things down was causing leakage, cleaned it all off, no effect. Replaced several caps, no effect. Any of you guys have any idea? Thanks!


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 Post subject: Re: Cobra Scanner SR900, weird problem.
PostPosted: Mar Fri 29, 2013 12:07 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5814
Location: Black Hills, SD 57745
Instead of a heat gun, try a soldering iron on a lamp dimmer so you can dial down the temp. Chill the entire unit outdoors or in a 'fridge to give a longer warmup time.

I'd almost suspect a poor solder joint somewhere.

Good luck with that tough dog!
-Ed


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 Post subject: Re: Cobra Scanner SR900, weird problem.
PostPosted: Mar Fri 29, 2013 12:23 am 
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Joined: Jun Tue 21, 2011 8:27 pm
Posts: 3206
Location: Virginia
Not sure about a bad solder joint.... I'm thinking a resistor or transistor heating up.
Of course a good check with a magnifying glass won't hurt.

Didn't Sam's have a schematic series for scanners? -- Hmmm....

~ Mitch ~


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 Post subject: Re: Cobra Scanner SR900, weird problem.
PostPosted: Mar Fri 29, 2013 1:38 am 
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Joined: Jan Tue 15, 2013 4:47 am
Posts: 324
Location: Buffalo, NY
That sounded like a good tip Ed. Narrowing down the heat channel sure would be the thing to try.

Paul


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 Post subject: Re: Cobra Scanner SR900, weird problem.
PostPosted: Mar Fri 29, 2013 9:28 pm 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 1363
Location: Hutchinson KS
Since the unit was so sensitive to just a few degrees difference of temperature I did as suggested, using a solder iron as a heat source, slowly going around the components till I found the culprit! A adjustable slug type coil just by putting my finger on it a few seconds caused the scanning to stop! Well, howinhell do you find a new coil for something built in 1989? I went on evilpay and there are about a dozen complete units with the same model number for sale for 10 dollars on up! I sent the customer the info suggesting to just buy one of those units instead of hunting down a part number and trying to find a replacement coil. Thanks for the suggestions guys!


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 Post subject: Re: Cobra Scanner SR900, weird problem.
PostPosted: Mar Fri 29, 2013 11:55 pm 
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Joined: Jun Tue 21, 2011 8:27 pm
Posts: 3206
Location: Virginia
Hmmm.... There's not much in an adjustable coil to fail, so I'd bet the coil is out of adjustment.
Without a schematic or photograph, its hard to determine whats going on from the forum's perspective.

~ Mitch ~


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 Post subject: Re: Cobra Scanner SR900, weird problem.
PostPosted: Mar Sun 31, 2013 4:20 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5814
Location: Black Hills, SD 57745
Glad my tip helped! Just for fun, reflow the solder joints around that coil. It may not cure it, but won't cost much to try.
-Ed


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