Cobra 25/29 receiver overload?

[SilverFox]

I work at a rock quarry that uses CB on their mobile equipment and in the scale house to talk to truckers. In a year or so of employment, there have been 6 Cobra radios replaced. All suffered the same problem; low receive signal. No static even with the volume and rf gain maxed. All would receive at short distance <1/4 mile max. A local trucker that runs 12 trucks in an out the quarry complained of the same problem. Problem mike cords and bad antennas/coax have been accounted for. We have a shelf in the shop full of "bad" radios pulled from equipment. Is "close talking" wrecking the receivers in these radios? Any ideas? Thanks.

PS: All radios were Cobra 25 or 29 with the exception of one Galaxy 929 that was purchased to help eliminate this problem and it too failed in less than 6 mos. with the same semi-dead receiver.

 

[wcsd106]

Replace D1 and D2 in those radios and they'll be good to go again. When you're close talking in the quarry, start running with the RF gain backed down.

 

[9C1Driver]

Agreed, fix em and run the RF gain turned way way down. Problem solved.

 

[hotrod]

agreed. id use only enough rf gain to hear them.shouldnt
be anymore then 1/2 or 12oclock for real close stuff.
now ya ruled out coax/antenna so id check your swr
to make sure there at 1.5 or less

 

[SilverFox]

Thanks guys for your interest. I'm looking forward to try out this fix. Looking at the Cobra schematic, the rf gain control is placed in the signal chain past Diodes 1&2. How then, will this control protect D1 and D2? The block diagram on my Galaxy indicate similar placement. Don't get me wrong, I really don't understand. This is such a common problem around these parts and there is no radio shop here in west Alabama.
BTW - Are there beefier diodes available for D1 and 2? Thanks again for all your help!

 

[AA1QD]

AGC

Funny, I remember when Cobra 25-29 Ltds had one of the toughest AGC circuits around. I mean I've had a dude running a 16 pill key within 100 feet of my old truck, my 29 was fine but the guy with the galaxy next to me had a toasted rx. Maybe the old stuff was better.

 

[dulley]

Replace D1 and D2 in those radios and they'll be good to go again. When you're close talking in the quarry, start running with the RF gain backed down.

Thanks for the info I had a 29 doing the same thing
Took some out of a parts radio and I was good to go
Thanks again

 

[wcsd106]

Glad to help

 

[SilverFox]

PS: All radios were Cobra 25 or 29 with the exception of one Galaxy 929 that was purchased to help eliminate this problem and it too failed in less than 6 mos. with the same semi-dead receiver.

Update:

The Galaxy mentioned above (DX929) has been repaired. A loose connector from the wiring harness to the main board seems to have been the culprit. Thanks for your interest.

 

[psycho]

Funny, I remember when Cobra 25-29 Ltds had one of the toughest AGC circuits around. I mean I've had a dude running a 16 pill key within 100 feet of my old truck, my 29 was fine but the guy with the galaxy next to me had a toasted rx. Maybe the old stuff was better.

Usually the single final radios will blow before a dual final radio in that situation. You were lucky!