Cobra 2000 recieve malfunction

[TonyV225]

where are the recieve limit diodes and the rf amp in this corba 2000 i am putting one together and have it done but the recieve is really weak and i get no movment on the meters on tansmit or recieve but transmit works put out rf power and sounds good. there is an odor of a burnt diode or resistor in the front of the radio under the meters or in that area.`

 

[nomadradio]

where are the recieve limit diodes and the rf amp in this corba 2000

Hi Tony,
Probably not the root of your problem. Unless someone put another radio on the other end of a coax jumper and keyed straight into it. Didn't blow mine out the last couple of times I was that sloppy. This COULD be the root of the problem, but the odds alone don't suggest it.

i am putting one together and have it done

Guess it depends on what you're interpretation of the word "done" is?

but the recieve is really weak and i get no movment on the meters on tansmit or recieve

Not exactly "done" in the "ready to sell or use" department, I'd say. The meters are just old and the pivot bearings are seized up. Tends to happen at the 15- to 20-year age category. If it says made in Taiwan or Japan, it's between 18 and 27 years old. Newest one you can get, made in the Philippines, will hit 15 years pretty soon.

The plastic in the meter housings will shrink as it ages. This places more tension on the pivot bearings, until they won't move. Loosening the tiny screw that sets the pivot tension is like working on a wrist watch. Use care, and try to scrape away the dab of lacquer that locks the screw in place before you strip the screw slot or break the tip of the jeweler's screwdriver. A half-turn often frees the meter back up.

The cause of the weak receiver can't be pinned down until alignment is attempted. If that doesn't bring it back, the position at which some of the slugs show a peak can be a troubleshooting tool. Some of them have a very broad peak when they're perfect. Others should have a peak that is very sharp, whrere the signal drops off with a half-or quarter-turn either side of the peak. If one of those is "broad", it's a hint of trouble in that circuit. Don't know where you'll find that "sharp/broad" guidance in print anywhere. If AM alone, or SSB alone works right, and the other mode is weak, this narrows things down a lot, too.

there is an odor of a burnt diode or resistor in the front of the radio under the meters or in that area.`

The meter lights are fed from half-Watt carbon resistors on the small circuit board where the toggle switches are mounted. A bulb that pulls more current than the original ones will overheat those resistors. Leaving the radio turned on 24/7 does that after a year or two, also.

73