Blown Finals

[redpsy36]

I found a Cobra 25 LTD at a yard sale for cheap. It was made in 1996 but it looked to be in good shape and there was no signs of damage that I could tell. However, after checking everything out, I believe it has a blown final. Without an antenna hooked up, or a mic, whenever you turn it on, the RF meter is pretty close to max to the left. Turning the RF gain to the max and/or squelch up or down doesn't change anything. There's no static coming from the speaker but I have verified it does work. I verified using a cb in working condition. Whenever you turn it on or off a few times, the RF meter will swing from completely left to completely right. Another thing, when it actually is hooked to an antenna (I haven't transmitted at all using it) It picks up fairly decent although when I touch any part of the cb, the power meter goes the rest of the way left.

I believe it has a blown final but I don't really know much about CB's. If anybody else has any ideas then let me know.
Thanks

 

[KX0Z]

Maybe cold solder joint. Take a non-conductive object and gently press/tap on the circuit board and see if affects the meter. I don't think its a blown output transistor. But have you actually tried transmitting with it? Do you have a swr/watt meter to check output. Thats the simplest way. You said it does receive though? That pretty much eliminates the PLL..

 

[prcguy]

A blown final transistor would usually only cause transmit problems and not the long list experienced by the OP. If it were shorted it could also keep blowing fuses but that's about it.

 

[redpsy36]

I don't have an SWR meter. But when you key it up, the RF meter shoots farther right and will stay there. No static can be heard while it's like this. After you key it up a few times, it will swing to the left and do the same thing. It now won't pick up anything. It barely will pick up static.

 

[wyShack]

likely one of the electrolytic capacitors on the board has dried out. These caps were mostly used on the power supply circuits (and or buses)to ensure steady voltage. In restoring an older radio many of us start by replacing all of the electrolytic caps as they are the shortest lived (25+ years) of the many components in radios.

If you are a tinkerer, you may find a service manual for the radio online.

 

[EddieHusak]

More than likely, this radio simply needs a few capacitors changed out. You can get an SWR meter for under $15 on ebay, or ask your friends if they have one you can use, as well as a dummy load. A dummy load is simply a 50 ohm load termination that absorbs the majority of the RF output. There are some SWR meters which have on built in, I just bought one for my test bench. There are capacitor kits available on ebay as well, all separated by part number.

As far as electrolytic caps go, anything built before 2009 or so would probably benefit from a recap. That said, I have some vintage gear from the 60's and 70's that still work just fine, but I know a recap is on the horizon at some point. Gear from that era was generally designed for long life, where gear from the modern era are designed to last ten years tops without intervention from the soldering iron.