Cobra 2000 gtl NO Modulation

[3puttbob]

Well, it was time to dig out the old base. After 13 years in a box I wanted to try the old Cobra. Sadly, there is no modulation. The mod needle shudders slightly and just sits there. Listening from another radio all you get is a dead key. Guess that means the finals are ok. Everything else is fine and receives fine on am and ssb. Before I start replacing capacitors, which is not wise since I'm a klutz around a circuit board, anyone have a suggestion on what to check first? I looked around both sides of the board for anything obvious, melted, broken or burnt. No cb repair shops around here anymore. Oh yeah, swapped the d104 lollypop with a d104 mobile. Same results.

 

[unit248]

I hate to say it, but based on what others have said, I think your best bet is just to replace all the electrolytic capacitors in one go. You might be able to run down the one cap that's causing you trouble now, but you'll just have another one go bad later. It'll turn into a long, drawn-out game of whack-a-mole. Having the radio go bad after being in storage for a long time is a classic problem description too.

The factory-installed caps are all rated for 10 volts but usually have 8 to 9 volts across them. That doesn't leave a lot of room for error; it would have been better to use 16 volt caps, but they were trying to keep costs down as much as possible.

The good news is that electrolytic caps are easy to get and fairly cheap. And they aren't that hard to replace. At least they aren't surface-mount parts.

-Bill

 

[555]

make sure the knob on the left side of radio is to the right
and maybe check battery in power mic

 

[3puttbob]

Thanks for the support guys. I decided to try the hand held mic one more time but, checked the wiring first. Oh crap, the red and blue were reversed (Thanks for the wiring code Robb). Then I remembered it was wired for another radio. After rewiring, my Cobra works. This means the D-104 lollypop has a problem. I put in a new battery, checked the wiring and looked at all the solder jobs. Everything appears ok. Is there another battery in the head? Something happened while sitting 12 years. Any suggestions?

 

[unit248]

There's no second battery in the mic head, but the D104 mic element is made using a Rochelle salt crystal. The crystal can be damaged by extreme temperature changes, humidity or physical shock. Humidity is a problem because moisture can dissolve the crystal (just like regular salt).

Unfortunately, there's no fixing the element if it's been damaged, and replacement ones are hard to get now that Astatic stopped making them. You can try a different kind of element or a 3rd party replacement.

-Bill

 

[3puttbob]

Oh wow. I never knew that. This was stored in a shed subject to 120 deg. for 12 years. Guess I'll have to shop around. Thanks unit248.

 

[unit248]

Yeah, the MC-320 crystal element in the head would not react well to being stored at those temperatures. As a last resort, you can try double-checking if the contacts in the neck just need cleaning (after 12 years they may have oxidized a bit) but chances are good the element is shot.

If you really want to make it work again, then your have maybe 3 options:

- Try fitting a Heil or other kind of element into the head
- Try to dig up a replacement element on fleaBay
- Try a replacement element from http://www.rbmicro.com

The RB Micro replacement element is supposed to be pretty good, but I've never heard one on the air. There is a recording on the web site with a comparison of an original MC-320 vs. their replacement so you can decide for yourself. They also sell the Kobitone crystal element that some people have used as a replacement for the D104 as well. The RB Micro XTLK replacement is more expensive but it comes with a complete mounting kit.

Alternatively, you can try getting a whole new lollipop from fleaBay. There are lots of them for sale there.

-Bill

 

[3puttbob]

I checked eBay and there are some for a couple bucks less but...... The one at Micro looks like the best deal. You're right about the choices, I think I'll just order one. It acts just like the one I tried earlier that was wired wrong so I rechecked that and it is correct. This mic used to work great before it sat in the shed so, I think I'll just order the element.
Thanks Bill.