Cobra 200GTL DX Transmitting problems

[Beakster]

Hi,

Got some problems with my rev3 Cobra 200 which has been working great for the last year.

Sometimes when I turn it on nothing happens for a few seconds, then it turns on. Then when I key the mic on high power the frequency counter and lights on front flicker on and off really fast and I can hear the relay clicking away on and off in quick succession.

This sometimes happens on low power too. If I keep trying to transmit then occasionally the radio sorts itself out for a time, but other times it gets worse. When this happens the lights on the front start to dim, and then go out all together, so it looks like the radio is off but I can still hear it receiving static.

The radio is about a year old, but the warranty is void because about 5 months ago it received reverse polarity (jump starting car) and I had to open it up to replace the idiot diode.

I am competent in soldering (although not surface mount) and would like to fix this myself. I just need to know which component is faulty. Any suggestions?

Thanks
-Chris

[Rick]

Jump starting car with any transceiver is not a good idea and can cause all sorts of faults

Check IC10, IC25, D83, Q83, R422 and R423.
If none of the above are faulty, try unsoldering the the collector's and base's of the the two finals (2SC2290) if you do this you should have about 8 to 12 watts output if you do then the finals are short and will need replaceing.

[Beakster]

Hi,

Thanks for the reply.

Please can you let me know how to check these components? Can they all be checked without desoldering them?

e.g. I assume I can just check them with a multimeter by,
measuring the resistance of the resistors,
making sure the diodes only conduct one way,
But how do I check the ICs and the transistor?

If they can't be checked without desoldering them, would I be as well to just replace all these components? I checked the price of finals and they are £25 each! I was hoping it wouldn't be them and given the problem is intermittent (although happening nearly all the time) is it possible for a transistor to fail in this way?

Also, can you suggest a supplier in the UK who will stock these components?

Thanks
-Chris

[Rick]

You will need to remove the components to test them.

Do a Google search on test transistor with a DVM.

You could just replace and hope for the best, but this is not the best way to find the fault.

I cannot help with UK components stockers.

[Beakster]

Thanks Rick, you're the best!

I'll pop down to Craplins and get a small pointy bit for my soldering iron and some soldering wick tonight to get those fiddly components out.

I'll give them a test with the mutimeter and let you know how I get on. Looks like I may have to order replacements from the US, but as they are small it shouldn't cost much for postage.

Thanks
-Chris

[timrim]

I recently found a bad on/off switch on a Cobra 200GTL DX with similar symptoms. (Resistance in switch). Bypassing the power switch should determine if this is the case, or check voltage on in and out side of switch and look for voltage drop.

[alexthegasman]

Hi Beakster, try Chris at mode componants for spares, if they're not in his catalogue, give him a ring, he's very helpful and reasonable prices.


www.modecomponents.co.uk

hope you fix it, alex

[Beakster]

Hey,

Thanks for all the advice, I've now solved the problem!

I tried connecting a 148 in my car to use as a replacement for the weekend. It was giving me similar issues! I suspected an electrical fault with the car at first. The power at the radio was over 12V though so I was sure everything should be good.

I then checked the blade fuse in the power cable from the battery to the radio. When I opened the little box the fuse has melted into a blob! I guess this means that even though the fuse hadn't blown it was not allowing enough current through even though the voltage was reading high enough. I put a new fuse in and the radio was good as new again! I'm very happy and relieved now.

Thanks again for the help!

[Animal]

Hi Beakster, try Chris at mode componants for spares, if they're not in his catalogue, give him a ring, he's very helpful and reasonable prices.

www.modecomponents.co.uk

I'll second that - got two mosfets for the price it would have cost for one at that shop which is open day & Knight!