New Cobra 29 LX

Turning up modulation, power or both will push more heat in the final transistor.

The one in the picture appears to have little or no heat-conducting compound under it. The clear mica insulating washer should have a thin layer of this stuff on *BOTH* faces of the washer, against the heat-sink surface and against the metal tab face of the final transistor.

Running it bone stock for the warranty period probably would not cause it to overheat and break down.

But if you "turn it up" without providing an efficient path for the added heat energy, that final transistor will fail prematurely. The compound fills the small voids between flat surfaces, and carries heat across that insulator and into the heat sink more effectively. Sort of a "thermal gasket" compound.

But that's why I think the factory does this. Every day the radio lasts beyond the warranty cheats them out of another sale opportunity.

73

Perhaps this can help?

From the old CB Tricks Forum Days....


Comes from a thread back in the CB Tricks...

The mod is a variant of the typical JP-36 mod of a FIXED resistor and Cap - only now, you have a variable supply for it.

You still have a resistor and Cap to fed this into - but the TIP-120 is made for the concept of driving more power and control of carrier - at the driver stage.

IF you do this mod, please be careful with a few things...

The TIP-120 will need to be mounted - to provide heat sink...

The Diode needs to be used to keep the TIP-120 from self destruct.

If you decide to go direct to the mod transformer - you'd better have your ducks in a row for excessive modulation will generate a lot of negative spikes than if not tamed - can blow up all this effort on yourself.

Best to keep and use values shown above for starters - then you can adjust as you wish.

Cap values adjust Volume of presence of audio in carrier envelope -
Small capacitive values = less audio but more carrier control (steadier swing to help with Amps)
Larger Capacitive Values = More audio and swing but can damage the Final and Driver from excessive audio drive

HINTS: - you'd be better off to use a LOWER VALUE - Higher Wattage resistor to swap out the effects - but that changes power levels to a point where you may be better off to compromise on the swapping of values...

For Educational Purposes Only

Grogan said:

Yeah if somebody has one that would be a big help. I posted about buying imports with a little extra power and found by the time you pay for the radio,Conversion and tune it's more than I wanted to spend for a work truck radio. I did buy the cobra for under 100.00 delivered new in the sealed box. I guess I need to know will it tune down to 1 1/2 to 2 watts for a small 50 watt RM amp without the variable power mod or will it tune to 5-6 Dead key 25 watt swing. Just wondering cause it's on the way. I can adjust watts and modulation pots but not a super tune.or variable power mod.
Thanks

Click to expand...

Hey! I bought mine from Bells CB in Florida. Screwdriver did a mosfet upgrade for me with the talk back upgrade and added an echo board. Keys 6 watts and modulated around 50 watts via my Astatic 636L mic. They are great over there. It costs a little more, but your radio is top notch when you get it back.

Or just get a Uniden 980
Velcro an RM ITALY KL203 to it.

AM/SSB + NOAA

Probably 70W or better.

Will all run from a 15A circuit.

Slowmover said:

Or just get a Uniden 980
Velcro an RM ITALY KL203 to it.

AM/SSB + NOAA

Probably 70W or better.

Will all run from a 15A circuit.

Click to expand...

The Uniden 980 will be the downfall of that company. Over 50% display failure rate according to many other forums and the company is not keen on their warranty either.
BTW, I spend more time 'un-modding' good Cobra radios as I usually get them with diodes and resistors cut, slugs missing from the final coils and off frequency due to the vast number of truck stop wanna-be techs ripping off the drivers by actually charging them to destroy their radios.

My 980 has been fine. Same with 880 and 885. Several thousand hours now on each.

kc4eoe said:

The Uniden 980 will be the downfall of that company. Over 50% display failure rate according to many other forums and the company is not keen on their warranty either.
BTW, I spend more time 'un-modding' good Cobra radios as I usually get them with diodes and resistors cut, slugs missing from the final coils and off frequency due to the vast number of truck stop wanna-be techs ripping off the drivers by actually charging them to destroy their radios.

Click to expand...

Id be interested in checking out those forum posts, got a link or 2?