Cobra 29 LX lower deadkey with 100 ohm resistor and no cap?

[loosecannon]

I was going to post the same diagram 543-Dallas did. it works well, and will not burn the pot out over time.

543-Dallas, you are exactly right about the audio not being attenuated going through the diodes.
since they are already forward biased once you key the mic, the audio will pass through 1:1.

pappas and beer- if you just want a 1 watt deadkey, get rid of the RF gain pot thing, and just play with the value of the resistor you added in place of JP36. lower resistance equals higher deadkey and vice versa.
LC

 

[Pappas and Beer]

I added the 220 ohm resistor and it brought it down to about a watt. But than things went south. The soldering job I did was loose and would work when I "pushed on it". (I did a half ass solder job to see if the new resistor would work) . So I impatiently unsoldered it to do a "permanent" job and in the process ruined the pcb traces by yanking them out like a bull in a china closet as my stepfather "affectionately" described me. Quickly moving to fix my error I scraped the some of the traces, but realized that is not the way and instead I jumped the traces to the nearest solder pad which both had a perfect continuity on the multimeter, but still no TX. I thought that would work but no transmit. Here are some pics if you want to comment.

 

[LeapFrog]

I have strung together a few 1N4001 diodes (the same rating used in the newest cobras I think, the older radios used a 1N4003 anti-spiking (3 amp rated rectifier) diode, the 1 amp diodes haven't failed in place of the 3 amp original. I only use that radio for testing and don't talk on-air with it so I don't know how reliable the 1 amp diodes would be under daily use but I think they would be fine as the newer 29 LX uses a 1 amp anti-spiking diode anyway.

 

[LeapFrog]

I added the 220 ohm resistor and it brought it down to about a watt. But than things went south. The soldering job I did was loose and would work when I "pushed on it". (I did a half ass solder job to see if the new resistor would work) . So I impatiently unsoldered it to do a "permanent" job and in the process ruined the pcb traces by yanking them out like a bull in a china closet as my stepfather "affectionately" described me. Quickly moving to fix my error I scraped the some of the traces, but realized that is not the way and instead I jumped the traces to the nearest solder pad which both had a perfect continuity on the multimeter, but still no TX. I thought that would work but no transmit. Here are some pics if you want to comment.

That final has a solder glob shorting the collector and base, clear as day in the first photo!
Look at the legs on the solder side, the two pins/legs of the final transistor that are on the same side have a ball of solder causing a short circuit.


After you fix that, check to make sure the resistor legs will not short out on the cover once put back in place, the final might be toast, remove the glob and test without the covers on, also you may have burned up a diode due to the short. (So if TX doesn't com back, look for a burned/failed diode, if it still doesn't come back you may need to swap in another final transistor (If your radio didn't come with a mosfet), a 2SC2078 is called for depending on the age of your radio, many others will work, 2SC2075/2SC2166/2SC1969/2SC2312 )

Something Like This will help save you tons of aggravation when a solder bridge is created, some desoldering braid/wick would prove useful down the road.

I remember the first trace I ruined, lol it was my first Cobra 29 and I was trying the same exact mod. Though, It'll work even if it's not pretty anymore!

-LeapFrog

 

[Victoriamichelle]

Try a 220 ohm/1 watt resistor . ..

Just did the variable mod. I used the 220ohm/1 w resistor. Dk is from 1w - 4w. Only lost 1w of pep. Added the swing mod for kicks. Wow! 1w dk to 16w+ pep. Good advice. On scope, looks good. So far so good lol.

 

[loosecannon]

sorry but no, it does not look good on the scope when you do a swing mod to any radio and swing 16 watts from a 1 watt deadkey.

That's not to say that i don't do them. It's what people want and i have no problem with it if the customer is happy, but the negative peaks are cut off and the positive peaks do flat top at some point if you turn the mic gain just a smidge past the "perfect" setting.

now if we could get some actual negative peak compression in a transformer modulated radio...
LC

 

[Uncle Ronnie 336]

 

[Uncle Ronnie 336]

the perfboard/adjustable dk is the only way i go about this on a transformer rig. it looks :::less worse::: on the scope i got than the standard r/c setup