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Cobra 142 gtl Transmit problem.

Alright, I replaced the voltage regulator, and after a little tuning, she receives great, but when I tx, my meter shows some swing in modulation, but my watt meter dosen't move, so I assume I have modulation without power? My neighbor can receive me even though I put out no watts. Maybe since we are so close he is picking me up for some reason. I tried ssb and lost modulation swing and still no power, but am has modulation swing. huh, weird. Could this be my finals? In PA mode it works fine.
 
Could be the output driver transistor is working; but the final transistor is fried.

Or, you might need to check the bias adjustments on the driver and final transistors.

Or, it might need an overall tx alignment.

Or . . .
 
Last edited:
Cable Guy said:
Alright, I replaced the voltage regulator, and after a little tuning, she receives great, but when I tx, my meter shows some swing in modulation, but my watt meter dosen't move, so I assume I have modulation without power? My neighbor can receive me even though I put out no watts. Maybe since we are so close he is picking me up for some reason. I tried ssb and lost modulation swing and still no power, but am has modulation swing. huh, weird. Could this be my finals? In PA mode it works fine.
Click to expand...

definately sounds like a problem around the final.could be any of the things Robb mentions, could also be poor alignment of the rf section, but as you have modulation and a neighbour can receive you,

most likely scenario is the driver is fine and the final is shorted which would allow the radio to tx a very low powered driver signal, which may be too low for your power meter's sensitivity but high enough for your neighbour to rx.
 
Thanks! That makes sense, the final being fried. The VReg was fried. Is there a good way to test the final when I remove it from the board? Can I test like a transistor? I dont think it's a fet.

Thanks,
Cable Guy
 
Cable Guy said:
Thanks! That makes sense, the final being fried. The VReg was fried. Is there a good way to test the final when I remove it from the board? Can I test like a transistor? I dont think it's a fet.

Thanks,
Cable Guy
Click to expand...
Just check the numbers on it. It is a 2SC1969 final - or it should be. Which makes it a bipolar transistor - and it can be checked with a diode checker. IF it was a mosfet like a irf520 or erf2030, it would need a few volts applied to it to open up the 'gate'. The 1969 is harder to get, and don't buy them from China/eBay; they are phony. Been there; done that.

There are a few places that still sell real ones on the net, like RF Parts for one . . .
 
best thing is to look for a CB20w transistor. they are made by workman.

they are whats being used in the new 148GTL's, and are about the only replacement available these days for a 1969. (besides a FET, but ive yet to hear of anyone perfecting that mod on this chassis)
LC
 
loosecannon said:
b(besides a FET, but ive yet to hear of anyone perfecting that mod on this chassis)
LC
Click to expand...

That a challenge?

Or, here... I'll let the cat outta the bag. You have to modify the bias circuit. Using a REAL zener diode, and set the circuit up accordingly. Those little 'circuits' consisting of a silicon diode and a pair of resistors fall apart when it comes to the dynamics of a class AB (read, drawing base current at times) vs switched mode.

The 29 and other chassis run the finals class c, as does the 148 chassis in AM (to an extent, it's really b).... On SSB, you need to be able to have ACCURATE bias, which implies LOW IMPEDANCE..... Meaning, those resistors that lift the diode above DC ground automagically cause the bias to drift, causing the SSB signal to sound like a 12 watt radio on sideband being fed into a 12 pill Davemade......




--Toll_Free
 
toll free,

you post the procedure and ill install it in my taiwan 148GTL.

i wont call it a challenge, unless that lights a fire under your ass. LOL
LC
 
I'll see if I have an IRF device still laying around. If so, I'll see what I can do about a relatively stock chassis.... SOMEONE in town has to have one they'll donate, I believe mine are all widebanded audio monsters with volted finals, etc.

COULD be made to work, but would NOT be indicative of a standard "148"

--Toll_Free
 
alright, newest updates. Replaced the final with a 2166 because I dont have a 1969 right now. Drive3r and final are both 2166 right now. It rx and tx fine, with about 2watts dk, but when i speak, the watts drop. Meter on the radio shows positive modulation, but once again, the watt meter shows backwards. hmm... where to go now?

also, checked sideband with a friend and its out of whack, they sound like a frog under water, so maybe an alignment will fix this.
 
the 2166 driver is too much input for another 2166.
you are probably driving it into saturation.

i wouldnt run it like that myself.
LC
 
okay its a temporary thing anyhow just for testing. I have another 1969 but its in my backup radio, but i might as well swap them. the backup will do fine with a 2166. it's a small mobile. I will try that today.

Thanks!
 
okay, replaced with a 1969 and after some tuning, got it swinging positive. I have noticed some of the inductor cans (2) have broke slugs, so I want to replace them. L27 and L29. They have to do with the TX and will undoubtably need adjusting since some screwdriver expert went in and broke them, they are probably wrong value (especially after they got broke). Thing is, I cant find anything online about replacements. They are Toko RCL... LA160 and LA254. I have some spare I can yank out of another radio but im not sure if the inductance range will be close or not, since the LA numbers are not identical, but close. Ever replaced any of these?

CableGuy
 
Okay, didnt know the slugs were double ended, but that got it. Yes, Toll Free, it is a mess, and I do use ceramic tools for adjusting these. Swapped the slugs around and bingo, I will let you all know what happens after I send it to get aligned, and thank you all for your help. A little direction can go a long way, and your help is truely appreciated.

Thanks again,
Cable Guy
 

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