Cobra 2000 GTL problem

[kb6led]

Ok here it is I am working on a Cobra 2000 with the following problem:

When the radio is powered up it will draw approx 6amps in rx or tx and the final transistor gets very hot and there is no output power at all. When I pull the TP8 & TP7 wires off their pins the short goes away. I have been removing and testing capacitors in the final section looking for any that may have shorted and so far all look good also I put a known good 1969 Final transistor in the radio and no change. Anyone ever run across a similar issue with there cobra radio? Any insight or thoughts would be appreciated.

 

[loosecannon]

just noticed this thread and that no one else has offered to help yet.

i would say you probably have a shorted driver, or pre-driver transistor, but dont just replace them without testing, as you could have a shorted component that will blow the new part.

start by going here:
http://www.google.com/url?sa=t&sour...z1eVFOmxA&sig2=km5f4eyZyI_l0XUJh2X3Yw&cad=rja

and learn how to test transistors with your digital multimeter.

test the old final that you pulled from the radio. if its shorted, then most likely you have a shorted component associated with the final.
the bad news is, it may have instantly blown your new final. check it too.

now check the driver and the pre-driver transistors.
if you find one of them blown, be sure to check their associated parts so you can be sure that a bad component didnt cause the shorted transistor.
if they seem ok, replace the shorted transistor, and you should be fine.

remember, the active devices, like transistors and IC's are the most likely to fail.
LC

 

[Toll_Free]

Pull all 3 leads from the transistor. Does it still pull lots-o-current?

Can you also pull the board (Or leads, depending on vintage) and test for bias current, IE, can you set bias, or do you pull like an amp of Ibias?

--Toll_Free

 

[kb6led]

Thanks guys for the reply back I got a bit side tracked on working on this thing but will start in again this week.

The 2SC1969 is bad for sure but it is not the cause it just the end result, so far I got as far as removing it and the short circuit has gone away so the radio will at least power up now. As I remember I was reading a constant 12vots on the base of TR36 (2SC1969) but that is as far as I got and put the repair aside for a week. I will pull the driver and pre-predriver this week to take a look.

 

[kb6led]

Just an update I had some time to work with the radio and found C152 capacitor was shorted I replaced it and the 12volts I was reading on the base of the final is gone. At that point I installed the new 2SC1969 and am now getting power out but only 3.5 watts but not able to get a bias reading(s) for the driver or final, I plan pulling TR37 and to see if there is a problem there.

It was late last night but one other thing I did notice is the AM power would dead key 3.5 watts and when modulated it would drop back to 2 watts also SSB dose not put out full power I am reading about 4 watts.

 

[loosecannon]

first you need to make sure you have a real 2SC1969.

if you bought it recently, like within the last few years, and you didnt buy it directly from RF Parts Co. then there is a good chance you got a re-labeled fake.

they are everywhere.

if this is the case, you have two options, pay the high price RF parts is asking for the 1969, if they even have any in stock, or look for a CB20W transistor.

ive never used one, but ive heard people say they work about the same as a 1969 power-wise.


if thats not it, then checking the bias transistor is a good idea, and you should check the bias diodes also.
they are the ones bolted to the heatsink near the final and driver.

i hate low power problems because they can be so many things.
could be as easy as re-tuning, or could be some weird issue in the mixer circuit that is affecting the level of the IF or VCO signal, or who knows how many other things.
good luck with it.
LC

 

[kb6led]

Thanks Loose cannon for the reply back, I hear ya on chasing the low power issues I have repaired a few of these radios for locals but this one is proving to be a bit of a challenge!

I did get the 2SC1969 from RF parts so I a am assuming that I got a good one, what I am seeing is a dead key max on AM of 4 watts and when modulated it will drop back to about 3 watts, On SSB I am seeing about 8 watts. At one point I did check the bias diodes and they looked ok but I will revisit that again just in case I missed something. Speaking of those diodes do u have a source on them? I also noticed each of them have a different part # do u know what the difference is?

 

[radiohobby]

I think I read on CBTRICKS about a problem with the voltage regulator chip causing something like you are describing. You might be able to get the technical manual from that site also if you don't already have it, they usually have some expected voltage readings at various points that might help you narrow it down.