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Cobra 2000 gtl low TX AM/ SSB

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slipkid
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Cobra 2000 gtl low TX AM/ SSB

Post by slipkid » Friday, 18 November 2016, 17:50 PM

Hello
I have a low tx problem with a cobra 2000 gtl on am and ssb. The radio puts out about a 1/2 watt on am and a little over one watt on ssb.
When checking through the radio with a o scope i found that the rf carrier from the driver transistor collector is attenuated on the output of C152 470 pf disc cap by about 1/5th. C152 has since been changed.
After the change of C152 the same problem presists.
This carrier level from the driver collector is fine before the cap. The Final has been changed with a fresh one from RF parts with no increase in output power.
Knowing that the only thing between the output of C152 and the final amp base is TR37 driver bias regulator and R181 10 ohm resistor and C151 .047 uf that is in parallel with R181.
R181 and C151 along with TR37 have been changed as well. The voltage to the pins of TR37 is at follows, 1.4v base, 6.3v collector, 1.2v emitter. 8 Volt regulator was also changed as well. Radio was completely recapped .
Base voltage of Final amp is about 1.0 volt.
The rf carrier that is present on the base of the final is being amplified properly by the transistor as the scope shows good transistor gain with the rf carrier present on the base.
The bias on both final and driver are set to maximum at this point just to get the output i am producing.

Any help with this problem would be greatly appreciated.




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Re: Cobra 2000 gtl low TX AM/ SSB

Post by MDYoungblood » Monday, 21 November 2016, 10:23 AM

Are there any mods done to it like variable power? Just asking cause it is a common mod. Another thing would be to check the caps in the transmit section for polarity since you had it recapped.

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Re: Cobra 2000 gtl low TX AM/ SSB

Post by slipkid » Monday, 21 November 2016, 14:24 PM

hello
all caps have been checked for proper polarity. When i received the radio the radio had the AM power control wired to the front of the radio on the tone control.
Control has been rotated in both directions and seems to work properly as far as raising and lowering carrier in AM mode The value of the control has not been tested. I will check that next now that i have thought about it.
The radio had this transmit problem before i recapped it and all caps were checked with a capacitor tester before installing.



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Re: Cobra 2000 gtl low TX AM/ SSB

Post by MDYoungblood » Monday, 21 November 2016, 14:50 PM

Did you check L20, that was a problem child in past radios. You could try sending CB Doctor a PM/email, he was the "Cobra Guru" on the forum even though he's not on much anymore he still might give you an answer.

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Re: Cobra 2000 gtl low TX AM/ SSB

Post by slipkid » Tuesday, 22 November 2016, 19:34 PM

Hello,I will check L20 and post back.
I emailed the CB Doctor and gave a description of my radio symptoms but got no reply back.
Maybe it is another person.
thanks for the reply.




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Re: Cobra 2000 gtl low TX AM/ SSB

Post by pabxone » Wednesday, 23 November 2016, 2:20 AM

I've had open circuit / high resistance L39 & L42 chokes on these 2000 GTL's causing low TX - The chokes are part of the bias feed via TP7 & TP8.

TP 7 & TP 8 pins go high res / open circuit due to poor soldering onto the pcb (very common)

Also had faulty S042P (IC5 Mixer) Check IC5 voltages against those listed in the service manual.

Also have had faulty AN612 (IC3 Balance Mod ) causing same problems.

TR37 & TR39 are both common failures.

Alignment of L44 , L45 , L46 ,L47 , L48 is all critical.

A faulty crystal filter can also give similar problems as can a faulty TR41

-- 23 Nov 2016 19:34 --

Varistor's D49 & D50 need to be checked also.



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Re: Cobra 2000 gtl low TX AM/ SSB

Post by slipkid » Sunday, 11 December 2016, 3:06 AM

Well i have checked out everything that you mentioned in your post and everything seems o.k.
I have noticed that when i remove the base of the final from the board that the signal at the output of C152 returns to normal amplitude.
The final transistor is new from RF parts so i am still stumped. I do not have a high quality out of circuit transistor tester but i have noticed that the transistor
seems to be operating properly as far as gain is concerned on the scope from imput to output . seems like the internal resistance of the final might be the cause
of the signal being drawn down. I have noticed in years past working on radios with some 1969 type finals that out of circuit that they have a internal resistance from collector to emitter
where as other 1969 transistors do not when measured with a tester. Maybe i will change the final again with another one with different numbers that follow the 1969 on the transistor.
This is the only radio that has ever been this much of a pain in the gluteus maxims that i have ever worked on and i have repaired many radios over the years. Now i know why i prefer my Courier Centurion. Never a problem.



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Re: Cobra 2000 gtl low TX AM/ SSB

Post by sonoma » Sunday, 11 December 2016, 4:33 AM

check C95 cap just above the an612 ic. it helps control the am dead key and ssb signal. if the cap is bad or the resister right by it you can lose just like you are stating. you may have put a bad cap there or in backwards. I know you said you recapped the radio but worth a try. will look for the resister in circuit for you when the cap goes bad it can knock out the resister also. R108 is a 100 ohm resister that can go bad from heat if the cap had shorted before the recap.



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Re: Cobra 2000 gtl low TX AM/ SSB

Post by slipkid » Sunday, 11 December 2016, 7:56 AM

I will check it out. All caps were checked with a cap tester. Maybe i put one in backwards,Could happen. will check the resistor as well. thanks.



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Re: Cobra 2000 gtl low TX AM/ SSB

Post by sonoma » Sunday, 11 December 2016, 9:17 AM

if the radio was like this before the recap it could have damaged the resister in that location. the resister gets hot from the cap being shorted so the cap could be good since you changed it already and the resister is bad.



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Re: Cobra 2000 gtl low TX AM/ SSB

Post by slipkid » Sunday, 11 December 2016, 9:44 AM

Radio was like i described before the recap. Will check the resistor. Got lots of radios around here to rebuild. Got 2 rooms full of radios i have repaired ,some completely restored over the years. I prefer tube units
but i have some transistor units that i like as well. Thanks for the help



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Re: Cobra 2000 gtl low TX AM/ SSB

Post by pabxone » Tuesday, 13 December 2016, 8:20 AM

I'd try replacing both the Driver and Final again , There are a lot of Fake 2166/1969 & 1306/1307 transistors floating around out there.
Having said that RF Parts are a pretty reliable source and it's highly unlikely they would knowingly sell fakes but their suppliers may have let them down ?

If you have an old radio kicking around with a working pair try swapping them out into this radio for a test , The 2000GTL runs a 1306/1969 pair , but a 2166/1969 or 1306/1307 pair would be fine.

I've got a tray full of Fake 2166/1969 and 1306/1307 that came via a well known Chinese Internet site, these put out little to none in the way of RF in circuit , They DO test up fine on a multimeter for a Go/No go test but they sure ain't 30Mhz RF transistors - I suspect they are just power transistors re-labeled to sell off surplus junk?

I'll guess you've had a look at D49 ( MV13Y) & D50 ( MV1Y) Varistor's in the bias compensation circuit ? These can be subbed out with 1N4148's for a test to see if they are part of the problem



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Re: Cobra 2000 gtl low TX AM/ SSB

Post by WIRADIOOP » Sunday, 18 December 2016, 21:34 PM

I just had a similiar problem with a cobra 139. It was the 7.8025 Crystal. I wouls start by checking to see it you have have all the correct frequencies at the TP(test points) as indicated in the service manual. Your 7.8 MHZ signal should be at TP3.
Let me know what you find.
Wiradioop



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Re: Cobra 2000 gtl low TX AM/ SSB

Post by basicii » Thursday, 20 July 2017, 11:14 AM

So, have you found a fix?
Sound like an audio issue.
I had a 2k that had the same problem. It was C174.
As a check, did you temporarily jump VR10 and key it up? I'm certain you did.
Now, C174, although at the end of the audio chain, plays havoc in xmit sometimes, and maybe check if the radio has been modded with the TR41 mod. Original value should be 1uF/50v. If it goes bad, you'll get the same symptoms, i.e., low output. The "low output" you'd witness is a low(1-2 watts), barely modulated signal. If you do the TR41 mod., when choosing a larger value(up to about 1000uF), watch for reduced rx volume. Sort of a catch 22.
Low SSB output again seems to point to modulation.
Have you injected an audio signal directly past C142?
Sounds like the driver and final transistors are just sitting there waiting for some decent AUDIO to come there way.
In your post, you mentioned replacing C151 with a .047uF. It should be a .0047uF. Maybe this was a typo?
Have you tried jumping R181?

Please post any updates. Thanks



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Re: Cobra 2000 gtl low TX AM/ SSB

Post by basicii » Thursday, 20 July 2017, 11:16 AM

correction, not C142, but C174.




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