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Cobra 2000 GTL Smoking on SSB (literally)

Well I can tell you the DC should not be present from a power supply unless the switch is turned to the DC position so if that is true then somebody wired it that way. This may or may not be a problem depending on what they did it could make voltage present on the DC socket while in AC mode?
 
OldTech03 said:
Well I can tell you the DC should not be present from a power supply unless the switch is turned to the DC position so if that is true then somebody wired it that way. This may or may not be a problem depending on what they did it could make voltage present on the DC socket while in AC mode?
Click to expand...
Thank you OldTech03,
this occurs only when switched to D.C. on the rear panel, but the radio is on when the main power switch is off, the frequency counter is off (displays clock) but the radio is on and running.

I think this ishappening becauset main power switch is switching ground (?), and the external supply is proving positive and ground when it itself is on? (I've yet to get the schematic open this morning)
 
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LeapFrog said:
OKay.. as the title says the radio wants to smoke (it won't blow a 4 amp fuse) itself when switched into SSB mode, I measured the voltage at the collectors of driver and final, and when I switch the radio to SSB the voltage shunts to ground and the current draw increase, a lot. This is in RX mode, it's enough to blow a 2 amp fuse but not a 4 amp fuse.


With VR10 adjusted for about 6 Volts the radio would only peak to 3 watts carrier and couldn't do 100% modulation, swinging backwards.
The best it'll perform is with 2 watts carrier going to 8 watts PEP, something is amiss!


I have a funny feeling that when I go to adjust the bias, I won't be able to.... lol
The radio will RX ssb, and TX a small amount on ssb.

Oh yeah, a little background on this particular 2000 GTL.

It was plugged in backwards with the fuse bypassed and the RPD fused itself, and a choke and a hand wound transformer, and several other parts fried before the owner unplugged it, eventually seeing the cloud of smoke.


So far I've rebuilt the power supply, recapped the entire radio, electrolytics & tantalums (phreak counter also, and diode update).
The audio IC was bad so it's been replaced, along with the PLL.



An awesome forum member helped me realize the poorly executed (major slide) clarifier mod was performed incorrectly and dragged the VCO preventing proper alignment "40CH. stretch", clarifier returned to factory and VCO + synthesizer aligned nearly perfectly.
It nows receives and transmits, but obviously with something seriously wrong.
I'm trying to save this radio & "get it out the mud", any (more) help from members is greatly appreciated!

The fine tune adjust on the clarifier operates in reverse?? I've switched the wires around to no avail!
Also the carrier was fluttering about an 1/8 of a Watt, strange..

I believe I should check the MB3756 for proper voltages on RX + TX.
And attempt the driver + final idle bias adjust.
I need to come up with an "at home solution" for some freeze spray, I don't have an I.R. camera to see exactly what is heating up when the radio is switched to SSB.
The current draw is over 2 amps as the collector voltage drops from "supply" to ground.

I've replaced two burned out chokes, and the bias transistor.
The final is a known good one, and the driver is operating.

Edit: it's been suggested to me that I should inspect the bias circuit closely.
Click to expand...
The only advice I could add is on the IR camera. I'm fairly sure they have IR apps. Whether or not they work I wouldn't know. But it's worth a shot. Good luck bringing back an excellent transceiver to its former glory. So far it seems like your devoted. I would guess that this mm has a good chance considering its being done by someone who's devoted to getting it completed. 73
 
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I forgot to update this thread, oops!

Thank you for the kind words, Sonar. :)

The radio is all done and works 100%, just waiting on decals & LED's for that custom look.
(the bezel on this radio is beat up) :cool:
 
If anyone finds this thread, and the radio you're working on has been hacked by an golden screwdriver, check the ferrite cores, this Cobra 2000 flaked out, VCO was constantly going out of lock, my friend was not happy... I changed every capacitor in that radio, but over looked the damaged ferrite cores.

After nearly two years of struggling to find issues in the radio, to 'make it right' for *Silent Key* i should have located an parts chassis Cobra 2000 and swapped the ferrites over.

I let down a friend, and now he's *Passed On*, i don't know where that 2000 GTL ended up, but i learned a lot about not mixing business with friendships.

I was well within my capabilities to repair his microphone cords, but at that time i wasn't ready to perform an complete restoration. I was sure that i could deliver, and i did not deliver. Something something, "know your limitations". 73
 
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LeapFrog said:
If anyone finds this thread, and the radio you're working on has been hacked by an golden screwdriver, check the ferrite cores, this Cobra 2000 flaked out, VCO was constantly going out of lock, my friend was not happy... I changed every capacitor in that radio, but over looked the damaged ferrite cores.

After nearly two years of struggling to find issues in the radio, to 'make it right' for *Silent Key* i should have located an parts chassis Cobra 2000 and swapped the ferrites over.

I let down a friend, and now he's *Passed On*, i don't know where that 2000 GTL ended up, but i learned a lot about not mixing business with friendships.

I was well within my capabilities to repair his microphone cords, but at that time i wasn't ready to perform an complete restoration. I was sure that i could deliver, and i did not deliver. Something something, "know your limitations". 73
Click to expand...
I was a kid when my Old Man's radio didnt work.. It was a 142 GTL radio.... No receive audio and im sure transmit struggled too......

He brought the radio to a local radio repair guy, and sadly... It is forever gone... This was in the 1990s...

I fixed a broken 142 GTL radio in 2017 with the exact same problem... Which then made me think I wish I was able to repair my Old Man's radio...

Fast forward to today.... My Old Man is here, but health isnt too great... I was able to show him pictures of the 142 GTL and had him sign the old Palomar 350z he had with it back when... Sadly... The Palomar 350z is not worth the time to invest in it as @nomadradio made it clear it is a PITA to even try to correct all the shit wrong with it....

So for now it sits happily on a shelf in the garage.... :)

Keep repairing radios. Less and less repair men for radios happens every year.
 
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yes, I am turning down repair jobs. too much effort and
time to get them working after all the other guys worked on
them before me. and not enough "compensation" to me.
I first must find their mistakes. bad soldering. solder bridges
across circuit board traces. components installed incorrectly.
counterfeit components installed. etc...
no more tube amps with "sweep tubes" or 811's.
first, you know that the tubes are blown out.

I just got the RF key-up circuit working on a Fire Bird F-100 amp.
now it keys up, 75 watts carrier, but just sounds terrible.
I told them to turn the drive way down to a watts or so,
and it may work.

yep, your Palomar 350Z would be another disaster to work
on. bad tubes, bad relay contacts, etc.
 
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