Cobra 2000 GTL Smoking on SSB (literally)

[OldTech03]

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?

 

[LeapFrog]

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?

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)

 

[Sonar]

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.

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

 

[LeapFrog]

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)