COBRA 2000 GTL AM carrier issue

[Unit 75]

sp5it, Im going to check with the rf probe shortly and post my findings.

Low_Boy...No carrier on SSB. I have talked to people on SSB and have had excellent reports about being right on frequency and good audio, Both USB and LSB are working fine with no issues.

 

[Unit 75]

Ok... here are my measurements all done in AM mode Receive and Transmit voltages:

Collector voltage TR36 is 12.950v RX 6.650v TX
Collector voltage TR37 is .887v RX 3.278v TX
Collector voltage TR38 is 12.950v RX 6.740v TX
Collector voltage TR39 is .499v RX 8.311v TX
Base voltage TR39 is 90.300mv RX 1.307v TX
All voltages appear to be stable

 

[sp5it]

Your RF probe is not working. There is 0V on RX on collectors.
RF voltage appear only on TX.
You need this: http://n5ese.com/rfprobe1.htm connected to DMM, than measure again.
Diode can be any silicon or germanium diode. 1n4148, 1n60 whatever...
Mike

 

[Unit 75]

Those measurements were done with my Rigol DM3068. Can't locate my RF probe at the moment, it still may be in a box somewhere. Just moved the shop and I'm still unpacking but I will look for it.

 

[sp5it]

With rf probe you should easily locate where power disappear.
See attachment. Red is AM tx path. Common with ssb left from mode switch. You wrote, that SSB is OK, so imo problem could be a mode switch or elements right from that - marked green.
There is high current flow, switch can be bad. Try to bypass for a test.
Mike

 

[Unit 75]

Ok. I will check it out and report back. Thanks

 

[Unit 75]

Ok I checked these with the RF probe
Pre-driver collector voltage with RF probe
0v RX
1.921v in TX when it is at 2w (initial key up) and 1.071v after it stabilized at 4w

Driver collector voltage with the RF probe.
0v RX
9.7v in TX when it is at 2w (initial key up) and 5.56v after it stabilized at 4w

Final collector voltage with RF probe
0v RX
45.240v in TX when it is at 2w (initial key up) and 19.8v after it stabilized at 4W

Bias regulator (TR37) collector with RF probe
0v RX
172mV in TX when it is at 2w (initial key up) and .233v after it stabilized at 4w

Base voltage of bias regulator (TR37)
0v RX
.337v in TX when it is at 2w (initial key up) and .795v after it stabilized at 4w

I'm going to look at the mode switch next.........

 

[sp5it]

Please measure both sides C163 am tx with rf probe. Looks like so42 could be defective.
Mike

 

[Unit 75]

ok. I'm beginning to think that this radio is possessed..lol
So I go back into the shop to check out what you asked about measuring both sides of C163. I plug the radio in and turn it on. When I keyed it up, it dead keyed at 8w and was stable. I tried it both on ch1 and ch40 and 8w and stable. IC5 (the mixer) was working properly when I was troubleshooting the radio the other day but I thought I would check it out again. These are DMM voltage readings of IC5 as found a few minutes ago.
pin 2 .469v rx 7.93v tx
pin 3 ,469v rx 7.93v tx
pin 5 .469v rx 7.93v tx
pin 7 .469v rx 2.85v tx
pin 8 .469v rx 2.85v tx
pin 11 1.41v tx
pin 13 1.41v tx

ok..so then I measured both sides of C163 with the RF probe
closest side to IC5 read 2.270v tx
other side was 3.5v tx
at this moment in time it appears that this thing is working. I will try it again in the morning to see if it still works....it's been a long day. Thank you so much for your help in the matter,sp5it. I really appreciate it man. I will post an update in the morning time.
-75

 

[Unit 75]

Just to let you know it's doing it again. Back to finding this problem that now seems to be intermittent.

 

[Low_Boy]

Possible bad solder connection on something you probed?

 

[Unit 75]

Oh, yeah. Sure enough, Low_Boy.... cause it was actually working last night. lol... I spent today trying to re-trace what I was probing to see if I could find it. Intermittent problems seem to always be the toughest to locate

 

[Low_Boy]

I know you wish it would just stop working.

 

[357magnum]

My 2k has worked flawless ! But then again I have one of those " Garden Hose " radios that people on her don't like ! I have big shoulders let it fly !

 

[Unit 75]

My Cobra 2000 GTL I use on the base works great as well. It's these scrap radio's I get off eBay that have all the weird issues. I figure if I work on enough of them I will know them inside and out.
I just got in a Cobra 148F with the exact same problem as this 2000 I'm working on...Go figure. It's doing the exact same thing. Maybe I will get lucky and be able to fix both

Here is a pic of my 2000 you can click on.....