146 GTL PC-833AD, Local RX (1 mile or less) will peg the meter and audio is clear, however, there is no background noise hardly at all, no needle movement. Just a little static hiss....weak signal (in this case 2 miles away) was crackily, not consistant.
Checked alignment, TX was off 2 kc but RX ok
Squelch is ok, not stuck on or misadjusted.
RF Gain switch is ok, by using a cb on another outdoor antenna, it will adjust signal up and down.
TX is great, on freq and with good audio,
PA works
No mods other than pot adjustments,
I took a known good radio and hooked it to an ant switch. when good radio had 6~7 on the meter I would switch to the 146, and it had very little....when a strong signal does come in, it "pops" into audio, as though it just reached a threshold and turned kicked in...
Any help would be greatly appreciated!!!!
73's
Cobra 146 GTL with weak RX
Re: Cobra 146 GTL Low RX gain.
Re-check RX alignment coils L3, L4, L5, L6, L7, L8, L9, and L10 if one is miss tuned the RX strip will drop like a rock.
If RX alignment is ok then check D13 and D14 (MC301) D15 and D16 (1N4148) and maybe check for cold solder joints as there radio are getting very old. Let us know how you get on.
How is the RX like on SSB?
If RX alignment is ok then check D13 and D14 (MC301) D15 and D16 (1N4148) and maybe check for cold solder joints as there radio are getting very old. Let us know how you get on.
How is the RX like on SSB?
Rick.
SSB is the same,
D13/14 show.650
D15 .534
D16 .540
Just for the heck of it, fired it up with the 4 diodes removed, same as before....
I dont have any new diodes here but I do have a 2950 parts board here EPTZ 295913Z If you know which diodes I can pull I can try diffrent ones in the 146....
Thanks Rick!
D13/14 show.650
D15 .534
D16 .540
Just for the heck of it, fired it up with the 4 diodes removed, same as before....
I dont have any new diodes here but I do have a 2950 parts board here EPTZ 295913Z If you know which diodes I can pull I can try diffrent ones in the 146....
Thanks Rick!
Re: Cobra 146 GTL Low RX gain.
Ok sounds like the front end transistor as gone low gain, try replacing TR12 (2SC1674) and maybe TR13 (2SC1675). If you have a scope with a 10:1 probe you should able to see which stage as low gain, this can also be a cap which leads to this kind of fault.
Rick.
Re: Cobra 146 GTL Low RX gain.
The caps to check electrolytic and disc ceramic cap, it is unlikely you will find them short but you may find that electrolytic that is leaking and disc ceramic have change value.
Have you checked how much current the RX stage is using; this can be a good indication as to the state of electrolytic caps and leaking diodes.
Have you check the mixer signals? If the 1st or 2nd mixer is low, this can affect the overall gain of the RX strip. Do you have a bad filter?
Do you have a signal generator or a 455 KHz ref source? If so inject it in to the 455 KHz stage just before the 455 KHz filter and after the filter, this will let you know if the fault is before the filter or after.
With the right test gear it not always easy. You could use the swap out method; this just takes time and is not foolproof.
Have you checked how much current the RX stage is using; this can be a good indication as to the state of electrolytic caps and leaking diodes.
Have you check the mixer signals? If the 1st or 2nd mixer is low, this can affect the overall gain of the RX strip. Do you have a bad filter?
Do you have a signal generator or a 455 KHz ref source? If so inject it in to the 455 KHz stage just before the 455 KHz filter and after the filter, this will let you know if the fault is before the filter or after.
With the right test gear it not always easy. You could use the swap out method; this just takes time and is not foolproof.
Rick.