Cobra 142 GTL, lots of issues

[mr_fx]

Well I just got a solid copy pegging the meter Independence, MO to Staten Island New York 27.7550MHZ

The other operator confirms that I am transmitting on 27.7550

however the radio appears to be jumping freq while transmitting, almost like it is transmitting on multiple frequ's

 

[ExitThirteen]

Ok, if the radio is showing 27.595 - 28.035, it has a different loop mixer crystal put in. The crystal that SHOULD be in there (from factory) is an 11.1125 mhz crystal. I bet the one that's in there is either an 11.325, or an 11.3258 crystal. This crystal WILL work, however, you need to isolate pin 10 of the PLL chip from the ground trace that it normally connects to. Isolating pin 10 should drop the radio down to 26.955 - 27.395, which is the correct band it should be in.

Also it shows 1 channel off on frequency, so it sounds like the clarifier is messed up also. Normal frequency for that 11.325(x) crystal should be 27.605 at channel 1, and 28.045 at channel 40 (With pin 10 isolated, it should be 26.965 - 27.405). So it sounds like the clarifier is not aligned correctly, and will need a re-alignment.

Hope this helps you get on the right path. First thing to do is isolate pin 10 of the PLL chip from the ground trace it normally connects to, and see if it "drops" bands down to the normal CB channels. Let us know how you get on.


~Cheers~

 

[mr_fx]

I got her talking... she is reading one channel high, so the unit displays channel 9 but is actually channel 8

so far I have talked way out...

also the click, pops, beeps up on 28MHz+ where actually morse code on SSB

 

[mr_fx]

Hope this helps you get on the right path. First thing to do is isolate pin 10 of the PLL chip from the ground trace it normally connects to, and see if it "drops" bands down to the normal CB channels. Let us know how you get on.


~Cheers~

good call

well it had a 11.325MHz crystal in it... not the 11.1125... so that is the issue,,, sorta

is there a quick and dirty way to shift the frequencies? right now the unit reads channel 9 when it is actually on channel 8... so is there a simple way to shift all the frequencies over that far?

or is a full realign needed? I do not have all the equipment needed for a full; realign (tone generator, osilliscope, super nice multimeter, I think that is about it)

 

[mr_fx]

I am not sure what happened I was moving the radio... and messed up... must have crossed something...

now channel 1 = 28.1400 MHz Channel 40 = 28.1180MHz

nothing seems to be able to make it go back down... things that SHOULD work dont (like pin 16 for +/- 10k)

did I blow up the PLL? I mean I can hear people just fine... but mostly I hear morse code... how the hell did I do this?

how to you know for sure that the PLL is functioning?

the only thing I did was messed with the trace cut on pin 10, tired to clean it up a bit...

used to be if I re-grounded pin 10 I went up around mid 27-28ish... now NOTHING

 

[ExitThirteen]

Sounds like you have another trace or something grounded... there is a possibility that the PLL is bad, it's hard to say what the problem is without it being on my bench. The reason it was 1 channel off originally was because the clarifier isn't aligned properly to match the new 11.325 loop crystal. The radio could be re-aligned to match that crystal with little trouble, but now it sounds like there's something else going on.

The VCO could possibly be out of alignment as well. L13 is what you might have to play with to line it up back in the bands. However that will probably need some good test gear to get the VCO lined back up to where it should be.

Re-ground pin 10 of the PLL, and see if it goes back to 27.595-28.035, if it goes back to those frequencies, the PLL should be ok, if not, the PLL may be bad as badly as it seems it's been hacked into.



~Cheers~

 

[mr_fx]

.

Re-ground pin 10 of the PLL, and see if it goes back to 27.595-28.035, if it goes back to those frequencies, the PLL should be ok, if not, the PLL may be bad as badly as it seems it's been hacked into.



~Cheers~

re grounding pin 10 does nothing...

 

[ExitThirteen]

Have you tried adjusting L13 at all? If doing some light adjusting of L13 doesn't do anything, most likely the PLL is bad.


~Cheers~

 

[mr_fx]

more on what happened... I had a wire soldered to PLL pin 7 I think it may have touched something when I was moving the radio...

this is what I found...

weather pin 10 is grounded or not does not matter... pin 16 does not respond to + or - or the absence of either

HOWEVER crossing pins 9-8 (I think) gave me somewhere in the 24 mhz range and crossing pins 9-7 gave me 27mhz just above US CB band...

however I also noticed this:

1.) If I turn the Clarifier CCW it raises the frequency... and it lowers the frequency going CW... also the range is wide than normal... channel 1 =28.1433 - 28.1256

here is the channel list...

1 = 28.1405
2 = 28.1419
3 = 28.1412
4 = 28.1408
5 = 28.1406
6 = 28.1403
7 = 28.1401
8 = 28.1409
9 = 28.1397
10 = 28.1397
11 = 28.1393
12 = 28.1388
13 = 28.1386
14 = 28.1386
15 = 28.1376
16 = 28.1329
17 = 28.1214
18 = 28.1215
19-28 = 28.1216
29-40 = 28.1215

 

[ExitThirteen]

I would double check for any joined solder joints, but judging by your last post, it looks at though the PLL chip is bad. Try to clean up any solder joints you can on all 18 pins of that PLL, and let us know how you get on.


~Cheers~

 

[mr_fx]

do these work?

CB Radio PLL Chip, MB8719 Replacement | New York | eBay Classifieds (Kijiji) | 4670259

 

[mr_fx]

I would double check for any joined solder joints, but judging by your last post, it looks at though the PLL chip is bad. Try to clean up any solder joints you can on all 18 pins of that PLL, and let us know how you get on.


~Cheers~

yeah I just got careless I guess... I have been kinda out of it since last night... the doc started me on some sleeping pills... well live and learn I guess

 

[ExitThirteen]

Those will work.... however! You can't modify them, if you try to modify them for extra channels, they will fail. What you need to find is either a genuine MB8719, or the newer RCI8719's that are used in the Galaxy 9x9 series of radios (919, 929, 939, 949, 959, 979). That's your best bet.


~Cheers~

 

[mr_fx]

well the new PLL is installed... nothing new has happened...

same as it was with the old PLL... does this mean I pulled (and destroyed) the old PLL for nothing?

 

[mr_fx]

new news, if I adjust the VCO coil the freq changes, both send and receive...