The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

Ok guys and gals... I am a little new to this troubleshooting side of the radio biz!! I have a little basic knowlege of how circuits work and how to peak and tune and all that and I follow directions well!!

So here is what I have that I need help with...

Recently purchased a 139XLR... the good: it seems to do really well on TX... AM, USB, and LSB all sound really good on another radio and shows good modulation on a mod meter!

the bad: It is pretty week on the RX side of things! I have another older comparitive base here..an 86xlr and there is a 3 to 5 db difference in the 'ear' on these radios! The 86 is stock so I don't think there should be that kind of difference with the same antenna, coax, meter, etc.!!

I have already gone through the schematics I found on another site and turned the AM RX gain variable resistor up... it didn't help much at all!!

Could someone please help me diagnose and doctor this nice little unit up??

And if I can get the RX side fixed I would be interested in knowing how to install variable power in this radio as well!

Thanks in advance to all for your help!!

Check that the RF gain is working?
If the RF gain is not working 100% then check C211, D13, D14, TR11, D1 and TR4.
I have seen TR4 with open base emitter junction this can cause low RX signal.
Also check that VR2 and VR3 are correctly setup.

Thanks a bunch Rick!!! I've checked RF gain and it is working 100%, checked the adjustments on VR2 and VR3 they are both fine! I checked D1, D13, and D14.... all are good!

Please excuse my ignorance here... but how do I test C211, TR11 and TR4?

VR5 from what I see on the schematics is the AM RX Gain variable.... at the point the radio has the most RX it is all the way counter clockwise and I get a reading of 167 ohms..... when I turn the variable clockwise and increase that number I lose RX gain as I should! So in theory if I pulled VR5 and replaced it with a resistor that yields a smaller value say a 50-100 ohm would this give me more RX gain??

How do you know that the RF gain is working correctly?

The value of VR5 is 100K.
If the RX strip is not working correctly then VR5 will have a very limited range, do not change the value of VR5 this will not help. As you need to fix the fault first.

As for testing, the transistors do a Google and look for something like (testing transistor with a DVM). As for the Cap just replace it and nope for the best, make sure you put the new cap in the right way round.

ok I looked up TR4 and TR11 and found them to be 2sc945 NPN type transistors.... so I tested them with the diode setting of my DVM and found the following:

TR4 shows
E-C doesn't register anything with + on E and - on C
E-C doesn't register anything with - on E and + on C
E-B doesn't register anything with + on E and - on B
E-B registers 702 with - on E and + on B
C-B doesn't register anything with + on C and - on B
C-B registers 694 with - on C and + on B

TR11 shows
E-C doesn't register anything with leads either way
E-B doesn't register anything with + on E and - on B
E-B registers 690 with - on E and + on B
C-B doesn't register anything with + on C and - on B
C-B registers 679 with - on C and + on B

I don't know if that helps or not? But those numbers are with the part pulled from the radio!

As for testing the RF Gain... I'm unsure how to test it other than turning it open and closed and seeing and hearing the difference! Is there another way I can check it with the DVM or something?

I haven't replaced the cap yet... didn't have one handy! If the fault isn't in one or both of the transistors I will try the cap next!!

Thanks again Rick!! From what I've seen reading through the forums you are the man when it comes to helping and being correct! I sure wish you lived close and were for hire on these old radios!!! I could sure bring you some business!!!

The transistor test looks OK.

It is maybe just a case of have an alignment job done.
This is where you need the right test gear and know how to use it. If one or more of the coils is out of tune it will cause your fault.

I think you will need to find some one with knowledge and the test gear to do a re-alignment job

Thanks a bunch Rick! Unfortunately there aren't any good techs in my area! All the good ones within a couple of hours of home have all passed on or retired!

I'm putting up a Shooting Star beam tomorrow.... if I don't get the RX gain from it that I'm hoping for then I guess I'll have to run a pre-amp until I locate a good Tech!!

I owe a big thanks to the man with the plan....

Thanks Rick!

I followed your instructions to the T and I have the 'Ole 139XLR recieve up to par!

It in combination with the Shooting Star I put up is really something to enjoy!

Now if you have an idea on how to install variable power in this radio I would be once again indebted to you for your vast knowlege and ever thankful for your assistance!

I also have a post in the amp section if you could take a look at it as well!!

Thanks again Rick

The TR5 is a high failure part in this radio.9 of 10 Cobra 138XLR and the Cobra 139XLR with a bad reception or no RX the problem is the TR5.The Dynascan Cobra company has a Service bulletin for the NO RX problem in this type of radios.I fixed one Cobra 139XLR today with the same problem.In this case the radio came at my shop with a very low RX.


Psssss;There are over 10 Tech service bulletins for this type of radio in the Dynascan homepage.This type of radio came from the factory with many problems.And this covers the Cobra 138XLR,Cobra 139XLR and in the other brand of radios with the same type of board.

Unit 67, do you have the URL handy for the Dynascan homepage? I have been looking for a source of TSB's on Cobra radios.

There is on link for Dynascan Tech, but you can find all the service docs are on CBTricks.
www.cbtricks.com/radios/cobra/139xlr/index.htm
Hope this helps.

Yeap!!!!!!

You can find out the same info in the CBtricks.com link too.