Cobra 139XLR ANL- ANL/NB Switch Wiring Hook-Up Help Needed

[CollinsMan]

I finally got around to working on a Cobra 139XLR (base station) "EBay Special" that was supposed to "work fine," but didn't...

I've just about got everything straightened out (new channel LED, final and driver, LED meter bulbs, etc.) except one thing… Sometime in my radios distant past, someone did a mod on the 858. They wired this to the ANL/NB switch, and taped the wires from that switch together.

Well, actually, they taped the little red and white wires in the shielded cable together. They left the big red and brown wires disconnected.

I was wondering if any of y'all have one of these. If so, would you PLEASE pop the bottom cover and let me know how the switch is wired up? I would really appreciate it as I've put lots of hours into this radio and would like to get the old classic properly working again.. I promise to return the favor if I can ever help!

Thanks and 73!

 

[loosecannon]

sure thing, glad to help.

i have one of these sitting right here.
i will give you the wires to each switch terminal, from the perspective of having the radio upside down, with the back of the radio facing you.
this means that the switch assembly is in the upper right hand corner, and the dynamike control is at the far left.
the SO-239 (antenna connector) should be pointing right at you.

there are eight contacts on the switch.
four on the left, and four on the right.
there is a space between the top pair of contacts and the next pair down.
is this making sense?
good.

first thing to notice is there is a resistor connected from the top left terminal to the third terminal down on the left.
the value of the resistor is 330K ohms. (orange, orange, yellow)

next is the brown shielded cable.
it has a white wire and a red wire coming out of it.
connect the white wire to the top left terminal, and the red wire to the second terminal down on the left. (right below the white wire).
now solder the red wire to the second terminal down on the right, and the brown wire to the bottom terminal on the right.

that should do it!

best of luck,
loosecannon

 

[CollinsMan]

Thanks!!!

I really appreciate the help, Loosecannon!

I wired-up the ANL/NB switch a few minutes ago. Thanks to your precise instructions, I am now able to get rid of most of that $%%*@ AC line noise that ruins my reception while using this Cobra.

BTW, are you by any chance a school teacher? If not, you would be a good one!

Thanks again, and 73

Steve / Warlock / AC5CH

 

[loosecannon]

thanks!
no, im not a school teacher.

i have to write very technical emails for my job quite often, so i know how things that would be obvious in person, need a detailed explanation using this medium.

BTW, if you want to improve the ANL a bit, try replacing R43 with a 1meg ohm, up to a 1.5meg ohm, resistor.
(if the radio is open, component side up, front of radio facing you; R43 will be 1/2" in front of, and about 2" to the right of the relay. right behind VR5.)

it should have been a 1 meg from the factory, but most 139xlr's have a 680k in that spot.
must've been cheaper.LOL
anyway, i tried a 2meg, and it seemed to cut down on the receiver's sensitivity, so i put a 1meg in, and i like it.
your results may vary, so buy a couple different values and "speriment".

have fun,
loosecannon

 

[CollinsMan]

Thanks for the noise limiter resistor information! You are probably right, Cobra must have got a FANTASTIC deal on those resistors...

I'll definitely do some experimenting and fine tuning. I live in the middle of the city, and the interference is severe. So, anything I can do to improve noise rejection helps.

By the way, have you tried replacing the AM detector and noise blanker diodes with schottkys? I did so in my Grant LT, and it made a huge difference.

If you have any other ideas on how to improve the Cobra, please let me know. I like the old radio, and have made it a "test bed" of sorts...

Thanks again, and 73!

 

[loosecannon]

the shottky's are great in every radio ive tried them in.
i like 'em.
ive not tried them in the 139xlr, but i dont see why that chassis would be different than any other.

there are a few common problems with this chassis. (other than age!)
C179 is a little blue tantalum cap near the final and driver.
it tends to blow causing buzzing, and fuse blowing on TX.
replace it with a 2.2uF 25volt electrolytic capacitor.
(watch polarity)

ALL of the 2SC1419 transistors (TO-220 case) should be replaced with NTE152's, or 2N6487's.
the 1419 is only rated at 2 amps and is underrated for its application.
you will find one of these in the power supply, (BTW, replace the big electrolytic cap in the power supply with a 6800uF 50volt cap.) one on the left wall, one on the back wall (upside down), and one on the right side in the PLL section. (this one in particular causes big problems such as freq. drift because of inadequate heatsinking.
the solution is to move it right next to the one on the back wall.
you have to drill a hole, and run the wires, etc...)
see the resistor in the teflon right behind this transistor?
take the teflon off. is it black?
if its burnt or close to burnt, replace it with a 56ohm 3-5watt resistor. mount the new resistor on the bottom side of the PC board.
this resistor gets very warm and overheats the crystals right next to it. thats why you put the new one on the bottom.

want more modulation?
find D21&22. they are in the rear right corner.
unsolder and lift the banded ends out of the board.
now place a 4.7k ohm 1/4watt resistor in the now empty holes.

now just give the radio a good tune up, and you should be all set!
(be sure to tune the TX coils for max. while saying "ahhhh" into the mic)

have fun,
loosecannon

 

[CollinsMan]

Thanks Loosecannon!

I've got the upgrade parts ordered from Mouser, and I went ahead and ordered replacement electrolytics for the whole thing due to the age of it.

I re-crystaled and aligned it for 10 meters yesterday, and have made a few good contacts there. I talked to a guy on 29.050 Mc, and the told me it was the best AM signal he had heard in a while! Of course, the DX 1600 didn't hurt...

Thanks for all of the helpful information; I'll let you know how it turns out!

 

[loosecannon]

very cool!
re-capping an old radio like that is a great thing to do IMO.
suddenly, things that seemed to work fine before, now work great!

cool 10 meter mod, but where in the heck did you have the crystals made at?
i cant find anyone to make small amounts anymore.

the PLL in that radio has more range for extra channels than any other ive ever seen.
with the right switching arrangement, you can get 26.085 to 29.805!!!

of course, i doubt the bandpass of the radio could cover more than 2mhz anyway, but still cool.

good luck,
loosecannon

 

[pathfinder259]

hello gentlemen,,,how are you doing today..fine i hope...got a quick question????where do you guys buy your schottkys??and can you buy them as an individual,,or you have to have a company tax number to buy????

 

[CollinsMan]

Loosecannon,

I really enjoy the old Uniden chassis units, and don’t mind putting some work, time, and $ into them. They are definitely worth it in my opinion! I sure wish Uniden still made SSB units. However, since they don't, I try to save all that I can!

I got the crystals from Ken’s electronics. He has a TON of all sorts of crystals for $12.00 or so each. Unfortunately, for the 858 SSB chassis, you have to change out 3 crystals to get all modes where you want to go versus 1 for a MB8719 chassis

I’ve read that these can be broad-banded, but it involves some major surgery on a “UHC” chip. It isn’t for the faint of heart.

Thanks again for all the help!

 

[CollinsMan]

Hi Pathfinder 259,

You can get the schottkys from Mouser electronics without a tax number. They are about .18 each if you by 500 or more at a time; a little more if you buy less.

If you only want to do a couple of radios, you can get “receive kits” from various folks on Ebay. These come with 2SC2999 transistors, four schottkys and instructions. These “kits” are usually under $10.00 each including shipping.

BTW, watch out for DL and Highway on channel 20 if you improve your radio’s receive; those folks will blow your speaker out if you don't watch it!

 

[loosecannon]

since i only need them once in a while, i usually just buy a kit off of ebay.
i buy from "wingwirxmodulator", im pretty sure thats the correct sellers name.
he has the right stuff.

mouser is also a great source. no minimum order!!!

the diodes you want are the 1N6263.
also known across the net as NTE583.
either one will work. they are the same part.

later,
loosecannon