Copper Talk » Modifications » Radios » Upd858 Cobra 21XLR (AM Only) need mod. « Previous Next »

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738
Posted on Wednesday, July 09, 2003 - 5:43 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

I came across this great condition old Cobra 21XLR AM radio. I would like to ad some low freq.s to it to make it usefull. All the sites I have found on the net only have reference for the SSB boards. Anybody have a link or info. on this one?
Thanks,
738
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Kc0gxz
Posted on Wednesday, July 09, 2003 - 1:42 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

738

See Tuesday, July 08, 2003 - 01:29 pm posting in this section on the Cobra 89XLR.

The main reason why you rarely see any mods on the 858 chip in AM radios is because it operates in a completely different manner in AM radios than it does in SSB radios. VERY limited in frequencies.

Jeff, kc0gxz.
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Kc0gxz
Posted on Wednesday, July 09, 2003 - 4:19 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

738

Ok, I found what I was looking for but I'm not posative it will work on the Cobra 21. I had to do a little digging for this one.

Here is the frequency modification you asked for.

1- Find pin 19 on the PLL chip on the foil side of the circuit board. It will be a grounded pin in the near center of the row of pins.

2- Isolate this pin by making a cut in the foil so it is no longer grounded and install a small 1/4 watt 4.7K ohm mini resistor across this cut.

3- Mount a SPDT (center off) mini toggle switch in a convenient location on the radio.

4- Solder a wire from the center pole of the switch to pin 19.

5- Next, solder a wire from the top pole of the switch to board ground.

6- Last, solder a wire from the bottom pole of the switch to pin 21 which is a constant 5 volt source. Now you're done.
--------------------------------------------------

When the switch is in the center position, you will have the normal channels 1-40.

In the UP position, you will have some frequencies above channel 40.

In the DOWN position, it will go 9 channels under channel one.

You're going to need a frequency counter to figure out where you are on the radio. Or, I guess I could make you up a frequency chart if need be.

This modification is far easier to do than I described it. If any problems come up, you may Email me at .

73s

Jeff, kc0gxz.
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738
Posted on Wednesday, July 09, 2003 - 9:52 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Thanks for the reply, I have not messed with 11 meters and mod.s since about 1989 or so. I will dig up the parts and give it a shot, at least I have a freq. counter now. A few down would be great should not be any AM on the high side anyway, so as long as it will go down a little that would be great. The last time I was working on radios it was all MB8719's 142, 148's, and 2000's wish I still had them. Planning to come back to 11 meters this old 21XLR got me interested again. Tried the ham thing and like 6M SSB, but everybody is so unfriendly and uptight...
73
W7XLR
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Kc0gxz
Posted on Thursday, July 10, 2003 - 5:24 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

738

Yes. I think that's because most of them are more interested in filling in the little squares on their maps than actually getting into a real conversation with anyone.

Jeff, kc0gxz.
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Kc0gxz
Posted on Thursday, July 10, 2003 - 6:35 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

738
I posted this elsewhere for a Cobra 89. It may work for your too.

I came across another way to do this PLL mod. If the first one doesn't work, then try this one.

This second one is almost identical to the first one I gave you but a different switch is used and TWO cuts are made on "PIN 20" instead of pin 19.

An x-acto knife works best for making foil cuts. After the cuts are made, do a continuity check from pin 20 to ground and make sure there is no connection to ground.
-------------------------------------------

1- Find pin 20 on the PLL chip on the foil side of the circuit board. It will be a grounded pin in the near center of the row of pins.

2- Isolate this pin by making a small cut on BOTH SIDES OF PIN 20 in the foil so it is no longer grounded.

3- Next, solder a small 1/4 watt 4.7K ohm mini resistor from pin 20 back to ground.

3- Mount a SPST mini toggle switch in a convenient location on the front of the radio.

4- Solder a wire from the center pole of the switch to pin 20.

5- Next, solder a wire from the top pole of the switch to board ground.

6- Last, solder a wire from the bottom pole of the switch to pin 21 which is a constant 5 volt source.
------------------------------------------------

Hopefully this is what you will end up with.

27= 26.875
28= 26.885
29= 26.895
30= 26.905
31= 26.915
32= 26.925
33= 26.935
34= 26.945
35= 26.955
I didn't list the few frequencies that you will get above channel 40 because that's all sideband anyways.

Evidently there are two versions of this mod. Obviously it depends on what radio you are playing with.

Jeff, kc0gxz.
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738
Posted on Thursday, July 10, 2003 - 10:18 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Thanks again will try them this weekend! Just found and ordered a Turner +3B for $25.00!!! This should be a fine little AM radio looks like it has a nice audio section with xfmer. Yes you are correct about the grid collecting, thats my complaint noboby wants to have a normal conversation and they are to cheap to QSL, and many times if they have your grid they just move on and don't want to talk to you,how rude! Although I have had a few very nice contacts on 6M. I may sell this 6M and get back on 11M SSB everybody is much more friendly. Got another post in the tech section just got to update myself on current radios with the best RX.
73
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Kc0gxz
Posted on Friday, July 11, 2003 - 3:37 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

738

What kind of radio are you looking for? Perhaps I can help you.

And by the way, I'm looking for another 6 meter radio. The one I have now is a RCI 5054. It's not very impressive for being a 6 meter radio.

What do you have for sale?

Email me at and let me know.

73s

Jeff, kc0gxz.
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2600
Posted on Friday, July 11, 2003 - 11:31 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Uhh, Jeff your e-mail address does not appear on your profile page. Come to think of it, I think mine does, but maybe I should check?

73
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Kc0gxz
Posted on Saturday, July 19, 2003 - 1:53 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

2600

Thanks Chris. I'll be a fixin that.

Jeff

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