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

I came across an old radio today, (Cobra 138XLR), and I want to install talkback in it. I tried the usual 64ohm resistor across the PA/EX SPK and didn't get any results. Does anyone out there have any ideas?

I should have also included that I'm not interested in an on/off switch or variable volume in the talkback. Any help is greatly appreciated.

That radio has a relay, so you will need to place a cap in series with a resistor across 2 of the relay contacts. I am not at work so right now I am unable to tell you exactly which ones, I will look it up later for you.

I sure would appreciate that buddy. I must admit, I'm a novice at this kind of thing and even though I realize it's not a difficult thing to do, I may need some details on how and where to make my connections. Thanks

Well Timrim, I guess the talkback is the least of my problems at this point. I had the radio on my bench and I peaked it out only to have it completely stop transmitting. The relay is still engaging, but no forward power or modulation is coming from the radio at all. I guess I need to take this thing to someone who actually knows what he's doing and give this novice stuff up.

I should note that all I did to the radio was tune VR8 from a six watt dead key up to an eight watt dead key and then.........poof. I have peaked and tuned many Cobra radios, added talkback, echos and such, but I have never tried to adjust a radio like this, I mean an older model with a relay in it, so I'm really not sure of where I went wrong. Sorry for all of the post on the matter.

Try re adjusting the dead key again, perhaps there is a dead spot in the control. Also press around on the board with something plastic or rubber and look for bad soldering. Tap the relay and make sure its properly seated. If this don't help, I can tell you how to start troubleshooting it.

An 8 watt dead key is a little steep on a single final radio, perhaps the regulator simply died.

Had the same thing happen to me with a 139xlr and it was caused by a resistor near the final went bad and took out the final also.
Bob

Thanks for the info guys, I'll take a look at it again when I get back from a trip I'm taking. Again, thanks

they have a 2816 audio ic in those radios if you blow it there not readily available so id leave it if i was you, thanks hamguy2 nsw australia

Actually the audio IC is a UPC1156, and you are correct, they are no longer available. Junk radios are the only way to get a replacement.

your right im wrong i know a guy here that has a few he wants $30 us each for them but trhere brandknew thats for the 1156 i was think of a upd 2816 which is a phased locked loop normally used to replace the unmodifiable 2824 in ax144s etc the 2816 will go in 5khz steps in the pins etc where the 2824 is rom (read only memory),thx dave