Cobra 138xlr issue

[Holydvr]

Hello everyone,

I figured I would post this issue, as I am sure there are a few of us with this model radio. When I acquired the radio, it barely put out any power. It turned out to be a bad final. The radio works well overall now, aside from the receive having an issue. There is a station around 14 miles away, and if he is on channel 20 and I am on 19, I get bleedover. This is not like what most of us get as as far as splatter etc. I can almost hear him crystal clear as if he is on the same channel, but obviously quieter than he would be if I click over to ch 20. But, as stated he is not on the same channel with me. It is almost as if the radio is receiving multiple channels at once in a sense. I hook up another 138xlr that I have and I do not have the issue. Just figured I would ask if anyone had a similar experience.

Tony

 

[Dmans]

Tony,
I have a couple of these chassis radios and have not noticed that problem.

See the website linked below. Unit399 has some great tips on improving the reception and adjacent channel rejection on the 858 chassis. I have applied his modifications to 2 of mine with great results.

I can't answer as to why one of yours has better rejection than the other but to suggest an alignment problem.

HOME | 858ssb

73
David

 

[Holydvr]

I was wondering about the alignment also. I am unsure, but I do not think it had this problem before it was aligned by a local tech. I may have to have a good look at it after reading through the website you linked.

Thank you!

Tony

 

[Handy Andy]

Another thing to think about.

Not everyone runs a "clean output" station.

Nor...

Does everyone runs a PLL - based CB radio - I'm referring to the Crystal synthesis that older models use.

So I didn't see anywhere in the posts if the guy is using an RCI, Galaxy, Midland, Cobra or (ulp) UNIDEN - and your "in between" channels issue may be intentional. It would help to know what is being used but honesty is not always the result, nor would it be worth it to even ask, you may be handed a story, buy it or leave it - scenario. It may simply be from that guy running their radio using a 10kHz switch and trying to beat the band on both 19 and 20 - using 19A as his "starting point" in a dirty boot, and you are 15~20 miles away - Sorry to see this also happen to you - you are not alone in this.

To stay on topic...

Crystal synthesis - you're at the mercy of the mixer results.

On a PLL based radio you may be in for a ride of your life trying to find a clean channel to get away from the guy. I've had various fixer-uppers that came across the bench with lots of mods done not to make it perform better - but to piss people off. So you may have an operator (for kind sake) that is angry about something and wants others to know exactly how they feel.

 

[Big Kahuna]

It's not you...it's me. Seriously though, more likely other guys is spraying his signal everywhere than an issue with your radio. I've had DX come in clear as a bell on channel 7 when they were on channel 6, some of those guys on 6....their signal looks crazy when you see it visually.

 

[Handy Andy]

Hmm, I do know that the 138's "IF Filter" - is only one filter. The 399 website has more on this.

But to help your situation your radio can be tuned to receive images both ABOVE and BELOW. and it's a little too easy to do...so you may be onto something - but a tech would have seen the changes in the AGC action and know they hit it (tuned to) the wrong side.

Here's the example I'm seeing in this...

The above is the 138's Front End - minus the PIN attenuation section...
​

This may make the meter look good, or the radios receive noise floor look like it's working, but it's only passing what it gets shifted to, not as the fault of IF filter itself , but as the Incoming signal your Mixer's IF generated 16MHz down mix from the 1st stage - gives you your 10.7MHz, but to claim more signal - he may have worked the wrong image offset to obtain that receiver's figure of merit.

The 138's Filtering AFTER the IF xtal lattice, is based on broad banded discrete Band Pass filtering that was supplanted later using Ceramic and Xtal filtering with tweaks to the IF and used the PLL's own 10.240 standard to make a separate image to 455kHz (hence the AM only 455kHz shift of their 2nd conversion based chassis) But, this circuit used something unique that was ahead of it's time - a FEEDFORWARD circuit used to help REMOVE noisy signals and spikes present in the received captured IF signal - the issue is using the RF side, not the IF side - they tried to Mix it out at the 1st IF amp - hence that 3-input 3SK45 mess up there...

The bane of this type of radio using only one true IF Xtal filter - makes the tuning of the images from your expected and then tuned-to intended - picking the right side of their intended image, gives you a quieter receive and AGC action that works - tune to the wrong image side - the results are a nosier receive full of other peoples stuff.

 

[Holydvr]

The more I think about it, I do believe this started after the alignment. Sad part is that since it had a blown final, I did not spend much time with it. The person on the other end is running a Galaxy 949 direct feed, studio mic, mixer etc. This is the only radio I own that has this symptom in this way. If he has power on with any other radio I own receiving, I may get a twitch of the needle here and there on his peaks if I am on the next channel, but nothing more. I have the alignment info, so I may very well have to go through it myself. We will see what happens I suppose. Worst case for now I do have a few more to run if need be that work well.

 

[unit_399]

Make sure that L6 and FT2 are installed in your unit. In some sets (TRC449, Cobra 138/139XLR) they were omitted and replaced by a jumper (JP-37).
These parts are absolutely necessary for best adjacent channel
rejection. Unfortunately, they are no longer available from Uniden America or Dynascan.
Just as an additional note to persons looking for these parts. Coil L2 on the Cobra 142gtl, 148gtl and 2000gtl models is the same LA-179 coil used as L6 in the 858ssb sets. FT2 on these same models is the same FL-046 as on the 858. So maybe they can be scrounged from an old junker you have laying around.
- 399

 

[Handy Andy]

@unit_399 - there your are!

Thanks for stopping by!

 

[Big Kahuna]

Don't ever assume someone is putting out a clean signal just because they have lots of audio equipment He could easily be overdriving the 949 (as the big audio sounds good in the headphones) and then feeding it all into a dirty box.

It's interesting you only have the problem with that radio not others, that makes you think channel rejection. If he's the only one causing that problem with the radio though that also is interesting - something about his signal and that radio.

I guess the last question is - is it really a big deal? Is your favorite channel 7 and he's on 6? If it's not a problem 3-4 channels away then I wouldn't worry too much about it. I used to know a local with a dirty radio that would bleed a crazy amount of channels away and I was 10 miles away from him. It was bad news.

 

[Holydvr]

Thank you for the input, Unit 399. I am out of state at work at the moment. I will be sure to check into your recommendations when I return home. I do believe that I may very well have the parts, if they are needed.
I believe that I have visited your website in the past, though it has been some time now. I will be sure to visit again, as I have a handful of 858 rigs that I have been slowly getting road ready.

Many thanks!

Tony

 

[Dmans]

I have done the mods recommended above (and more) by 399 to 2 of my 858 radios. One of which I used 2-FT2 crystal filters in "piggybacked" together. The results are unbelievable in that radio. It is my "go to" 11 meter rig now.

73
David

 

[Holydvr]

I pulled the parts from a junk 148 I have here. (They were not installed in my 138XLR) The problem is now corrected. I thank you all very much for your help!

Tony