SWR and LSB Freq. align on Cobra 135XLR!!!

[15minigrass]

OK this is 2 separate issues. 1st off, SWR. Im using the radios internal SWR meter. antenna coax straight to radio it has a reading of 1.4:1 on Chan 1 and a reading 1.6:1.

Now when a install a meter or an amp inline w/jumper SWR shoots up past 3.5:1. This is with the AMP inline. and 3:1 SWR with the meter. The SWR meter itself reads 2.5:1 thou.

Could it be the antennas or the radio itself. After talking locally with a friend for 4-6 mins. Than radio started to heat up while running the amp so I know there's alot of reflected watts coming back to the radio.

Im gonna try to play with the antenna a bit and see what happens.


2nd Prob. AM and USB are dead on Receiving freq. LSB is way off. with voicelock all the way to the right, everyone still sound like a DUCK but still legible. Just need to help on what pot or can I need to tune to re-align the RX on LSB.....



Mahalo, 1-5

 

[Robb]

Two things come to mind off the top of my head:

*The amp's input tuning/impedance is off.
*Try using a different length jumper between the amp and the radio - like longer by a few feet. Won't cure anything; or it might. But it will point out if the amp's input impedance is off and may need to put in the shop.

More comments to follow . . .

 

[Captain Kilowatt]

It does the same thing with just an external meter put in line? Sounds like you need to tune the antenna. Any time the antenna is not a perfect 50 ohms changing the length of coax cable (which you do by adding a meter or an amp or even just adding a length of cable) the SWR will change at the radio end of the cable. Ideally you should place an SWR meter right at the antenna when tuning it. To prove or disprove this simply add a few feet of cable between the antenna and the SWR meter and see if it changes the reading on the meter.

 

[15minigrass]

Thanx guys its night here, will have to do it tomorrow. It will come down. As of now I have a 107" aluminum rod in the driven element or main ant. Im gonna try to put the original element from the starduster and see if it changes anything. Too bad I dont have an MFJ here.

I will keep everyone posted. Any comments on the local of the alignment point for LSB RX?????

 

[joejoenikk]

Cobra 135xlr

Same board as a Browning or Tram, both so pricey when new that I'd do my best restore it to exact factory spec. There obviously wasn't a better SSB board avalable. I have the same problem you did, little to no factory information.


Frequency adjustment is kinda weird. The relationship between transmit cans, T8 and T9 is definatley plastic screwdriver turf with USB the right place to be.

If you take the bottom off you'll find the Bias pots for driver and final. It has a 1307, it costs more than a good radio nowadays, so don't hump it up too heavy.

There is no measuring the oscillator frequencys, factories at the time could afford the expensive amplifier for that and wrote their instructions accordingly. If it goes "out of lock" it's probably from "accidently" bumping t301-t103.

Actual frequency is what you asked for(Last year.) I assume you noticed that somone published a rather nice ident of the board, there is two of them available as free downloads actually, as usual one author just copied the other. C82 SHOULD BE what we would call frequency, it's located rather close to the transformer next to an equally tempting trimmer resistor R72 which controls part of SSB offset. That part of SSB offset we usually call "carrier balance", not quite true with this radio. What is true is that like any SSB rig you simply turn R72 to keep LSB and USB wattages the same.

The guys who wrote the avalible downloads both called T7 frequency and C56a modulator. I wouldn't call either anything but SSB slope but I've also seen on different radios "C56" being called a TVI filter. Of course, if you really want the thing to work right, you have to do better than the factory, and all of that confusion really doesnt matter as much as setting yourself up with a good radio to monitor it and way of playing a nice tone or some music into the mic. You'll never do better on SSB than your choice of monitor radio allows, so try and save a virgin rig that you like and a factory aligned for that job.

Party on.

 

[15minigrass]

So what your saying is 301 and 103 is what I should play with or is it T8 and T9. I cant adjust AM power so I just need to bring it in to freq or center it. I do have a rig here with a dead on center freq on both USB and LSB which is my trusty 146GTL. I might do it this coming weekend since its a long weekend. I been wanting to send it out but Im affraid that the shipping co. might bang it up some more during the trip and return trip. Right now its just sitting in the closet. Will be waiting for your response.

Mahalo, James.