Modulation looks fine on dummy load, not the antenna.

[JoeDirt]

I shared this in another thread that you may have saw. My scope pick up is a bnc connector and piece of blue wire mounted in the final amplifier. Simple and works where more complicated pickups have given me issues.View attachment 19595

I use that exact method inside my watt meter. I can run 100mw to a few KW without issue. All %100 shielded is important.

 

[JoeDirt]

I could perform a transmitter alignment I don't have a signal generator for the receiver side but how do I go about adjusting transistor bias on a Cobra 29? lol I understand the process for the export radios far as I know there is no bias adjustment on the AM only Cobra radios.

JoeDirt, how would I go about checking to see if the radio is "oscillating" & I don't mean the 10.240 megahertz reference oscillator?

Learn something new every day I suppose!

Thank You JoeDirt
73

Sorry I forgot it was a 29, I doubt its a bias issue with that model unless it has been upgraded to a mosfet. I have has problems with ELK companion parts, learn how to bias without using the ELK scheme. Even Magnum radios with mosfets use ELK parts and that was one of their biggest downfalls.
Can you post pictures of what your seeing on the scope? Are you using a amp? You need a decent freq counter to see the 10.240 oscillator, that wont affect the scope anyways.....

 

[LeapFrog]

No worries JoeDirt, It happens. Two of my RCi rigs have mosfet conversions done, but I've not had this problem with both, only one of them, I understand the companion part can be flaky if you got a bad one.

I'm going to try Captain kilowatts suggestion maybe tonight but more likely early in the morning, if I don't have any luck I will post a photo of what it looks like on the scope, I'm leaning towards it being a an RFI / ground problem from reading what CK posted.

Thanks You Gentleman.
(Results will be posted early in the morning, I'm beat lol )

73
-Leap

 

[LeapFrog]

Update: Sorry guys I my 1sT SDR came in the post today I have been setting that up and testing all day.

I have yet to sort out the r.f. pickup, but I will get to it very soon.

 

[LeapFrog]

I followed what 543_Dallas suggested, I wrapped the wire around the center conductor, and left the enclosure as it was so I could still use BNC jumper cables for convenience.

Now the modulation looks great (appears the same) on the dummy load and the antenna for all these radios but one that has a real issue!
Thank You Gentlemen!
73

 

[Captain Kilowatt]

I followed what 543_Dallas suggested, I wrapped the wire around the center conductor, and left the enclosure as it was so I could still use BNC jumper cables for convenience.

Now the modulation looks great (appears the same) on the dummy load and the antenna for all these radios but one that has a real issue!
Thank You Gentlemen!
73

Sounds like a ground loop problem. My suggestion would have in all likelihood shown the same thing but at least you have a working sampler now.

 

[JoeDirt]

Grounding does weird things. I know on my setup everything is isolated from earth ground*. I ground the last piece of equipment either the watt meter or antenna switch to the ground rod (amp if its 3 prong 220v). My computer is also earth grounded but that's it. I had every imaginable ground issue on my base, computer monitors, metal shelves, antenna grounded on separate ground rod looping back through my garage service panel...... Everything plugs into the same DC power supply and same ac power strip (minus 220v stuff and or high amp 110v).

For example my computer I use for SDR and audio processing needs to be earth grounded (3rd prong) I had issues without it. Since the potential for a ground loop through the computer to radio via a USB audio interface is present I have found if run yout computer power cord through a longer cord it offers enough resistance to keep the ground loop to a minimum. If you had a true balanced audio setup ground loops should be less of an issue through the audio side but I aint got time for that.....lol

 

[Captain Kilowatt]

For example my computer I use for SDR and audio processing needs to be earth grounded (3rd prong) I had issues without it. Since the potential for a ground loop through the computer to radio via a USB audio interface is present I have found if run yout computer power cord through a longer cord it offers enough resistance to keep the ground loop to a minimum. If you had a true balanced audio setup ground loops should be less of an issue through the audio side but I aint got time for that.....lol

You are misinterpreting something. The resistance of even a looong power cord would be in the milliohm range ie. insignificant. A longer cord however will act as a better antenna to pick up RF however. I suspect that altering the length to a long cord caused an effect other than increased resistance. Five feet of standard 16 gauge wire is only 0.02 ohms. Going to a ten foot cord would only be 0.04 or 40 milliohms. Insignificant.