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2995dx modulation on a scope

GnG8d

Well-Known Member
Jun 28, 2010
2,037
351
93
Ohio
Whats this look like to you? I don't open radios very often, but whats going on here? 30w peaking 150 into a dummy load.

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Your first picture looks like an AM carrier w/o modulation, and the second one looks like a modulated sine wave.






First picture is over-modulated AM signal.
Second is asymmetrical AM modulation
.
 

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You have a swing kit in that radio? Does your negative modulation percentage lower when you lower the dead key?
 
Nope, nothing clipped, nothing added. I can pinch the carrier off down at around 10 watts, but then you can't even keep the positive peaks on the screen.

I've had this scope for 6 months or more .......... I just knew I should have left it in the closet instead of hooking it up :blushing:
 
Nope, nothing clipped, nothing added. I can pinch the carrier off down at around 10 watts, but then you can't even keep the positive peaks on the screen.

I've had this scope for 6 months or more .......... I just knew I should have left it in the closet instead of hooking it up :blushing:

Are you sure nothing has been added? Thats the characteristics of a swing kit!

R292 is a 560 ohm resistor, you can change the value of that to control the negative modulation. You should probably try a 1K resistor.

I usually go to Radio Shack and get a 15 turn 5K trim pot and install in the place of R292. So if i change my dead key I can still adjust it without picking up the soldering iron.

Just change the resistor out its an easy fix!
 
Well, it wasn't supposed to :sad:

I just checked and my 2970 looks eerily similar on the scope.

Change that resistor out! Its in the back right corner of the radio if your looking from the top.
 
Change that resistor out! Its in the back right corner of the radio if your looking from the top.
Well, I just popped the 2970 open and found a diode across R292/C258 that I'm pretty sure isn't supposed to be there :eek:

I kept thinking I wasn't triggering my scope right, or it was junk. I triggered it from both the positive side, then the negative side. Then I went as far as feeding the raw signal into B, triggering from B, and looking at the modulation on A. At least now that I've wasted all day, I know how to use the scope and I know that my radios aren't what I was expecting.

JD, I'm not sure my eyes can handle this SMT bullshit. I need one of them lighted magnifying glasses :angry:
 
Well, I just popped the 2970 open and found a diode across R292/C258 that I'm pretty sure isn't supposed to be there :eek:

I kept thinking I wasn't triggering my scope right, or it was junk. I triggered it from both the positive side, then the negative side. Then I went as far as feeding the raw signal into B, triggering from B, and looking at the modulation on A. At least now that I've wasted all day, I know how to use the scope and I know that my radios aren't what I was expecting.

JD, I'm not sure my eyes can handle this SMT bullshit. I need one of them lighted magnifying glasses :angry:

Crap I forgot they were surface mount radios!

Thats why a scope is a handy tool to have, most radio techs dont put the radios they mod on one. On your 2970 the diode is pretty much bypassing the resistor and making your scope look the way it does. There suppose to be ran in series with the resistor, some people remove C258 and add a resistor diode combo but its the same mod.

R290, R292 or C258 On your 2995DX are the popular places people put diodes.
 
Opps I forgot to get to my point :tongue:

If that diode is bypassing that resistor, changing it is not going to have any affect on the radio.

I would try and remove the diode, or have someone do the mod the right way.
 
Here is the same radio, same scope, same 1kHz tone but with 1/2 the carrier. I suppose this is how it was meant to be ran, but it sure kills the average power.

1e69a2ad.jpg




I guess it's not in series, I took the cover off again.

bb0c785e.jpg
 

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