Negative Peak Compression boards for Cobra 29

[Shadetree Mechanic]

Ok, here is what I found. The new board will only do a maximum of 2 watts dead key, so all radios in this test were set to 2 watts. Maybe someone else will have higher power with an external control pot, I used the jumper. All radios also set to 125% modulation with a 1 khz tone.

The new:

 

[Shadetree Mechanic]

The Nomad, please understand that it is not meant to be an NPC. Just allows the radio to swing full power from low dead key. Just like the capacitor and resistor mod. Will look great at 100%.

 

[Shadetree Mechanic]

And finally a real NPC, RWOB Texas Ranger 936. Does not have a modulation transformer, so this mod will not work on a 29.

 

[Shadetree Mechanic]

So the new board does not pinch the carrier, although it looks flat bottomed. I wonder what this looks like on a spectrum analyzer?

 

[dave457]

All three look bad

 

[Low_Boy]

All three look bad

Agreed. Never saw any kind of swing mod that looked good.

 

[Eldorado828]

@Shadetree Mechanic
Thank you for taking the time to test and further giving us the evidence to see it for ourselves. It's very appreciated as not all of us have the proper equipment to do so.

Interesting stuff.

 

[Handy Andy]

While you have that thing on your bench, measure the DC component of the JP36.

Then SOLDER a cap across the JP36 where it goes into the Cobra, then do this again.

• That cap is the missing component of any Swing Mod using the Variable carrier.
• See post # 80 - (Oh man! this threads gotten long...)
• - remember the Diode may be a Zener - causing a condition of "quick and fast hold" clamp action - it would explain the ramp you have at the end of the signal. The effect is amplified because the AMC it's self has a Peak and Hold - so you see that trailing slope like those shown on the other shots from the YT bench vids.

 

[nomadradio]

Haven't had time to put one of these into a radio here yet. Customer traffic just won't let up. Pretty sure I'll want to trace out the circuit and draw up a schemo before I do. Just not enough round tuits.

73

 

[Shadetree Mechanic]

While you have that thing on your bench, measure the DC component of the JP36.

Then SOLDER a cap across the JP36 where it goes into the Cobra, then do this again.

• That cap is the missing component of any Swing Mod using the Variable carrier.
• See post # 80 - (Oh man! this threads gotten long...)
• - remember the Diode may be a Zener - causing a condition of "quick and fast hold" clamp action - it would explain the ramp you have at the end of the signal. The effect is amplified because the AMC it's self has a Peak and Hold - so you see that trailing slope like those shown on the other shots from the YT bench vids.

I just installed a 100 micro farad capacitor across the JP36 wires. It doesn't seem to have made any difference. It still has the funny flat bottom DC levels.

 

[AudioShockwav]

Thanks for taking the time to test and report back.

73
Jeff

 

[Shadetree Mechanic]

Forgot all about this diode mod until it popped up in another thread. Has anyone tried it?

 

[Low_Boy]

It actually only does about 20 watts. The limiter is working. I cant figure if it is a hack mod or not.

 

[Handy Andy]

Forgot all about this diode mod until it popped up in another thread. Has anyone tried it?

I've had fixed a lot of bad caps on radios that use this mod

I've had to take and fix regulator issues due to the problems it can cause.

The thing multiplies voltage - careful on this because people tend to "volt up" radios - which adds to this effect.

Can damage VCO/PLL due to spikes the thing can make.

This Diode mod Reminds me of the "fine print" in those "contracts" during a Job interview...

"This list is incomplete and will be supplemented as necessary"

​

 

[Low_Boy]

I sort of figured it was putting added stress on something.