NPC test results....

[ExitThirteen]

Came across a thread a short time back about how to compress the negative peaks while increasing the positive peaks. There was a link to another site that shows how to do it on the Cobra 148/2000 chassis without the use of using a diode.

Here's some pics of the results.

I did two things. I took R196 out of circuit to allow me to turn my deadkey down to approx. 2W. The other mod involves adding a 10uf 35V electrolytic capacitor between pin 9 of the audio IC and the solder trace junction of D63/R194/R228.

This test was done on a President Madison, with the MB8719. It's almost identical to the 148/2000 chassis, just different part locations.

First pic below is of the radio with just a carrier, set at 2W and calibrated on my Tektronix 465 scope.

Second pic shows modulated output before the modification. It shows the radio at approximately 90% modulation.

The third pic shows it after the mod was done. I did not remove ANY modulation limiters for this test, nor did I change the AMC adjustment any.

Major improvement! This pic clearly shows positive peaks at about 175% while negative peaks showed very little change. I was quite impressed on how it looked on the scope, no real flat-topping that's noticable, and the negative peaks look good still. Initial radio reports back tell me, "Nice loud clear audio" "Doesn't sound overdriven" "Great sounding radio". Just for kicks I flipped on my X-Force amp, and the reports stayed the same, I was told my audio stayed the same, just got a little louder.

Kinda neat, I thought.

Anyhow, thought that people might want to see some actual results of this mod posted. Enjoy.

Side note, SSB output did not change, nor did did SSB clarity get changed in any way.


~Cheers~

 

[sonoma]

I have the npc mod done to my grant LT and cobra 148 and both have good output and they sound good on the air. listened to them through a backup radio system I have set up in the garage. they both have been done this way for over 10 years now.

 

[ExitThirteen]

Was any of the modulation limiting circuitry modified in any way on your radios? Just curious.


~Cheers~

 

[Robb]

V.Cool!

Now try the audio mod with this mod if you want. I think you will be very pleased. I was. The combo of these two mods should work even better - as far as an overall improvement even using a stock mic.

Try the 1.5k ohm resistor on the AM Limiter collector/middle leg - as an experiment. My results come up with modulation that has to be adjusted to get the right spot. It is touchy/fussy to get the sweet spot. You get more gain; but at an unneeded cost IMO. But I think you showed enough improvement over the stock modulation/carrier that it isn't really necessary.

Did you need to reduce the carrier/dk?
If so; how much?

 

[sonoma]

the only thing I did was increase the voltage to the final as most of the npc mods show it done . I got the mod from the old uniden forums years ago, which shut down about 2004 if I remember right. their sight was like rogerbirds. they only had uniden XL mods on the sight.

 

[ExitThirteen]

Robb:

On this particular radio, I have a significant amount of adjustment to play with on the AMC, as it's an actual vertical mounted potentiometer on the main PCB next to TR26. That's why I didn't use the 1.5k resistor mod on TR26. Didn't see a need to change out anything when I could go from stock modulation to full blown nasty by adjusting the AMC...

Also wanted to mention that I use a D104 head for a mic, the head sits on a boom and is non-amplified.

I set the carrier at 2W deadkey and left it there for the entire test. No change to the deadkey was done, and it's still set at 2W right now. Keeps the deadkey wattage down on my X-Force amp, but shows nice forward drive. Even tho I'm using a Bird 43A wattmeter, I'm showing about 260-270W deadkey and close to 400W forward drive. That's with the amp running wide open. And the Bird has no PEP kit in it.

Sonoma:

I didn't want to increase the voltage to the final, as I didn't want to stress the power supply with the extra current draw. I did change out the AM pass regulator (2SC1419) to an NTE152, which is a higher amperage regulator. I do that on all my radios anyhow, which is why I didn't mention it.

Perhaps I should make mention of it, since I'm most likely hitting the regulator a little harder with the extra PEP output VS. stock output.

~Cheers~

 

[sonoma]

on SSB you already hit the 13 volts.I just changed AM so it gets the full 13 volts also. hook it up right at the input to the radio. does not seem to hurt it any. both of my base 142gtl's are done this way and the power supply runs the same.