OVER 130% MODULATION!!!

[loosecannon]

i have this mod in my madison now, using a .22uF cap (ceramic).
also using 1.2K for R228.

getting great reports so far. this one seems like a real winner!

nice job exit13.
very cool.
LC

 

[ExitThirteen]

LC,

What are you seeing for positive peaks with the .22uf capacitor? I'm curious.



~Cheers~

 

[loosecannon]

sad to say, im still considering which piece of radio gear to sell in order to finance a new scope, as the one i bought a year or so ago never did work right.
My radio budget after doing all the AMTEL upgrades to my madison is all but depleted, and summer here in Vegas carries with it some of the highest electric bills in the US.

So, no scope readings yet, but you can bet this will be the first thing i measure when i get one this fall.
LC

 

[Robb]

The Rigol digital storage 100mhz oscilloscope would be something to consider if you can find one used on Craigslist, LC. Even purchased new, they are a pretty solid scope for the price point; so says E.E./Dave Jones @eevblog. Other than that, there might be a used analog scope in LV somewhere for cheap; happy hunting. Like to get a DSO too; but funds are not there for it yet either. Buying the Rubidium 10mhz clock took away play money for another couple of months before I could even could consider getting a DSO.

See where you are going with this, Xit. But you have already tested a number of caps already and must have a pretty fair idea how cap values are affecting % of modulation. What does it look to you at this point - albeit premature? You are using mylars IIRC. How much has the dissipation factor worked into the type of cap chosen for this mod?

 

[Tallman]

Dagnabit, I avoided all of this electronic crap shoot just by buying my Magnum s-45-hp with the "Top Gun Modulator" circuits. I feel dirty but still some what satisfied.

 

[ExitThirteen]

Robb,

I'm not seeing any noticeable difference between the different types of caps. I have used non-polarized electrolytics, metal film, mylar, polyester film, ceramic disc and mica so far, and at least on the scope, it's not showing any real difference. It does make me wonder if there may be differences in the actual audio. Things that make you go "hmmm...".

The only important thing is that it has to be non-polarized, and must not exceed 1uf. The reason you don't want to exceed 1uf is because the final and AM pass regulator transistor just won't have enough headroom to support the extra current draw from the higher positive peaks. I tried a 2.2uf capacitor and it knocked out the stock AM pass regulator transistor pretty quickly, it overheated (2SC1419). I currently run 2SC1061 for a pass regulator, and that's a bit beefier than the stock 2SC1419 and even an NTE152, even though all 3 of those cross reference to each other. If you use a 1uf capacitor, I'd highly recommend installing a 2SC1061 or an NTE152 in place of the original 2SC1419.

I have since modded my Madison with an MRF477 final. This allows me more headroom to open up my AMC a little more to get more on the positive peaks before I start clipping on the negative peaks. However, I set it back to where it was when I had the C1969 final in it and left it alone.


~Cheers~

 

[David Waits]

Guys, how do I do this to my 142GTL? I need cap-size, diode numbers and any transisters. Somebody already clipped the AM limiter out so I have to replace it first.

 

[ExitThirteen]

For the Uniden Washington/Cobra 142GTL:

Remove R104 (AMC limiter resistor), and replace with a 20k ohm trimmer pot, you'll have to solder this on the solder side of the PCB.
Remove R204 (560 ohm), and replace with a 1.5k ohm resistor.
Install a 1uf, non-polarized capacitor between pin 9 of IC4 (TA7222AP audio IC) and the junction of R204/D62/R121. The capacitor can be electrolytic, mylar, metal film, polyester, mica dipped or ceramic.
Remove the 2SC1419 at TR34, and replace with a 2SC1061, or NTE-152.

Set your deadkey to 2W at VR6.

Using a 1khz tone, and a scope, set your modulation on AM with the 20k trimmer pot you soldered in at R104. Adjust this trimmer until your NEGATIVE peaks are approximately 90%.

At this point you should have 180~200% positive peaks with no flat topping.

Wattage should be 2W deadkey, with PEP output at about 13-14W.

Good luck!

~Cheers~

 

[David Waits]

Thank You!

 

[Tallman]

I have a Magnum S45HP that has the top gun modulator built in to it.
It is using a process called "Asymmetrical Modulation." I have had nothing but good reports and many offers to buy it.
Hold back on the negative wave form and boost the positive along with the power setting and you have modulation that will punch through.

It is running a RFX-75 on the back. Setting the dead key at 2 watts and with the front panel selector in the proper position it will swing to 70 or 75 watts.
I know Any modulation over 100% is impossible. But if you set your modulation meter with 2 watts and shoot a signal of 70 watts is misleading.

 

[ExitThirteen]

Here are pics from my Uniden Washington that I recently modded. Below is a pic of a 2W carrier on my scope.

The 2nd pic is the carrier modulated. Positive peaks are about 180-190% and negative peaks are 80-85%.

Enjoy.

~Cheers~

 

[Tallman]

My Magnum S45HP will do that very well. But it is adjusting the rf output and is not just modulation.

 

[Stellasstillarat]

[QUE="GnG8d, post: 457807, member: 13838"]130-150% modulation can sound great actually.[/QUOTE]
My sonar fs2340 with the audio limiter board bypass can hit peaks of almost 200% modulation and stays loud clean and clear. the only problem is it has to be exstramly (backround) quiet. As the d104 picks up the old lady snoring and she 1/4 mile away. usually run it peaking about 150-%. At least ten people have made me crazy offers for it but it's been with me since 82 and it's not going anywhere.

 

[w9cll]

[QUOTE="Stellasstillarat"As the d104 picks up the old lady snoring and she 1/4 mile away. usually run it peaking about 150-%. At least ten people have made me crazy offers for it but it's been with me since 82 and it's not going anywhere.[/QUOTE]
Your running the mic gain to high. Turn it down and talk closer to the mic.

 

[AudioShockwav]

""I know Any modulation over 100% is impossible.""

Have you been reading this thread?
The pictures on the scope clearly show positive peaks over 100% with no flat topping, if that is impossible you better let the AM broadcast guys know because they have been doing it for years.....
Xit is showing a very nice waveform on the scope.


73
Jeff