a swing mod (more involved)

[Uncle Ronnie 336]

note the darlington needs to be heatsinked. the tip142 is to-247 style. heavy duty

 

[nomadradio]

Thought the TIP147 was bigger than that. If it's carrying the full current to power the final and driver both you'll need a heatsink.

73

 

[Cable Guy]

the only other solution to this problem would be to rewind T1 with 3 separate windings, one for the primary, one for the modulator, and one for the speaker, which is how it should have been done in the first place

I'd like to see a workup of that, I never cared for the low end response of that transformer. I also can't find much on the net about changing out that transformer either. I may re-wind one somehow as my next project, or start fresh with a beefier core, a little more iron, you know, for lower response and better saturation point. I guess I need to tear one apart, count the turns and go from there. What do you think?

 

[Uncle Ronnie 336]

Thought the TIP147 was bigger than that. If it's carrying the full current to power the final and driver both you'll need a heatsink.

73

142* but yea the original used a TIP120 which i don't really trust with a hot final

 

[711]

When I started installing Mauldulators, I kept burning up TIP120’s once the finals went above 2 watts. I researched and tried the MGH6284 and stopped losing TIP120’s , passed the info to the Mauldulator guy lol now it’s discontinued. I’ll try the TIP142 for Laz’s circuit.

 

[Uncle Ronnie 336]

yea, mosfets generally draw more, esp with a bias. don't give this info to anyone who's gonna dry the supply up this time lol. i have lots of old 23ch rigs i want to do this to so i can bask in fake wood grain and incandescent glow

 

[Uncle Ronnie 336]

ok i can confirm this works with mosfets. i have mine biasing on. and i had to mess with the resistor on the low end of the pot to get the deltatune pot to put out a carrier at all with it turned all the way down. ended up with 15k with a dk of 20ish milliwatts at the lowest setting of the "deltatune" pot. the center indent is 1w on the money, all the way up we're at about 5.5w and swinging up to 8RMS mfj watts all around. i do believe the biasing of the fet got me the linearity needed to key 20 milliwatts,swing 25peak (mfj watts) and have it look nice on the scope. will post pics of the 2nd specimen tomorrow

 

[Uncle Ronnie 336]

so how many percent modulation is it when the dead key is about 25 milliwatts and the p.e.p. is 25 watts?

thats how much modulation this radio is capable of cleanly delivering with the repurposed deltatunepot at the lowest setting

 

[nomadradio]

It's all about the waveform. Negative modulation percentage is not the same as positive modulation percentage. The lab-grade modulation monitor required for AM broadcast transmitters will have a switch to select one or the other, or two meters one for each.

Measuring modulation percentage beyond 100% can only be done for the positive (forward) half of the modulation envelope. By definition 100 percent negative modulation is always the same. Zero amplitude on the carrier. Not sure how you measure negative modulation beyond 100 percent. Only for positive peaks.

73

 

[Uncle Ronnie 336]

that math is above my pay grade lol

 

[Shadetree Mechanic]

If 100% negative modulation is a "pinched carrier" or zero volts, then if it doesn't pinch, it must be less than 100% negative? Can an AM receiver pick up a dual sideband signal? Or does it have to be able to pick up some carrier signal with it?

 

[711]

If 100% negative modulation is a "pinched carrier" or zero volts, then if it doesn't pinch, it must be less than 100% negative? Can an AM receiver pick up a dual sideband signal? Or does it have to be able to pick up some carrier signal with it?

Yes, most all AM recievers pick up the carrier and both sidebands. Remove the carrier and you still have the information thats contained in either sideband. When you get fancy receivers with built in Synchronus Detectectors, it acually fills in the missing "carrier" and helps to reduce the fading effect on weak signals. You can use the cheap SDR's to see this.

 

[Uncle Ronnie 336]

Cobra 29 delta-tune repurpose:

2x 10k resistors needed to mimic center slot on the DT pot.

R4 resistor to ground from pot 15k for something stupid-low like 25mW

the center slot indent on the DT pot is a hair below 1w

 

[Uncle Ronnie 336]

darlington upgrade is vindicated:

on the home stretch of a 25gtl buildout, (paint, LEDs, the whole 9) and the audio transformer gave up the ghost and was dumping power through it. darlington got wicked hot. if it was a TIP120 that wouldve cooked instantly. if it works when i partspig a new modj trans then i'm vindicated in upgrading that TIP142