irf520 mosfet for cobra 29

[nomadradio]

The bipolar 2SC2078 or whatever they're using this week will usually stand up to SWR better than the IRF520, 13N10 or such switchmode FETs.

A high enough SWR will croak a bipolar transistor, but not as fast.

73

 

[CbRadiosPRONTO]

Mosfet Final Upgrade: Cobra 29



Parts Needed:

Quantity 1: Mosfet Final (IRF520, EKL2030, or similar)
Quantity 1: 470 ohm resistor (1/4 watt)
Quantity 1: 1n4148 diode
Quantity 1: 3.3k ohm resistor (1/4 watt)
Quantity: 47uf Electrolytic Cap

•Replace final w/ IRF-520 (or similar) MOSFET transistor (i.e. Ekl-2030 mosfet)
•Change R56 to: resistor~diode combination 470ohm~1n4148 w/ banded end towards front of R56 hole.
•Move C53 to C59
•Change C53 to 3.3k ohm resistor (1/4 watt)
•Move C61 to C62
•Remove C126
•Replace R55 w/ 22pf Cap from C126
•Change C71 to a 33uf—47uf Cap (16v—25v-ish)

Above made from this older description found on forum decades ago.
MOSFET Supertune Replace stock final with IRF520 MOSFET transistor (pin-for-pin match when installing). Remove R56 and in its place install a 470-ohm resistor in series with a common switching diode (1n914 or similar), with the banded end of the diode towards the front of the radio. Remove C53 and solder it across C59. In C53's place solder a 3.3k resistor. Remove C 61 and solder it across C62. Remove C126. Remove R55 amd solder a 22pf cap in its place. Remove the slug from L14. Replace C71 with a 33-47uf cap. Turn VR4 (AMC) counterclockwise fully. Gently spread L13 and L12 and tweak the spreads for best PEP when modulated. Tweak L20 and L17 for best PEP when modulated. After tuning is finished, set VR4 to half-way.

If gate voltage is low, ad 10k or less resistor between gate let you 8v source.

 

[Uncle Ronnie 336]

How about the cap and resistor mod? work with the mosfet?

the cleanest way i've found is the TIP120 type of rc mod, and run a 4.7k from +8v tx line to the gate to bias the final a lil

 

[Enterprise312ok]

Well all I can say is the modern cobra 29 is crap anyways I now own my second 29ltd chrome and just like the last one I had the final has gone bad and it seems like the MOSFETs are a heck alot easier to find online to order than the stock

I think the new cobra 29 is probably The best 40 channel radio available. I am a cb tech and work on radios every day and hands down the Cobra 29 is the most servicable radio on the market today. I like my old xlr gtl and side mic Ltd 29s, but the new 29s are not that far behind.

 

[Enterprise312ok]

The bipolar 2SC2078 or whatever they're using this week will usually stand up to SWR better than the IRF520, 13N10 or such switchmode FETs.

A high enough SWR will croak a bipolar transistor, but not as fast.

73

I was supprised by the hfe number on the 2078 I pulled out of the new am/fm 29 was very similar to the mitsubishi 1969 I was going to put in it so I just reinstaled the 2078. Aound 230 hfe

 

[OddBall1958]

this?

CB Tips and Tricks

MOSFET Supertune (29LTD classic and newer): Replace stock final with IRF520 MOSFET transistor (pin-for-pin match when installing). Remove R56 and in its place install a 470-ohm resistor in series with a common switching diode (1n914 or similar), with the banded end of the diode towards the front of the radio. Remove C53 and solder it across C59. In C53's place solder a 3.3k resistor. Remove C 61 and solder it across C62. Remove C126. Remove R55 amd solder a 22pf cap in its place. Remove the slug from L14. Replace C71 with a 33-47uf cap. Turn VR4 (AMC) counterclockwise fully. Gently spread L13 and L12 and tweak the spreads for best PEP when modulated. Tweak L20 and L17 for best PEP when modulated. After tuning is finished, set VR4 to half-way.

Note: Older 29's (LTD and GTL) use the same basic procedure, but instead of re-using the caps, place a 100pf across C62, a 68pf across C59, and 22pf across the spot where you removed R55.

Channel 9 switch to Channel 19: Cut D32. Hard, huh?

Is that 470 ohm resistor 1/4 or 1/2W?

 

[CbRadiosPRONTO]

Is that 470 ohm resistor 1/4 or 1/2W?

1/4 is fine

 

[OddBall1958]

1/4 is fine

Thanks I take it that they are all the same 1/4 or 1/2.

 

[OddBall1958]

Wow....a whole 40W. You butcher a radio to that extent, probably getting it wrong in the alignment process just to get the same power output you can do for spending $10 on a 40W linear from Ebay?

Is there really any point to it?

Oh and BTW that mod will kill the finals you do put in because nobody seems to have noticed that you need to install a heatsink.

The point I see is that if I run a 2w DK with 40w swing into my 4 pill it does about 800 maybe 850 on a good day. And it doesn't cost $10 bucks for a useless 40w amp.

 

[Uncle Ronnie 336]

when you drop the DK the thermal concerns aren't much of an issue with a FET, even though I run a resistor to the 8vtx line to bias em for a more clean swing. the dk will heat the final up more than anything, and yea, I dont add a mere 10-20 peak watts to just drive an antenna, the reason for hot final mods is what sits in the chain between the rig and antenna. like archin your back and givin er the extra half inch she deserves.

 

[OddBall1958]

when you drop the DK the thermal concerns aren't much of an issue with a FET, even though I run a resistor to the 8vtx line to bias em for a more clean swing. the dk will heat the final up more than anything, and yea, I dont add a mere 10-20 peak watts to just drive an antenna, the reason for hot final mods is what sits in the chain between the rig and antenna. like archin your back and givin er the extra half inch she deserves.

I understand your point. I'm no radio electronics wiz. I've mostly had people work on my radio's. However, I had a 148 with all the goodies back in the day That did 45w all day long and never seemed to get hot. I ran that into my Texas Star 500V and pulled 800w out of it no problems. I ran that for years unti the guy that ripped it off from me apparently needed it more than I did. I miss that radio! Now I'm looking to retire and get my radio's back in order so here I am learning more about it. I have done swing kits before and mosfet mod's but never really tuned em up much after that. I'll be here asking more questions. Thanks everyone for your support.

 

[OddBall1958]

I understand your point. I'm no radio electronics wiz. I've mostly had people work on my radio's. However, I had a 148 with all the goodies back in the day That did 45w all day long and never seemed to get hot. I ran that into my Texas Star 500V and pulled 800w out of it no problems. I ran that for years unti the guy that ripped it off from me apparently needed it more than I did. I miss that radio! Now I'm looking to retire and get my radio's back in order so here I am learning more about it. I have done swing kits before and mosfet mod's but never really tuned em up much after that. I'll be here asking more questions. Thanks everyone for your support.

And No! I don't want to go into the radio business! Just want to play around with my own.

 

[OddBall1958]

Is there a preference on the 520's? I see we can get them with a 48w or 60w output. Does it matter?

 

[OddBall1958]

this?

CB Tips and Tricks

MOSFET Supertune (29LTD classic and newer): Replace stock final with IRF520 MOSFET transistor (pin-for-pin match when installing). Remove R56 and in its place install a 470-ohm resistor in series with a common switching diode (1n914 or similar), with the banded end of the diode towards the front of the radio. Remove C53 and solder it across C59. In C53's place solder a 3.3k resistor. Remove C 61 and solder it across C62. Remove C126. Remove R55 amd solder a 22pf cap in its place. Remove the slug from L14. Replace C71 with a 33-47uf cap. Turn VR4 (AMC) counterclockwise fully. Gently spread L13 and L12 and tweak the spreads for best PEP when modulated. Tweak L20 and L17 for best PEP when modulated. After tuning is finished, set VR4 to half-way.

Note: Older 29's (LTD and GTL) use the same basic procedure, but instead of re-using the caps, place a 100pf across C62, a 68pf across C59, and 22pf across the spot where you removed R55.

Channel 9 switch to Channel 19: Cut D32. Hard, huh?

Voltage for the 33-47uf cap? Does it matter

 

[Uncle Ronnie 336]

16v cap at c71 will work. what that cap does is controls the release of the AMC limiter so a smaller value makes it more reactive to the peaks without continuing to clamp down on gain after it passed