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Realistic TRC-427 Mosfet Modification (IRF520N/ ERF2030)

LeapFrog

Wielding Hanlon's Razor
Feb 15, 2016
1,709
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Anchorage, Alaska
[photo="medium"]4775[/photo] .
The little ferrite bead on R44 is L14, on one mod I reused this bead (on the hot lead of EN-1230) and on another mod I didn't use the ferrite bead, the radio without the L14 bead has more carrier, don't know if it's related, I'm not bothered enough to pull the bead out and compare results.

Remove R42, R41, C53
Place EN-1230 Companion part @ R42, w/ neg lead towards back of radio.
Place C53 on backside of C55, or just use a 390pF cap @ C55.

I changed R44 to a 4.7 Ohm from 6.8 Ohm, I did this on only one radio, not both; don't know what this did, if anything appreciable.
One radio had 2.25 watts carrier, the other 3.5 watts..

For giggles I "opened up the AMC" and whistled into the mic, let's just say this little mosfet works as a final... "swanging to 5,6,7 average dirty watts of splattering over-modulation".. Now I have the AMC adjusted lower than when it left the factory, but It is still near 100%, not splattering! (y)

This was without realigning the entire radio, "I just peaked L12, L15, & L16".
AMC needed realignment due to change in the carrier level.

The carrier was too low without going over the inductors, and ultimately adding capacitance at C55 did the trick (Total 390pF).

R41 looks like the "static discharge resistor", I say to pull it, only because I was looking at Cobra 25/PC66 install instructions for ERF-2030 by EKL to create my mod, and they say to pull it.. so I did.

http://www.worldwidedx.com/threads/40-45-watt-erf2030-mod-for-cobra-25-uniden-66-68.33400/
http://www.cbtricks.com/miscellaneous/fet_papers/ekl/graphics/erf2030_cobra_25_mod.pdf
http://downloads.cobra.com/CB/Models/25-SERIES/25LTDschem.pdf

In that thread Moleculo suggested to replace R38 (cobra25/pc66) with 10K-15K Ohm, instead of pulling it. If I've compared the TRC-427 schematic correctly against the (cobra25/pc66), "R38 in my mod is R41, I could be wrong though, so compare the schematics if you like".

Thanks Moleculo, EKL, CBTricks, and the WWDX forum members who have discussed using the mosfet parts in the past. (y) Cheers -LeapFrog


[photo="medium"]4776[/photo] .
[photo="medium"]4777[/photo] .


I have done two Realistic TRC-427 radios with these steps, I only just came up with the idea to mosfet convert the TRC-427 a few days ago, so the mod could be refined by someone, anyone feel free to comment or suggest a revision!


Maybe I'll try more voltage at the gate to get a 4 watt carrier, also next time I may try FQP13N10!
If anybody is interested in seeing the RF Peak to Peak Voltage, let me know..

I can re-visit the scope and look at the volts/per div. then measure for max with audio before I see distortion in the positive peaks... That'd be more accurate than most all consumer level P.E.P. meters, "true peak reading" or not.

I had about 1 volt on the gate of the mosfet after these steps, if someone did more and added voltage the carrier and total output would increase (assuming)

I'm not worried about the "Max Watts", i'm just glad I have a cheap alternative to traditional transistors, and they have "headroom" over the factory part, maybe!?.
One radio I keyed up a few times w/ no antenna to see if the part would "instantly fail", nope it still works fine, as far as I can see.

Cheers!
Me and this duck pluckin' Realistic, are back out. :sneaky:

-LeapFrog
 
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I will check it for in-band splatter on my RTL-SDR, tis all I can do w/ my budget at the moment.

I'll check at 54 MHz w/ RTL-SDR also, hopefuly the stock TVI trap is still "doin' the business", even though the mosfet is not an RF device. :)

I don't have any screen capture software, but I can snap a pic or film a quick vid.
 
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Alright gentlemen, here it is..
Please forgive my presentation, the video screen rotated 90 degrees unexpectedly.
Also I did not prepare a tone to inject, so once again please forgive my hooting and hollering! I know it's not very scientific or professional; but it shows the IRF520N can be used as a suitable replacement for an RF final @ 27 MHz, after component changes & fine-tuning.

Also, I apologize, for the SCREECH and SQUEALING from feedback in the beginning of the video.
I can do better for the presentation, and next time I will try a little harder.

~Cheers!~
73
-LeapFrog
 
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I recommend the ERF2030+, they are tough cookies!
I had set a (constant) bias voltage too high and saw one of these mosfets "take-off" into thermal run-away while I was testing, something wild, the heatsink was VERY HOT, and I saw a few smoke trails rise, immediately I unplugged the unit.

I thought for sure the little mosfet was cooked, nope! After it cooled back down I lowered the bias voltage from about 4.2 V to a more reasonable 3.6-3.8 V :p it didn't fail, I tested it after everything cooled down and it is still alive! :cool:

ERF2030+ get a thumbs up from me, I have tried FQP13N10, IRF520n, and so far I prefer the 2030+; a plain-Jane SiHF520 mosfet will be used for the next test.

While the TRC-427 is very similar to the 21 LTD, PC66 /Cobra 25 GTL radios it has a different audio chain, the pre-amp is not the same, you get an antenna warning indicator and mod indicator on the 427, something the others don't have.
 
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The 2030+ took a lickin' and kept on tickin'? Woah! I'd seen regular mosfets blow up just like fireworks!

BTW - what software are you using in that video?
Yes, I honestly thought it was done but these new generation of mosfets are tougher!

That software is called SDR#, it is freeware that I use with an RTL-SDR dongle.
https://airspy.com/download/
 
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Updated!
Realistic TRC-427 Mods


Schottky Receive Mod Location:
Replace D7, D8 with 1N6263 Schottky Diodes
Replace TR2 (c1674) with 2SC2999E
{TR swap might cause overload or increase noise floor}
{it is recommend to replace just the diodes}
{unless you have sophisticated receiver alignment technique/equipment}

Little Power/Audio Boost:
Change C147 to 2200uF 25V cap (main power/filter capacitor).
Replace C76 a 1000uF with a 1uF tantalum on bottom side of pcb in parallel. (audio coupling out of high-level I.C. TA7222)
Lower R99 to a 2.2 K ohm from 3.3 K. (lets a little more mic audio in)
The Mosfet Mod:
Install EKL EN369 DR @ R42, negative lead towards the back of chassis.
The ferrite bead that is on R42 has been left out & that makes L14 effectively removed, it is a tight fit to include the bead on the leg of the EKL "companion component", I usually leave it out,
If you want to include it, put it on the positive lead of the EN-369 DR.


Replace final transistor with ERF2030+ (or IRF520/FQP13n10/whatever)
USE THERMAL PASTE
[ERF2030+ In stock for a $1.00 over at RFpartsDOTcom]

Simply change C55 to 390pF after removing C53/C55!
Alternatively you can place C53 on bottom side of PCB in parallel @ C55 to equal 390pF.

Remove R41 replace it with a 10K 1 watt. You can use higher values like 15K+ if you want.

Optional:
*Recommended to change the driver transistor to a Sanyo 2SC2314E!!
Remove R41 & leave it empty!

If you like to live dangerously replace D15 with a jumper then run the Radio on 14.5 Volts, for a quick test! :)
Pull the jumper back out, that 0.7 Volts is not worth your transformer melting in the advent of a final transistor failure.

If you want to experiment in order to Increase Gain/Drive
(decreased attenuation) through pre-driver/driver/final RF stages...
You can try these:
Change R61 from 1 Ohm to half Ohm (or you can experiment with a jumper)
Change R44 to a 4.7 Ohm from 6.8 Ohm resistor, or go lower yet.
Change R45 to a 50 Ohm resistor <- haven't really tried this one yet
.
**Shorten L15, L12 each by one half turn {Not recommended}
These last few may cause stability issues, experiment at your own risk!
Correction! in my first post of this thread I mislabeled R42 as R44, when I discuss L14, will need mod to edit the post.

Optional Bias Voltage
if not using correct "companion component circuit".
Grab the switched (upon TX) voltage from the
center lead of TR21 and use resistors, trimmer pot
(bleed some off to ground) to get anywhere between 3.2 to 3.8 Volts
at the Gate pin of your Mosfet;
Another method would be adding a L7805 or LM317 circuit and support components + potentiometer.

I don't mind if you copy this somewhere else as long as you give credit to WWDX's Moleculo post on the high-power MosFet mod for a Cobra 25/PC68, & a thanks to CBtricks, & EKL, for the mod install sheet for Cobra 25 LTD,
they also inspired this TRC-427 Realistic Mod Sheet.

All the best regards!
-LeapFrog Tampa Bay
 
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I have one of these radios. Never knew it was a stable radio. It works like it should. Not a real fancy radio but same chassis as Uniden I'm told. Moth balled it on a shelf, maybe I'll get it out. Now what did I do with the dammed mic for it?
 
Pulled mine off the shelf and thought about the same thing. That 5 DIN is the same as what's on this old Commtron CXX, which needs a 4 pin, also. Now.....wher's that damned mic at. Shouldn't be hard to find, it's the only one with that wimpy DIN. I did copy your text about the Mosfet and mods. Good info.
 
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