Realistic TRC-427 Audio Mod 'GoldFinger'

[LeapFrog]

Realistic TRC-427 'GoldFinger' Audio Mod Locations
for the neophyte


RX:
C33 = 2.2uF Mylar or Polypropylene
C62 = 2.2uF Non-Polar Electrolytics or Mylar
C63 = 2.2uF Non-Polar Electrolytics or Mylar
C76 = 1,000-2,200uF 25 V Electrolytic
D7, D8 = 1N6263 Schottky Diodes
Tr2 = "First Receive Amp" Location
leave Tr2 alone.

TX:
C66 = .22uF, or .068, or .033 Mylar (Lower Value = Less Bass)
C124 = .001uF Ceramic Disc or Mylar (Maybe try .01 instead)
C130 = 1.5uF - 2.2uF Tantalum Capacitor (+ side to base of TR18)
R99 = Stock Value is fine at 3.3K (Mic Audio input), could lower to 2.2K, but no factory mic gain control..
**C132 = 22uF

Inspired by: GoldFinger of WWPDX
Written up by: LeapFrog of WWDX

This radio is very similar to the PC-66, 21 GTL, and 25 LTD radios.
The Realistic TRC-427 used a PC-392AB, or the PC-392AD PCB revision depending on year.
manufactured by Uniden in Korea & Taiwan for Tandy/RadioShack during the late 70's.
As a rule of thumb: if it's not NIB mint, I swap in a Cobra/Uniden 4-Pin style mic jack for these radios.

It's a real talker after this mod, I used a "medium fidelity"
(Fet modded) amplified D104 and even tried a stock Galaxy hand mic, the SDR samples sound great.

Of course the modulation transformer will have a frequency roll off of what it can pass, high & low,
so whatever audio gets fed into the pre-amp and amplified by the high-level audio I.C. (stock TA72222AP)
will ultimately be "choked" at the transformer.

An AM regulator mod
(I call it the Wabi-Sabi) remedies that and allows the radio to go WIDE..
with an optional External input control, but that's for another post...

Hope someone gets some use from this, just to clarify these are the "GoldFinger" cap change locations for the
Realistic TRC-427, you can go wild changing
transistors if you want (TR11, TR18), but this is just a basic cap change process for beginners..

I don't have time to run a frequency sweep, but if you own one of the these radios and try the mod, it will make a measurable difference.

DISCLAIMER: **Remember to follow all rules and regulations where you live & operate**

Best of Regards & Enjoy!
-LeapFrog

 

[TheRealPorkchop]

I don’t know who this Goldfinger fella is but he’s got a pile of “mods” on that site. It’s a shame the site seems to have extremely low traffic and postings. Lot of good info there in my opinion.

 

[Shadetree Mechanic]

I am a member there too. There is a ton of good information but the only regular member seems to be GoldFinger and he has mods for any radio. And if he doesn't, just ask and he will figure it out and let you know. More power to him, I only stop by once a week if that.

 

[loosecannon]

I like the sound of a slightly expanded radio, but Goldfinger takes it to the extreme.

What needs to be understood is why he chose such high and low values for the caps.
He chose those extreme values because his goal was to get a flat response from 20-20K Hz.

He wanted his radios to be wide open so that he could EQ the audio externally.

many people have done his mods not knowing this, and then been a bit disappointed with the extra noise.

I have found that moving the decimal point over one place to the left or right yields results that can be enjoyed without external equalization.
obviously that is a sweeping generalization, but that's where i start.

i hope this didn't come across as 'bashing'. it is not intended that way.

LC

 

[TheRealPorkchop]

Yup, the external EQ thing is what I thought when I started fooling around with some of his mods too. Some places he clearly states to play with the values if you want something different, more high or low.

 

[LeapFrog]

I like the sound of a slightly expanded radio, but Goldfinger takes it to the extreme.

What needs to be understood is why he chose such high and low values for the caps.
He chose those extreme values because his goal was to get a flat response from 20-20K Hz.

He wanted his radios to be wide open so that he could EQ the audio externally.


LC

LC,
Yes it is rather extreme, the reason I posted this mod is because no one has listed the locations for this radio.

A newbie could and should experiment with different values, and also examine the rest of the circuit to get an idea of what they're doing. I bought a Sam's manual for this radio because I couldn't find a schematic for free.

My plan is to "Wabi-Sabi" AM regulator mod a TRC-427, and using this modified pre-amp, pass some PHAT audio without external processing, frequency sweeps will be performed.

The goal was to get really expanded audio without needing to go to a PC (on a budget, so no rack gear) for the audio source.
Can it be too much, yes it can.
Experiment cautiously, perhaps a little Behringer EQ is in order.

Best Regards
-LeapFrog

 

[TheRealPorkchop]

I have seen the previous debate between you and an unnamed individual about this wabi-sabi thing, explain. What is this mystical wabi-sabi you speak of?

 

[LeapFrog]

I have seen the previous debate between you and an unnamed individual about this wabi-sabi thing, explain. What is this mystical wabi-sabi you speak of?

Wabi-Sabi is just a pet-name for the AM regulator conversion with supporting mods.
We take an AM only radio that uses a modulation transformer, remove it, and install a darlington configuration (similar to the Cobra 148) in its place. This removes the audio bandwidth restriction of the transformer, but it is only "half" of the mod.

If you want to retain your power output before and after the AM regulator modification, you will need to do "more" because of the extreme difference between the voltages present at the collectors (of driver/final) before/after the AM regulator conversion.

If you study the Cobra 148 schematic and compare to the Cobra 29 LTD schematic you see the SSB radio has a different PA configuration, the Cobra 29 LTD (or 427 or w/e) will need PA modifications to increase gain through some stages.

 

[TheRealPorkchop]

Interesting. I really got to get out from under the rock I live under I guess, I’ve never heard of that before.

So what’s the greatest gain to doing it? What’s the downfall of it?

 

[LeapFrog]

Well the greatest gain, is carrier control.
Asymmetry can be had and at a HUGE price reduction compared to the competition.

The audio response is awesome, blows the transformer out of this world.

People don't really believe me until they see it, that's fine!
The biggest drawback is that if you don't do the support mods you will not be getting the same output (4 Watts carrier to 16 Watts for 100% or whatever).

Asymod manual states to expect a (serious) drop in output power after install, well you can expect that same drop in performance if you don't do the "other half" of the mod.

We aren't the first to do this, RevBow's BME comes to mind, but I do believe my cohort and I are the first to get 4 Watts DK-16 Watts @ 13.8V out of the AM regulator conversion. We have progressed beyond that now, we have asymmetry controls, peak & carrier adjustments.

So if you don't mod the PA stages, expect a loss in power.
But the upside is asymmetrical modulation and audio frequency response.

I think the unnamed individual is jealous or just does not believe that a 25$ radio with 20$ worth of discrete components can perform with a 600$ Asymod modified Stryker radio, the audio and modulation.

 

[TheRealPorkchop]

Hmm. And where might one find this information for this conversion and/or more reading material on the subject?

I’m curious to see this, I don’t know why it’s sparked my interest but it has.

 

[LeapFrog]

I will try to find you the post (by Reverend Bow I believe)
that sparked my interest. It was on a different forum and they do provide a "starter" schematic, but it is not what we are using.

A lot of testing and swapping and burning up parts and more testing to get the idea working; it's been a fun process.
The best way to perform this mod is with a PCB, keep the wire lengths short as possible, reduce stray capacitance, use bypass caps where needed.

I can tell you a MosFet final transistor is recommended (like in the Asymod install), you can run the radio off of 13.8 Volts, or you can use a 24 Volt power supply with additional support circuitry.

Basically, you are transplanting the Cobra 148 GTL modulation scheme into an AM only radio.
That part isn't too hard, but getting the watts can be difficult.

I would say for a first time, expect to get 3/4 of a watt carrier, swinging to +200% modulation, that's about what I got in the beginning, and what Bow got, if I remember correctly. Now with more PA mods, the power increases, a main goal is to increase the voltage at the collectors; a more "extreme" route is to use 24 Volts input and seperate the PA section from the rest of the radio, maybe use an 7812 and a LM350, now you've got the secret...

 

[TheRealPorkchop]

Sounds interesting to read about. Whenever you get a link just let me know. I don’t know that I’d actually do it but it sounds very interesting to read about. Really interesting the 24v thing.

 

[LeapFrog]

Sounds interesting to read about. Whenever you get a link just let me know. I don’t know that I’d actually do it but it sounds very interesting to read about. Really interesting the 24v thing.

Yes 24 Volts input gives more output. The radio can be run off of 13.8 V, it just takes a lot of effort to make power at 13.8 V without the transformer.

The mod is more about audio & asymmetry than anything else.

 

[TheRealPorkchop]

Haha. Drake the kind of...well you probably know the rest so I won’t label myself by finishing that.