• You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

Seeking Realistic trc452 advanced audio tweaks

Uncle Ronnie 336

Well-Known Member
Apr 28, 2020
246
151
53
41
I have just recapped a friend's 452 and had to change a final out. I had a 2sc2166 Mitsubishi on hand so I put that in and changed the bias resistor to a 15 ohm with a ferrite bead. Currently I am getting around a 20 watt peak off of a 6 watt carrier with the AMC pot cranked wide open. I am looking for just a little more corn in the chute with this audio and I do not want to disable the limiter completely. I am not looking to do a NPC mod on this, but I would like some expert advice at how to go at this somewhere in the audio section. I already have this radio talking quite well for something that was not transmitting and had real scratchy audio everywhere due to leaky electrolytic caps.
 

20 watts peak from a 6 watt carrier ain't to shabby for a 2166.
Remember this is used as a driver in a lot radios.
That's about all I would expect, from that radio to be realistic ( no pun intended )
I think you got it working well.

73
Jeff
 
Yeah I don't doubt that but I feel that despite the limiter being cranked wide open I should be looking to put a little more gain into it. The end user is using the stock mic and I'm sure I can get a few more peak out of it ramming a power mic into the mix. I'm just looking through the schematic to see what I can do to get a little more audio compression. I feel like changing out a resistor or capacitor somewhere around the limiter circuit or in the preamp stage of things would make the limiter work more bringing up my overall loudness.
 
If you want more gain without inviting any RF squeal, change R272 (47 ohms) to a 33 or 22 ohm. This resistor is the bootstrap and determines the gain of TR 220. The lower the value the higher the gain.
 
Change D212 from 16V Zener to standard 1N4148 - 50PIV Min 1 Amp rating - see what she'll do.

This uses a Push-Pull Audio Amp design - be careful with it, it's pretty novel and Rare -in design- these days...

Go here for more Info...
http://www.cbtricks.com/radios/realistic/trc_452/index.htm

To add more...

Change C270 from a 0.47uF (or 1uF) to 0.1uF 50V or 473 Disc (To remove problems with Electrolytics')
Change C350 0.47uF (By Final) to 0.1uF or 393 Disc - to improve tone.
Locate C351 330pF - remove it and REMOVE C280 - 0.01uF (103 Disc/Mylar) REPLACE it with the 330pF at C280 location.

Other mods?

R272 - 47 ohm, Try 33 or even 22 ohm - no guarantees on stability so work in steps.

Of note...

AGC works in both IF stages and RF gain area - so AGC is aggressive - R287 is 10K so if you need "More receive" Increase the 10K to as much as 18K to reduce it's "punching down" tendency.

To improve Receive tone, Replace C222 (AF Det. output smoothing filter) from 0.0022uF (222) to a 0.001uF (102) Disc
 
Last edited:
Pretty much, but noise factor is relative - RF gain is the control you use for the "Adjacent channel Rejection" as well as Noise level to a more tolerable level.

It actually has some pretty good numbers...upload_2020-10-5_15-32-15.png


Some people like higher tone so in a way, you're hearing the "white noise hiss" but only as a tonal effect, not a washout.

IF you want to hear White Noise, then replace that 222 0.0022uF with a 220pF - that will get you your "White noise standard" every newer radio has in their receivers.
 
  • Like
Reactions: tecnicoloco
So I have changed out c270, and c280. That alone has given me around 24 Watts on the peak and it sounds good but as soon as I hook the speaker up another problem has arisen where I get full audio and feedback like there is a TalkBack function cranked all the way up
 
Yes, that's not good,

Replace the parts back to original position then start over...one part at a time.

This could be from working in areas that may short out due to molten soldering proximity.

D212 - as you know, is a Zener - so replacing it with a typical 1N4148 keeps directional - only it will not clamp on peaks. So again, caps that are rated 10 or even 16 volts - need to be upgraded to 24~25WVDC or higher rating in and around this area.

But to encounter the talkback, usually indicates shorted or failed Disc cap from Board ground to Chassis ground (case) .

Now a concern...

Is the speaker coil output winds - SHORTED to ground?
  • To check if that is the case,
    • unplug microphone,
    • locate speaker wires,
    • follow wires back, back to R298 120 ohm 1/2 watt,
    • MEASURE from one of those solder pads to Case metal, and again to Foil Ground.
    • IF either of the two readings show a short, there's' why you hear audio - it's found a return thru foil ground. Audio Coil is shorted.
  • If shorted, this can only occur if you placed too much power (volted power) to obtain the wattage - the coil wind arced internally.
Also - if you locate C350 - just remove it, they have a 0.039 (393) disc there - just like Cobra 25 or similar design. So you don't need to swap...
 
I found the solder bridge and rectified the problem so we are back to audio mod. So I am having a little trouble locating c350 and 351
 
Luckily I did not have to undo the work that I did already and just found the solder bridge that did not belong. So since I am cranking up the gain here what does that 10K resistor do for the receiver? If I drop in a 15K or higher does that just put the receiver closer to stock parameters after I do all these mods? I didn't find where it was on the schematic last night but it is displayed prominently in a wide open area in the radio
 
The 10K makes the receiver "less sensitive" it punches down the receive when you have ambient noise that gets into the AM Detector - your C222 but there is another "quirk" you won't recognize unless you've had to fix a noisy receiver due to a vehicles noisy ignition.

upload_2020-10-6_17-59-46.png


This board is a decent performance board but they did some stuff that - well...doesn't make sense unless you are trying to understand the "ambient noise versus Birdie" the radio can generate.

On the bottom (Foil side) of the board - the BLUE arrow points to a 330pF cap that straddles the wire that sends power from the CB/PA switch on the front panel (a proximity to noise source location) - the way this works, when it's in PA mode, the radios IF is simply cut off by applying power to the Emitter leg, raising it's voltage , so the thing can not amplify any signal - quieting the receiver.

So what is the problem - 330pF doesn't cut it when it comes to noise sources getting "rejected" more like injection into a radio - maybe you don't have this problem, but others did.

The fix is pretty simple, remove the 330pF install a 0.01uF where the 330pF WAS, then change C219 from a 0.047 (473) to a 103 - ON TOP OF. R303 gets split in two - the R303 is a 1.5K so you'll need Two 680 ohm 1/4W resistors and one gets the pigtail from the CB/PA switch - free end inserted where the pigtail went, The resistor is IN series with the pigtail wire from the front panel CB/PA switch. The R303 location gets the other 680 ohm resistor and you solder the 0.01uF (103) Disc cap where the 330pF was.

Now, that 10K - was part of this carborundrum it's pretty aggressive, so you may want to now back off that value to, say - 15K, to allow the mod done at the last IF stage, which sends your AF Detector signal for the output and noise fix for CB/PA switch - which goes back to why I recommend that C222 changes to a 0.001uF (102) to reduce the dampening effects and losses from the weaker signals are getting sucked down into that 0.0022uF (222) cap instead of thru the ANL filter. Less capacitance helps the brightening of the audio, lessens the losses, and seems to offer a good trade off from the white noise hiss versus the quieter receive the radio originally had.

Interesting you had mentioned earlier about a Wide versus Narrow filter - you'd have to go to the 1st IF stage to fix that problem and that too is a 10K or 15K (should I?) issue. The trade off in making ANOTHER MOD to the IF strip is the changes to an otherwise nice balance the radio had to recover some sensitivity that was lost in the original design for them to obtain the REJECTION (Read this as - Selectivity ) as listed on the spec sheet.

Locate C207 - change that value, Lower or Higher to achieve the Narrow to WIDE response (Lower cap value - smaller less signal - Narrow response) The Resistor (R208) can also provide some control, but not as a value to obtain Narrow or Wide, only Signal Strength thru to be mixed in the IF of the next stage.- the cap handles the RF - the Resistor simply limits it's power or strength into the next stage.


So if you've read this far and still are with us, and haven't run out of the building screaming...Well, Welcome to Radio...
 
Last edited:

dxChat
Help Users
  • No one is chatting at the moment.
  • dxBot:
    Tucker442 has left the room.
  • @ BJ radionut:
    LIVE 10:00 AM EST :cool:
  • @ Charles Edwards:
    I'm looking for factory settings 1 through 59 for a AT 5555 n2 or AT500 M2 I only wrote down half the values feel like a idiot I need help will be appreciated