Navaho TRC-431 tvi trap.

[LeapFrog]

I just picked up a sweet Navaho base station, someone's been inside it before.
I don't have a Sam's photo fact on this one, can anyone tell me what adjustment point is for the 54Mhz tvi trap as I don't have a spectrum analyzer and rather not turn ferrite cores without knowing what they are.

Thanks

 

[LeapFrog]

CB Secrets and cbtricks points to L910.

 

[loosecannon]

you can tune L910 to peak the final transistor.

do not tune F901 or L911.
LC

 

[LeapFrog]

Thank you LC, i swapped the final from a 2sc1909 to a 2sc2166 as I read in a data sheet the original was only rated at 1.5 watts! I may put a 2sc2314 driver in it as well because while the 2sc1957 driver is okay, I'd like to see at least a 3 watt carrier from this "budget" base station!

I will get the lids off it again tomorrow and see what I find!
Thanks

 

[loosecannon]

put a mosfet in it. It's an AM only base station so you shouldnt see any distortion issues and you can definitely wake the radio up.
LC

BTW- the 2166 is used as a driver in SSB radios and most likely wont show the power gains you were hoping for. try using a 2078 or a 2029 from a 29LTD and see how that works for you.
LC

 

[LeapFrog]

The thought has crossed my mind! I'd rather save the 2SC2166 for a vintage SSB Uniden/Cobra. To be honest with you I have not learned enough yet to decide if a homemade companion part would be suitable like in the Cobra 25/29 radios, I would have to compare the boards/study & learn some more. I wouldn't want to have to smell any nasty blue smoke lol!

Should be a fun learning exercise though no doubt!

I'm gonna tune it for max output with the transistor first, and if i'm not satisfied I will figure a way to mosfet convert it. I suppose a small trimmer pot will allow me to set the bias voltage at the gate of an irf520n or fqp13n10..

Some of the Navaho radio p.c. boards were made by Uniden, unfortunately this one was not.
I actually found the old 2SC2166 being used as a final in a Uniden PC68XL (Philippines), with what appeared to be original thermal compound on the heatsink/insulator and thread locker/glue on the screw, I was shaking my head in disbelief but everything looked original!

I will throw a 2SC2078 in it and see what see does!

 

[222DBFL]

Man the old cobra 25's were great am radios that a lot of guys used here locally long ago. They always were a loud audio radio with no distortion!! Keep the positive peaks at about 95% and set the dead key up for an amp if wanted. They were good radios to add to amp! Seen a couple drive 4 pill Dave made amps and they rocked!! Be nice to see what you come up with. I am having trouble wanting to part with my cobra 29lx le with a 1969 final installed!! It is a sweet radio. Got rid of all the cheap knobs and out some old chrome ones on there that look new and man it helps with messing with the radio. No loose arse knobs LOL! Keep up the good work LeapFrog!! Fun to learn! Have a great one.

 

[LeapFrog]

Thanks 222 have an awesome Saturday brother!

I started learning on the Cobra 25/29 when I first got into radio, they will always hold a place in my heart. I have four of each and I use one 29 and one 25 as guinie pigs for modifications, even though they are almost the same radios minus the features found on the 29's.

I picked up this TRC-431 Navaho hoping it was made by Uniden, although it's not It still is a 40 Ch. PLL synthesized radio, and has potential, plus I think it looks kind of cool!

I was not pleased at the factory output (under two watts).
So because they are not sought-after I'm going to modify it to perform more like a Cobra 25/29! I know if its local talk and or conditions are right 2 watts can get out & talk, I have heard of some people who practice QRP have talked (CW more likely) over a thousand miles with One watt & a wire!!

But if this radio is to become my daily AM talker i'd like at least a 2-3 watt carrier and right now she just wont do it!

So I'm going to have to stock back up on solder as I ran out last night, and put it back on the operating table lol!

73
-LeapFrog

 

[loosecannon]

I have always liked the TRC-431. great looks and the ones i used to hear were very loud.

i do know that you can easily go too far trying to get audio out of them though.
turn the VR up, but don't remove the diode.

if you are looking for a schematic, there is one in the back of the owners manual on cbtricks.

here is a quick tune up list and a couple of things to try for power and audio:


receiver: L101, L102, T301, T302, T303, and T304. tune these for max on channel 20. use L102 to balance gain between 1 and 40. (might need to re-tweak L101 a bit afterward)

transmitter: L901, L902, L903, L904, L905, L907, and L910. tune these for max modulated output on channel 20. use L901 and L902 to balance power between 1 and 40.

as for mods, for the receiver, try replacing D301 with a shottky diode.

for modulation first try just maxing VR7. you might try removing R519 (270 ohm) and see how that effects the operation of VR7. if removing it seems like too much, then try a 100 ohm resistor in it's place.

for power, you can increase the power to the driver by reducing the value of R921 (8.2 ohms). i would try 4.7 ohms, and then 2.2 ohms and check for the driver heating up.

you can increase power to the final by paralleling R922 with a 1 ohm resistor.
if you change the final to a 2078, you might be able to get away with just jumpering it, but again heat might be an issue.

you know me, im going to tell you to re-cap it first, but that's up to you.

this should really wake that radio up.
LC

 

[LeapFrog]

Thank You LC for the extensive help on this one, beyond the AMC diode I hadn't found much info on this radio! Although in Secret CB Vol. 20 (published Sept. 1984) I found Custom Conversion #10 Code name "Wild Goose" for the TRC-431 the mods mainly deal with custom channels, although they do explain how to add RF Gain and something that functions like Mic Gain!! Nothing like the power tips you provided though, I feel with this knowledge you shared I can truly make this radio a great (working/non-original) addition in my collection.

Now i'm off to get some 60/40 or Kester 44!
I've got a nifty little aluminium heatsink (I checked it will fit!) that should help the driver out.
After I get some solder flowing I will post back with an update.

As a side note the first thing I did was 4-pin Cobra conversion for the mic input as an original din-style realistic microphone was not included and I'm not a huge fan of pin adapter plugs.

Thanks Again & 73
-LeapFrog

 

[loosecannon]

you won't find that info anywhere online. I made those up from looking at the schematic.
please take that into consideration when you are trying these mods.

you probably did, but if not be sure to use bypass caps on your new mic plug to keep all the nasties away.
also, a ferrite bead on the mic audio line right where it leaves the plug and starts toward the PC board is always a good idea to keep the mic from squealing.

best of luck.
LC

 

[LeapFrog]

you won't find that info anywhere online. I made those up from looking at the schematic.
please take that into consideration when you are trying these mods.

you probably did, but if not be sure to use bypass caps on your new mic plug to keep all the nasties away.
also, a ferrite bead on the mic audio line right where it leaves the plug and starts toward the PC board is always a good idea to keep the mic from squealing.

best of luck.
LC

I will mod at my own risk, no worries
As far as the mic jack, the original din plug was soldered into a little pcb with bypass caps on it so I desoldered the plastic din plug and ran short wires to the 4-pin jack where the corresponding holes were on the little pcb.
I should add a small ferrite on the audio line as the shielding of the audio line never attached to the pcb or ground at least this end of the shield brade (not the main pc board side) Thanks for the suggestion, currently the radio does squel when I tested the public address function but some of that is because of how close I am to the output speaker (right next to me) and I expected some feedback.

 

[357]

theres a blue tantulum on the 431 in the middle of the board towards the back.
replace it now.

 

[LeapFrog]

Update: I ended up getting some lead free junk solder (I don't want that on this pcb) so i'm going to look into ordering the tantulum as well, I have electrolytics on the way!!

 

[357]

kester 66
60/40
.02"
best you can get