Cobra 29 LTD Classic

[Signal Kicker]

Hello One and All!

Any chance of getting some help on a Cobra 29 LTD Classic?

This radio belongs to a friend who I am only trying to help. I will not be
charging for any parts or even my time, I am just trying to do a good deed as he does
not have a lot of money anyway.

This unit has had the TFX75 module installed. I would like to put the radio back to stock, all
but the final transistor. I have not been into Radio for a long time and I really don't know what the latest
mod or upgrades that are out there. Also where is the best place to go for parts? (Mouser, Newark, Rf Parts, Master Electronics, Allied Electronics, any others ?)

I am thinking an IR520 Mosfet as I do not have the original final to put back.

Also I have searched a Cobra 29 schematics and I believe I have the parts list to replace the original parts,
all but L12 which is an inductor of an unknown value.

C8 15pf
c57 2pf ???
R56 22 ohms
C42 1000uf at 25 volts ( will install a higher voltage though)
R48 100K
L12 with no idea
Date of manufacture is Feb 07

Thank you to anyone who can help.

 

[BJ radionut]

Cobra 29 LTD Classic Schematic (tripod.com)

Cobra 29LTD ST, 29WX ST (cbtricks.com)

 

[Signal Kicker]

Thank you BJ Radionut.

I found the Bill of Materials (BOM) list as it was a pain to find the parts but it did work, thank you.
Also I was able to find C53 68pf which I could not find on the schematics, so that was
a good help there too.

I found the L12 being Kel-S026 coil spring 0.8x6x7.5 HA-S026, but it doesn't state
the value as in any number following nano or micro henries.

Are these part numbers actual numbers any part supplier will understand?
The description of L12 makes me believe its just a coiled wire with no ferrite core.

Again any help is much appreciated as I like to put this back to stock
other then a mosfet final if this is whats popular?

Many thanks so far!

 

[Handy Andy]

​

 

[Signal Kicker]

Hello Handy Andy,
Its nice to see a pic of that coil/inductor, thanks!
It looks like some wheres around 6 maybe 6.5 turns. Maybe turn some enamel wire
around a very small tuning screw driver might get it close enough to work to experiment!

Any chance could you tell me if the part numbers are good to use as lets say Mouser?
Will they have an idea when I call them and tell them the part numbers from the "BOM"
list of Cobra Manufacture?

I also discovered I will need L26 KEOE-OT118-1 Ferrite Bead T5 RH 3.5x51.3.
Any chance of having a pic of it?

Whoever installed the RFX75, I wished they would kept the old parts in a tiny envelope and left them inside the 29 ltd for possible future of this de-install!

Are you one in the same Handy Andy from a long time ago on Tricks?

Thank you for your help.

 

[Big Kahuna]

Is the radio working currently with the RFX75? Putting that back to stock would seem like a headache to me and just not worth the time when you can get a used one cheap. Unless you're just looking for a project.

 

[9C1Driver]

I agree. The RFX units are no longer made, it was a great mod for more power. If it's working why screw with it? Cobra 29's are easy to find used, have your friend just buy a stock one.

 

[Handy Andy]

@Signal Kicker - Wow, lots to answer for...

Ok, for simplicity, the L12 and L13 are 0.8mil (~16-20AWG approx) and you're better off using a simple drill bit shank or the "Stem" of a Philips head #3 or #2 of the right shaft diameter that once L12 slides on, that's what you "re-wind" the old wire on it - as a form, to get your needed shape and diameter.

• No matter what, many times I've had to straighten a coil wind even from a spare donor radio to obtain the proper "diameter and working value" to re-install. Seems that even a minor thing of the thickness of the enamel used on some solid wire is why some wire seems to be sold as a bigger AWG value than the actual amount of copper in the spool you really have.

Yes, that coil wind is actually 7.5 turns - look carefully, count the number of bumps, it's original 8 "wind" but remember that 8th wind is incomplete, it's stops at the 7. 1/2 stop of turns - so you can insert it into the board.

When you go for 6 turns you're selling yourself 1 turn short and get a big impedance bump - and you can't get that value back, once you've taken off too much of that which was trimmed off, you'll have to get another wire and wind that one all over again and tensilize it to make it work - with the right number of turns.

L50 and L26 both being Ferrite beads, both are used even in the PC-78 and PC-66 - but they are simple Ferrites - have a diameter of a Torx #5 (pretty fine) and feed thru - permeability? Anyone's true guess, they just list it as a simple ferrite bead

Here's some photos, some are installed on the lead of the Limiter for Cobra, while another lays flat - per Uniden style. and a better shot of L12 showing the siderail winds

A typical Cobra 29's TX strip...

​

Hope this helps!

 

[Signal Kicker]

Is the radio working currently with the RFX75? Putting that back to stock would seem like a headache to me and just not worth the time when you can get a used one cheap. Unless you're just looking for a project.

I agree. The RFX units are no longer made, it was a great mod for more power. If it's working why screw with it? Cobra 29's are easy to find used, have your friend just buy a stock one.

Thank you for your thoughts on the RFX75, but No it Does Not work at all.
Besides the man who owns this does not have a large enough supply to accommodate the current that the RFX75 draws to be used inside of his home.
Again thank you for your consideration..

Andy, thank you for your valuable information and of course the pics, this indeed will help get me started. I am not sure about L26 though, I may have to search for a parts rig if I cannot find
the actual part from a supplier.

Can someone please point me to the best supplier for parts?

Thank you to all who are participating in giving me a hand here.
The man who owns this is a very appreciated guy and everyone on the CB who knows
him gets along with him, he is very likable.

Again, Thank you.

 

[Handy Andy]

I am not sure about L26 though, I may have to search for a parts rig if I cannot find
the actual part from a supplier.

​

Well, L26 can be "borrowed" from a limiter - the L26 part itself just blocks some of the RF from going thru that 22 ohm resistor. In removing it, it will affect the power thru that section...

Do you have a 5.6uH to 6.8uH choke around?

Why?

Similar reason....that seems to be the APPROXIMATE value you need...put it in - in series with that resistor like you'd use the bead, just the Choke takes its place.

Seen this choke used directly (in value) in several other radio chassis in the cobra line, Realistic, and Midland.

This takes it's place in a 3 part divider system and the "bias resistor" is the one fed directly with RF but also provides the working value to take some of that NPN's Rectified RF power away from the junction and into this type of circuit...

It's RFC 303​

To keep the description short, they use a ferrite bead, just to choke off the RF that would otherwise be lost to ground thru that 22 ohm resistor - they keep it to the feeder line to the Final.

The ~5.6uH to 6.8uH choke provides the DC drain-off to release otherwise pent up RF energy getting rectified, that can (and eventually will, without that DC short the Choke provides) make the Final "Latch" - it keeps it from blowing up. The 68 ohm Resistor and 330pF Cap serve to work as a selective filter to drain off the 2nd order and above harmonics arriving to and found within this strip.

As you can see that 3.3 (3R3) resistor is pretty low, and makes this part drive pretty hard, this value when you decide to experiment with it, you RAISE this ohmic value - and you'll see it drives less into Class C and more into an AB2 realm - but since you're "Class D" the modulation part makes the use of the AB2 realm more of a swing event. It's what you do when you mess with the JP36 as you'll find out soon enough looking for repair work out here in this forum. So for safety's sake, just use the 6.8uH in SERIES with that 22 ohm resistor.

IF you feel it's necessary to go the IRF 520 route, see attached...

 

[nomadradio]

Now THERE's a parts kit someone needs to produce. Turn your RFX Cobra 29 back into a stock radio when the RFX torches itself the last time.

73

 

[Signal Kicker]

Now THERE's a parts kit someone needs to produce. Turn your RFX Cobra 29 back into a stock radio when the RFX torches itself the last time.

73

That would be a great idea! Perhaps for other transceivers as well.

Andy,

You are making this to easy for me, thank you!
I will have to look in my parts bins or I may have at least one junker of a radio
that I had saved, maybe I'll find something there.

Using an inductor calculator with a wire at 3.5 inches long with 7.5 turns
at .125 diameter and I think it was 18 gauge, it calculated 6.179E-003 microhenries.

It sounds like I am using the dimensions for L12 that you had suggested, imagine that!

I found a few chokes at first glance, but they are the typical 2.5uh that can handle more current, something you would use in a tube circuit.

Anyway great info you are sharing, this will help a lot.

Thank you.

 

[Signal Kicker]

Scratching my head here!

I didn't mention this before, I thought it was a easy fix. The fuse was blown in the RFX75 and when replacing the fuse and powered on D8 popped and of course the fuse blew as well. Obviously I should had used a smaller value fuse!

Anyway I replaced D8 and did the Mosfet transplant and added parts that was missing from the RFX75 install.

When powering on, D8 popped again and T1 Mod Tranny really had got hot! I thought maybe I shorted out the T1, but the Pa still modulates and I am able to control it with the mic gain. Any clue what would be taking out D8? There's definitely a short somewhere!

Also there are different schematics available, the 29 classic is 2007 series but the schematics are not created equal! They are very similar though!

Thanks for any insight.

 

[Handy Andy]

Ok, I don't have your unit in front of me, so what I can work with isn't much...

Ok, this D8 issue usually tells me that the MOSFET work done, is shorting out the Tab of a MOSFET to ground.

Pull D8, on the Solder side, D8's two holes, check with an ohmmeter - look for a dead short from the BANDED side, to board ground.

Do the same for D8's hole that had the UN-banded side, to board ground.

Convenient place to use ground is the Transformers tabs that solder and hold the coil in place. That's foil ground - just check from D8's holes - both holes one at a time - one lead to hole other lead to ground - to ground from there.

And which side shows short?

If it's both, we have a problem.

 

[Signal Kicker]

Hello Andy, thanks for jumping in here again.

Okay I have continuity on the Cathode ( - ) side and no Continuity on the Anode side ( + )
Its been a while but if I am thinking correctly this insures me there's no short in the transformer of T1 ? Is this correct?