Cobra 29WX ST with several mods.

[JDE]

No pictures yet, but I've already removed most of the components I've identified as mods or part of what appears to be a MOSFET kit? Here's what I've found so far;

-The final was replaced with one that says IRF520N on it.
-R56 was replaced with a 410ohm resistor and 1n4148 diode added to it.
-C53 was removed and replaced with a 3.3k ohm resistor, appeared to have a second resistor on the foil side(forgot to get value).
-R100 has an 1n4148 diode added to it.
-C61 was removed.
-D4 and D24 both replaced, haven't pulled the diodes yet to determine type, small and bright blue, not glass.
-R48 appears to have a second resistor added on the foil side, 150k ohm.
-L14 slug removed
-Talkback resistor was added.
-Bandit Echo Board was added.

Most of the work I've identified on this radio I think may be explained by the "MOSFET Supertune (29LTD classic and newer)" mode on this page - https://www.dtbradio.com/tips.html- or something similar to that mod.

Before working on it it did receive and transmit, albeit poorly on both according to my tuned/aligned General Lee monitoring radio. On the Bird 43 it looks like it was dead keying at 4w and swinging around 15, but I think the peak kit I installed isn't being honest about modulation. Was spiking to 30w+ in peak mode on the Bird.

I believe this is a '97 model and I have a '98 or '99 laying around but it's a different board version so I'm not sure how identical it would be for me to use it as a reference to figure out the components I need to get this one back to stock. For some reason I always have a hard time trying to figure out proper replacements for those little orange disc caps. I'm guessing the missing slug is not something I can easily find other than from a parts radio, and how important is that slug?

Will get some component and foil side photos of this radio next time I can get out to my workbench.

 

[JDE]

Got component and foil side photos of this '97 29WX ST, they're attached. Trying to figure out if there are any other modification components remaining so I can remove them.

Still trying to find the factory component values for the final, R56, C53, C61 so I can get replacements. I think the resistor at R100 is the factory value, they just lifted one end to put a 1n4148 diode on it.

Also need to know why they would have replaced D4 and D24 with a different type of diode.

Already replaced both 1000uf caps, and D11 because it was clipped and resoldered at some point in its life.

Note the solder blob on top of the big audio transformer. Whoever worked on this removed the protection diode and the ground wire, then put a diode between the ground lug of the power socket and the top of that audio amp shielding. I put a new protection diode across the power socket, then tried to run a new power wire but the hole in the PCB was shot/burnt so it's just going to the transformer shield. Tried to clean up the top of that shield and remove the leg of that old diode they had there, but couldn't get the solder to melt very well because I think the shield is acting like a heatsink and I didn't want to cook it.

 

[999]

R56 - 22 ohm
C 53 - 0.001
C 61 100P/SL

That's what I got off the schematic.

http://73.235.100.209/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29ltdwx-st_29ltd_sch.pdf

 

[Handy Andy]

Ok, several things going on...

1st - D24 and D4? Changed to Schottky-styles - Simple "N" junction - but noisy and since you don't have the original Germanium the OEM would be, you might be stuck.

Germanium has a Voltage drop of about 0.4 to 0.3V DC - pretty low making it effective for "detection" - Schottky is similar but Silicon and noisy due to the construction and its bandwidth.

Bandwidth? Yes, Germanium looks like a very leaky diode at High Frequencies - so even though it's PN construction - gave the diode a desired trait of a built in hi-cut filter when it came to noise detection and Detector output.

Check TR7 - looks like someone worked on trying a 2SC2999 mod, might want to review that...

gotta' eat dinner - been busy...​

Forgot to mention,

TR7 is OEM-wise, a 2SC1674 - does not look like they did much else to the section - but D24 is part of the Noise Blanker circuit, it simply "looks" for Broad banded noise and then uses a Diode for peaking that noise to a working level of voltage, then amplifies it non-linearly - more like switch mode - which when you use a Germanium diode - tended to look more for the lower-frequency spark noise like you'd have with older cars. So, the Schottky is - to me, not really an upgrade - just a swap of a part that pretty much worked for what it has to do, and this was more of a waste of time for the tech than a working upgrade.

As far as the power supply, the "Minus lead" soldered to the Power Choke is where I would re-position the Minus lead - Why? Esoterics mostly - the Choke is the main filter - looks like a cap might have been removed by someone. It can be left in there - or replaced with another - I use 1000uF or 470uF of a 35V rating. Whichever will fit the lead spacing.

 

[Handy Andy]

Other things?

Well? See where they did do so work to the "Pi-filter/52MHz" strip - mostly to try and peak using tweaks to the capacitance used at specific spots of the Pi Filter - so schematically it seems to follow what the board has.

I'll let you discover those "Varactors" - you have two and this was a older board with the "WX" piggy board that has the tone detect and the multi-channel selector. So you did get a more "prized" one that most - these usually don't survive long because they get butchered or sold off as parts.

Why? The older 258 board is their (KEPC) earlier works which cloned the Cobra Uniden brand more closely and used similar parts - so that is why it's prized, more "faithful" to the original design...

However, I digress, the board looks pretty healthy otherwise - look for "fresh work" on the solder side by looking and finding shiny work versus the older "glazed over" ones that show age and oxides. nothing stellar to report but the fresh stuff is where you need to focus on your efforts of bringing back to stock.

It's a keeper, labor of love - of course, but worth it to keep, use and learn from.

One of the things I learned from these kinds of board, the KEPC versus Uniden - was the noise floor you originally had.

IN my research, the noise floor came from the direct supply from the Power input versus the 8 volt regulation side - this changes the noise level present in a typical C-29 of KEPC as Uniden took on noise differently than KEPC did.

You'll see where the RF Amp strip that is the RF, IF and AGC/Detector section - is not regulated - only filtered and sent to the 13.8V battery rail in several sections of the radios' receiver.

This choice has me scratching my head for the lack of concern and to tell the buyers it is considered an improvement (indirectly) tends to mislead a customer into thinking all that extra noise is signal I can now hear...

Not really - but then who am I?

 

[JDE]

TR7 has altered solder joints. Has "C2999 041" marked on it.

C71 has altered solder joints, currently has 33uf 50v in it.

R48 has altered solder joints currently has 220k ohm resistor in it.

C53 I found where they moved that cap, put it back in C53 spot.

Need replacements for R55, R56, C61, and the slug for L14. I don't have any radios dead or damaged enough to be used for parts, yet. This radio is actually the worst so far.

Not sure if I should remove the 1n4148 they added to R100 or if it's safe to leave it. There is a 2.7k ohm resistor there, unsure if factory, and its leg is no longer long enough to reinstall through the board.

What model(s) of transistor can I use that would be 'factory spec' to replace the mosfet final I yanked out?

Definitely want to try saving it. I have another, a '98, that works fine and only needed a meter. Both of these are actually going to be used, one in my XJ and the other in my buddy's pickup.

 

[Handy Andy]

Final? Uses a 2078 - just like the Cobra 25, Cobra 18 II - Cobra 19 DXVLVII - Superbowl Edition - pretty much the "Standard" for CB radio is the 2078.

See Attachment

I recommend, to get your radios receive back - buy and install a 2SC1674 and replace the 2999 with it.

As far as the "other mods" R100 needs a 2.2K or 2.7K, but this is for a power regulator. R100 controls "bias" to TR21 the TX switch.

Are you sure its R100? Not R71? D11?

• - the Diode was designed to "trick" the power flow across the resistor - making it act like a one-way valve - trying to imitate a compressor - making the volume of the voice peaks which can swing negative - if the user wished to keep using or keeping the limiter in - flatter to make the audio volume appear to push (swing) upwards.

 

[JDE]

R100 was definitely lifted to add in a 1n4148 diode.

Because D4, D24, and TR7 were replaced it looks like they did the schottky receive mod I've seen posted in some places, including here. Pretty sure that mod was debunked as not being beneficial at all. Not sure how much of a pain it will be to find OEM replacements for those diodes, but TR7 is all over ebay.

The vendor on ebay I use for parts has plenty of the genuine 2078 finals in stock.

I do have two late 90s 29NW STs on the shelf that both work, but these non-glowing nightwatches are a dime a dozen all over ebay so I could butcher the worse of the two for parts if needed.