Cobra 29 LTD : 40w mod, now no TX/RX?

[office888]

Well, I had enough with getting walked over like crazy and my range being terrible, even with a 102" whip.

So, I decided to do the EKL 40w mod.

Now, I don't have TX or RX. When I first turned the radio on, it had feedback through the speaker (LOUD!) for about 3 seconds, then shut off.

Then I realized that I didn't insulate the new ERF-2030.

so I installed the little plasticy piece of insulation and tried again, still no tx/rx.

Think I blew something up? Any ideas for me to check?

I really want a semi-decent radio setup, but I keep having setbacks. =(

Thanks,
Richard

 

[fnkycoldmedina]

if you forgot the insulator its very possible you blew the final as soon as you turned it on .it can be blown and may not look it .
someone who knows how to check it will chime in i,m sure .

 

[loosecannon]

did you also insulate the screw that holds the transistor to the heatsink?

you probably did blow your new final.

remove the transistor from the circuit by unsoldering the legs from the board and make sure they arent touching anything.

do a google search for "transistor testing" and also add the word "diode" in the search.
you should find quite a few tutorials on how to check a transistor for opens and shorts using the diode function on your DMM .(digital voltmeter)

or you could just buy an IRF520 for about a buck or less and try the new part.
they are the same as the ERF2030 for all intents and purposes.

make sure you use the companion part correctly also.

good luck,
LC

 

[Robb]

He also stated that he has no recieve. That shows another -or more- failed part as well. Once he does the replacement final correctly; he will still have no way to determine if it is working.

888 - you need to hook up a speaker to the PA jack - and turn the PA function on. Test it for audio. If you have sound; your audio chip is OK.

Figuring out what part of the receive circuit isn't working may be problematic.
Perhaps loosecannon can guide you thru that troubleshooting and repair?

Turn the radio on, and turn it to a known busy freq. Are you seeing any movement on the RF meter when the antenna is hooked up? If it is moving - then the receive is probably working. If you cannot hear the receive; then there is a failure in the audio outpuit device...

 

[loosecannon]

LOL Rob, im not much of a troubleshooter. pretty good guesser though!

yeah, i missed the part about his receive not working either.
i think the audio chip is a pretty good guess.
especially with the way he described this happening.
loud squeal, then silence. sounds like an overloaded audio chip gave up the ghost, but, again still just a guess.

office888, go to cbtricks.com and find the service manual for the 29LTD.
in there you should find the voltages for each pin of the audio chip.
(the audio chip is the one bolted to the side of the chassis)

then you can turn your radio on, and check each pin with your voltmeter.
if the voltages seem way off from what the service manual tells you they should be, you might try replacing it. (just make sure you insulate the new one LOL)

there is a chance that other parts down the line may be the cause, so if replacing the final and the audio chip does not fix it, then you will have to find a tech to troubleshoot the problem for you.

good luck to ya,
LC

 

[Moleculo]

Just for kicks, try checking TR17. I had one of those that was shorted on the solder side from the factory. That transistor is the tx/rx "switch".

 

[office888]

Wow, thanks for all the support, I'll throw on the multimeter and see what happens.

I just threw on a 2SC2999e + Schottky diodes last week, made a huugge difference in my receive.

Funny part is I just did an audio amp on my other radio, a 29 LTD Classic.

I bought 2 x 40w kits, so I've got another ERF2030. If it turns out to be blown, heeyyy, I've got a spare. I'll be sure to remember the insulator this time around. Everything else was correctly installed, even still had the insulated screw.

I really do hope that it turns out to be a blow final + audio amp, that I can fix. I've got a 300 mile trip to take on Friday, and don't know what I'd do without my CB!

-Richard-

 

[office888]

No go.

Replaced the final with another ERF2030, and also replaced the TA7222AP audio amp.

Still no receive, no transmit. PA function works. Weak static on external speaker when I plug some headphones into it.

I followed the instructions EXACTLY.

I removed R55, R56, C61.
EN1230 @ R56
68pf cap @ back of C59
Removed L14 tuning slug

Am I supposed to put jumpers in between the things I removed or something?

When I removed L14, I desoldered the ends and removed the entire cylinder + copper wire.

-Richard-

 

[loosecannon]

put L14 back in.

i cant say this is why the receive wont work, but the transmitter wont work without L14 in place.

dont use any jumpers where the parts you removed were. just leave them out.


after you have it working, tune the slug in L14 for max PEP wattage while speaking into the mic.
you might find that removing the little black slug from L14 increases the power some.
this works only in some radios.
if power is less with it out, put it back in.

LC

 

[Wire Weasel]

No involving your immediate problem....but there is one more step in installing the TO220 transistors. All the parts stores will also have a little tube of Heat Sink Compound. This should be applied on back of the final, then put on the plastic insulator film, then apply compound again to back of insulator film so the compound will get applied to the body of the radio where the final mounts.

Sometimes you see plastic screw & nut sets which is fine. If using metal, then there is little plastic separators that go around the screw to prevent electrical contact.


Good luck!

 

[jazzsinger]

First thing to do is go here and download the schematic:
http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29ltd_sm_sch.pdf

Without going into too much detail i'll explain why you have lost rx.the incoming signal from the antenna passes through the swr meter sensing strip first,then on through L12,L13 and L14,after L14 it veers to the left (TO THE RIGHT IS WHERE THE TX SIGNAL COMES FROM THE FINAL AMPLIFIER TR14) and up to c8 and is coupled into the front end rf amp (tr7) via C10,L2 and C13,at tr7 the signal leaves the collector and heads too L3 and onwards to the fet mixer where it is mixed with a 16 mhz range vco signal and the resulting 10.7 mhz product is taken on through the IF strip.


As you can see when you removed L14 completely you broke the signal path for rx.i don't know about the mod itself as cb mods are generally a lot of crap,but when it said remove the slug they meant the black ferrite tuning slug inside the L14 coil,not the coil itself.

to be honest with you i don't see the point in doing that mod,as the power gain is probably minimal and 2sc2078 final you are replacing is still commonly available and relatively cheap,messing with power amplifiers without at the very least an oscilloscope but better still a spectrum analyser is unbelievably foolish,you may see a bit extra power on your power meter but the harmonics could be all over the place and be the real reason for the extra power reading,power meters CANNOT distinguish power on different frequencies,they only read total power irrespective of frequency.

Best thing you can do is replace everything to the way it was and forget that mod.if you want more power buy an amplifier as changing the final transistor in a cb will rarely give you any noticeable increase.

i hope that helps you.

 

[Moleculo]

Yeah, when you said you removed L14, that pretty much made it clear what the problem is. When you put it back in, remember that the coil wire is coated. Assuming that you didn't clip or damage the bottom part of the wire, you won't have anything to worry about. However, if for whatever reason you find that you need to solder on part of that wire that wasn't on the underside of the board, you'll need to scrape away the coating.

 

[office888]

I think I'm about to give up on this radio and throw it on the scrap pile.

Reinstalled L14 coil.

Radio works like complete crap, pulling 0.1w deadkey, 0.5 swing with +120% modulation for about 1 second.

With or without the ferrite in, those are my results. No idea what's causing that. Need to source out some parts and bring this Cobra back to stock.

I'm afraid I might have fried something else that was grounded to the chassis, but it's beyond the scope of my electronics abilities to figure out what. You guys are my last hope.

Probably going to dump this and buy a Connex or Galaxy or something.

-Richard-

 

[Robb]

...Radio works like complete crap, pulling 0.1w deadkey, 0.5 swing with +120% modulation for about 1 second.

With or without the ferrite in, those are my results. No idea what's causing that. Need to source out some parts and bring this Cobra back to stock.
-Richard-

Sounds like the output driver is working; and the output final isn't.
That is what I usually see when a final isn't working...
...Dos centavos...

 

[DTB Radio]

I think I'm about to give up on this radio and throw it on the scrap pile.

Don't do that yet. Many CB shops do not charge to do bench checks, and some often include a bit of brief troubleshooting to get a better idea of what the cause/cost would be. I run my Carlisle, PA shop that way. It would be worth a bit of yout time to stop someplace and have it checked, even if for nothing more than to find out exactly what did happen, and learn from the mistake(s).