Cobra 29 LTD Classic NO RX/TX HELP!!!!!!!!!!!!!

[Buzzard78945]

Hmm. You've got a great head start - don't stop now!

You know you can make $$$ with those tools!


(... And I don't think anyone has ever done up a Tuning manual for the 29 like this... I hope it helps!)

Oh don’t think for one second I won’t be absorbing up all the info you give me like a sponge... it is an extremely great privilege to be working with you sir and learning... this kind of info is almost unobtainable nowadays, I’m a life long fan of yours for sure... you’ve put more effort in this for free than I’ve ever seen before in my life, I’m not letting this info go to waste and hopefully I can retain this know how and apply it to my hobby and keep it fun and interesting... I really appreciate it Andy... I just don’t know why no one else is commenting or taking advantage of all of this good info

 

[Handy Andy]

Keep going!

I hope to hear the good news you saved the radio - I hope you can use what's been put into this thread to help you get it working - else if you get any hiccups - be sure to do continuity checks when you come across a dead circuit - newer radios with the silver solder - they don't always make the best connections - Cold Solder joints are a major problem with the new NON-Pb standard...

​

Even a Simple Circuit tester with the older Lantern style 12V lamp inside the handle can help track down troublesome circuits and open solder joints or bad traces

Best of luck!

Enjoy your weekend!
​

 

[Buzzard78945]

Keep going!

I hope to hear the good news you saved the radio - I hope you can use what's been put into this thread to help you get it working - else if you get any hiccups - be sure to do continuity checks when you come across a dead circuit - newer radios with the silver solder - they don't always make the best connections - Cold Solder joints are a major problem with the new NON-Pb standard...

View attachment 35952​

Even a Simple Circuit tester with the older Lantern style 12V lamp inside the handle can help track down troublesome circuits and open solder joints or bad traces

View attachment 35954

Best of luck!

Enjoy your weekend!
​

THANKS!

I may hollar back at you tomorrow with where I’m at on the radio

 

[Buzzard78945]

Ok so far kinda good I guess... I’ve been playing with the receive I have it receiving Chrystal clear but I don’t know how far exactly or how sensitive... reason I say that is I’m not getting much static with gain all the way up and squelch all the way off....

BUT... I still don’t have a carrier or any output at all, I’ve chanced the driver and final with known good ones robbing parts from this other radio like a bandit ( I know they are good that radio RX and Tx fine just has an annoying buzz/hum I’m thinking it’s the night watch ) anyways I’ve spent an hour doing voltage checks and cap checks found D11 clipped prolly not good I put it back ( is that the limiter diode? I didn’t have time to find it on the schematic

Where do I go from here?

I’ve checked lots of main caps traced voltages checked diodes... I need a little shove in the right direction I don’t want to just start turning pots in the TX section I’m thinking may be voltage not getting some where...? Or component failure because of no carrier no out put at all...

Yes I’ve used different mics and checked the very basics couple times

 

[Buzzard78945]

I have red RX light come on the speaker goes quiet... it makes a speaker noise when you do key the mike... during the cut off of RX I guess... no movement on the meter at all, no carrier. I’ve verified the mike plug and connections are good and solid I’ve resistance checked The Dynamike pot and verified with Pa mode that we do have audio...

Again I plugged in external speaker to Pa port, switched to Pa mode and you can hear your self speak... what’s weird is when I install a resistor jumping Pa jack to external speaker jack I don’t have talk back...???? ( when I’m Regular CB mode )

so I’m assuming I have good audio chip function? And if I get a carrier I should have audio output ???

I JUST NEED ME SOME CARRIER!

 

[Handy Andy]

Well, CB/PA switch - if the radio wont TX the CB - PA switch may be faulty but first we need to even see if you get any power to the Driver and Final...

The Carrier would occur if the Modulation Transformer (that big transformer in the corner) was able to send power...

But it also needs to have the TX switch (TR21) send power to the Pre-Driver to turn it on...so you will have to do this check in TX mode and check JP32 - to see if it's sending voltage to the Collector of TR16 - the Predriver - it needs power from the TR21 TX switch to even send a carrier thru into the Driver.

Hmm, you have a voltmeter - so check the Final and Driver CENTER leg - to Foil Ground - see if you get 12V or nearly the SAME voltage as your power supply. ALL THE TIME...The only reason it doesn't TX, is because the Predriver is setup to send a signal that is strong enough to power up the Driver at it's Base, then the Driver sends it's signal from it's COLLECTOR to the Final's Base to power it up. and all the added power from these stages behind the Final gives the Final the ability to develop your carrier. If there's anything wrong with the above, it won't send a lot of power out so no carrier.

You also need to see if the Predriver is getting voltage...It gets 8 volts from the Big transistor I showed you near the first part of this thread. That big TO-220 - you said you got voltages from and said it was ok - well the proof is here at JP32 - when you TX the radio - does JP32 show 8 volts?

A photo below... of the Predriver, Driver and Final section on an older Cobra 29 - this may help you locate the test points you need to check...

 

[Handy Andy]

Then if those tests pass, then the radio may have an issue wiith LOCK or the Predriver (TR16) is not getting a signal from IC3 thru L20 - or TR16 The Predriver is ok, but CANNOT send a signal to the Driver because a resistor may have blown - see photo above and refer to your guide from the Service Manual you got earlier.

It is not uncommon to find radios with Cold solder joints but this is a 2003 so it may have other issues including age against it...

When you look for and check TR16 - it is a 2SC941, a simple low-power VHF RF amp - but needs a FULL 8 volts to send a carrier that IC3 makes from L23 and L19 - that goes into L20 - then onto L17 afterwards


Now I use a tool not everyone seems to know but I shine a light to help me decipher the connections the Traces under the board, are in RELATION to the COMPONENTS soldered to them above in the other side of the board.

Shining a strong light can be rather intuitive in finding the traces and their relative position and how the section even works - like I use below...


I'm working on more documentation to help you and anyone else still reading this...
BRB

 

[Buzzard78945]

Thanks

both driver and final driver have supply voltage properly I checked there and I was going down the line of schematic and I did a lot of voltage checks earlier let me get back with you in a bit on what else I checked but I spent bout an hour and a half checking items

 

[Buzzard78945]

What are the transformer voltages suppose to be???? I can’t find them on cb tricks

 

[Handy Andy]

Mostly they are DC, and for RF stuff, you need to use the scope and an RF probe ( it has a built in DC Blocking cap ) The DC side sends bias thru the coil into the Transistor or FET or IC depending on where the transformer hooks up...

It is safer to use the Transistor pinout voltage chart, and check it at the transistor - for if you do seem have a problem GETTING a voltage there to the Transistor - the Feed line trace to it - usually thru the Transformer - may have opened up - even at the transformer (coil) itself.

Remember the chassis is old.

Was it recently recapped?

• - remember C116 is possibly too old and needs replacement.

 

[guitar_199]

* grin *
Don't think for a second that these jewels go unnoticed!

I went to school for electronics back in 1975-77. About 7 years later I morphed into operating system software, then software development. As I am approaching retiremtne age I want to get back to my electronic roots. Still have LOTS of book learning and theory rattling in this old head... just no "practical application" to go with it.

That is why my "chief belief" is that a closed mouth gathers no foot! I stay quiet much of the time but watch for these pearls.

I follow these things, learning the "methodology". I do not care to be a "shotgunner"... I want to understand what is going on and use logic and thought-power to tie symptoms to a likely radio section, then circuit, then component. These sessions are unbelievably educational.

Lately when one gets really deep, I have been copy/pasting text and saving images, and making up a Word doc that follows the flow. When it is done, I save it to a PDF ( I just like PDFs for storing and keeping track of!).

These things really serve to 1) help me confirm what I was thinking as it progressed, or 2) show me where I went off the track and enhance my knowledge going forward. I am open to EITHER!!!!!!

I just don’t know why no one else is commenting or taking advantage of all of this good info

 

[Handy Andy]

As a secondary note,

Remember you have that Galaxy 55 - so you can use it to help see if the Cobra can even make a signal for carrier...

How?

Well, the Circuit Tester shown above? It has a nice long wire on it - you can touch the probe tip to L17 or even the L20 traces and see if the Galaxy - in SSB mode or AM - can detect the "scratchy" sound of carrier as you tap along the TX strip. Caution - pay attention Possible shorts can occur. The tip is capable of shorting out two close traces together - so watch your tapping, but LISTEN for the scratchy sound the Cobra makes in your Galaxys' receive. Leave the free-end of the wire long and away from any other sources of power or grounding - make it a cheap antenna for the weak carrier signal the Galaxy will be listening for...

I've even used Paper clips pinched in my fingers to detect the carrier or INJECT a signal into the radio at certain areas to use my capacitively coupled body as an antenna element

• Just don't do it around Tube powered or high voltage line equipment - that may kill you before (or When) you find the problem.

Now, another aspect, when you check the Voltage across the Transistor - like say TR16 - that Pre-driver - remember to look for Base voltage as well as Emitter Leg voltages too.

IF the Transistor does not have a strong enough signal on it's Base? IT may be from an open resistor or a bad cap in a divider network used to send RF into the stage. Yes, RF divider to control the amount of RF going into a stage. You want some but not so much - keep it from overloading the Base of the transistors' in that stage - you'll get spurries and some possibly hot parts - you'll see a difference in voltage form the Base lower or the Emitter side too high or too low - from their Service Manual specification. IF you get skewed or different results - time to investigate...

Now I've seen conditions around the Pre-Driver from ...

Blown Pre-driver - sees 8 volts but 0 on emitter and Base has .0.8 or 1.2V DC on it - a little high - meaning the Transistor blew open - replace...

What about 0V in TX?

• Check TR21 the 2SA733 PNP - if it's blown there ya' go...
• Look for L17 - possible open coil wind
• JP32 - rare but it can work loose from cold soldering done at factory - more of an intermittent problem.
• C116 - a Cap that shorted out dried out, due to age - pulling the power down in the circuit.
• Nothing can be found wrong with the Pre-Driver -scope the output of L17 - sometimes it is the RESISTOR into the Base of the Driver that goes - so it appears to be a bad stage, it's in the following stage where the problem really is.

So yes, pay attention to the Transistor Voltage charts...

 

[Buzzard78945]

i have no output of L20 at R63... pin 9 of IC3 is showing 7.5V

and its is in TX mode

i played with L20 a bit but no change...

 

[Buzzard78945]

ok im having a hard time understanding these voltages and why they should be that voltage...

TR16 is an NPN Transistor
the voltages at TX are suppose to be...

TX RX
S D G S D G
0 0 0.5 .9 7.7 0.5


whats with the SDG thing?
Is S the collector
is D the Base and
G the Emitter???/

and what does SDG stand for????

i understand the transistor is like a switch or a relay in the automotive world i work in... but why are there 0 voltages when the transistor should be switched on in TX mode?????????????????????????????????????

 

[Handy Andy]

Ok that's what I needed to know...

TR21 is a PNP a 2SA733 - it's your TX switch - it powers those jumpers shown earlier - like JP32 - that sends power to TR16 - your Pre-driver...which is an NPN

SDG stands for Source Drain Gate - and no, it's not an MOSFET - which would be the SDG. TR16 and TR21 are Transistors so just look at the test for "Voltage" a different way - you need BCE or Base, Collector Emitter - and the only "zero" you should see is when the TX Switch is on - the 'Mikes Socket - Pin 3 gets shorted to Ground - that connection made when you key the mike - all that the switch does - is to put power frorm Collector thru the Base - to Emitter from one power rail to another to turn on - or - apply power to turn on - the TX side.

It's easier to review the photo...


You just need to make sure TR21 is sending power to the spots it needs to go, thru those jumpers.

Look in the photo - you'll see I've pointed out TR21 and there is a wire that comes off the board, routes to the front panel - to the CB / PA switch and then onto the Mic Socket Pin 3 - Transmit.

What the CB/PA switch does is reroute power from Pin 3 TX at the Mike socket. thru into the BASE of TR21 which turns it on and then it sends power to the TX side of the radio - TR16 included along with IC3.

So just "tap" your Positive Test lead to JP32, and the NEGATIVE test lead to any Case of any Coil and note the reading when you key the mike. Does it go up to 8 volts? Then you're good - you have POWER going to the parts for Transmit - but if it doesn't key up in Transmit - then we have to figure out why.