Cobra 29LX power issue

[longhaireddwb]

OK, I'm not talking about a blown protection diode or anything normal here. I hope.

A guy brought me this radio asking if I could fix it. He said that when he turns the key off in his truck then turns it back on the radio resets back to channel 1. It sounds like something that some older radios had where they had two power wires. One constant and one switched. But as you know the cobra is not this way anymore. OK, maybe the 19 or a couple of the throwaways are still that way.

Would you think this is normal, or is it possible a cap is not holding enough to keep the memory, or...
Any help with this would be nice. I don't see an issue but he says it wasn't this way when it was new. I can't say it was or wasn't.

So what do ya say?

 

[longhaireddwb]

I called a buddy that uses one of these radios and he says it holds the memory of the channel overnight in his truck. So, I see there is an issue. But no idea about where it might be. Any hints???

 

[9C1Driver]

I would imagine they use a cap to hold the channel memory when the radio is without power. Should last for several days at least, maybe this cap has failed.

 

[longhaireddwb]

Thsnks for confirming what i was thinking. Now if i could figure where it might be in this circiut. Guessing connected to ic1?

 

[999]

My guess is that its not going to be on the main board, but with the control IC. Pin 8 is a RESET pin that should get a 5V signal from IC302 (5V regulator). My guess is the fault will be in that circuit somewhere.

 

[longhaireddwb]

Thanks for pointing me in the right direction. When i get a chance i will test/replace a few caps. 303, 306 show in your pic so that should get in close. I will post what i find.

 

[Handy Andy]

I concur with 999...

The LX schematic shows D25/Delta Tune...

So if you're having a "reset" problem...

The question I ask is, was there any mods done to the radio - if so, can they be the ones hat cause the Reset on the CPU?

Why mention D25 - that line is monitored by the CPU to say Transmitting or Receiving...PTT overrides, but the two lines I'm talking about are it's own WX function.

IF the Delta Tune mods been done this affects D25 line to the CPU - you'll still see TX and RX - but the CPU looks to D25 even for WX to blare out the speaker....

OR, Do those mods, DRAIN off the power to the CPU's own? Not sure - but if D25 line from the Delta - tunes' been modded - it can affect operations like Power Down and thinks it's need to clear a function and so it reboots fresh reload next power up.

Now B+ aka Switched power - still goes to the main PCB TO CPU board.

It's supposed to execute a power down sequence to SAVE settings when the power is switched off. The 5V feed is supposed to help provide that "delay" of time from the power off to the power DOWN save sequence. Look for BAT_DET - you'll see it's the only line left unprotected - ouch...

That's internal to the CPU, that has it's own Keep Alive memory - so if that's gone - there's your issue. that would be at IC302 - using cap C302 and output cap C303 - now if the truck spiked the power line with 18 to 25 Volts (starter issues) then the caps that take in power from battery feed from Main PCB - blew - no longer takes in a charge.

Or worse case scenario - and possibly more likely - the CPU got spiked by the Voltage spike - for Pin 13 (P63)MONITORS for Battery - Battery DET function - so it decides whether to SAVE or Reset - and if the CPU can't seem to store any data - it simply "reloads" on power up because the internal memory is corrupted.

If possible, look at that lead to the CPU - that 120K resistor that is supposed to "Buffer" any spikes, may have opened up - so then you may be able to replace the chip resistor and then just refresh supply of caps for the 5V+ regulator and you may be good to go.

It is my hope that the 120K resistor opened up protecting the CPU from any further damages.

...OR...​

The input smoothing filter cap C302 47uF / 16V <<Big Clue for 24V spikes - is potentially toast.

So the Resistor - being blown/or cap - then tells the system that the BAT_DET line is "Dead" so the thing would presume - everytime - to boot with root to reset.

 

[longhaireddwb]

I will need to read thru that post a few times to make since of it. But i can say its a stock radio. Has not even had the covers off until i looked at it.

 

[Handy Andy]

Hate to add fuel to this fire but there is also the MAX version which when you look at the CPU layout - this CPU is the upgrade to the original.

Base functions - still there and pin out doesn't vary by much so the CPU looks be more of a drop in upgrade...IT IS NOT...

Wanted to point out though...

That 120K resistor / 39K resistor divider - IS NOT on this schematic. They did away with that Memory Keep Alive feature.

So if you cannot find the parts I'm referring to in the earlier post - look at the CPU number in this post to see if that matches your radio.

Then also for a heads up to everyone else...

Remember a few months ago I was working with my former boss and they were discussing the AT-6666 where he tore the ribbon in trying to show others how it works. IT can use a USB to program memory quik-access features to help you in setting up the radio with your favorites.

Another post was talked about the USB ports and some CPU units in these radios are programmable - your USB cable communicates with the CPU for this - Cobra has a similar function but as you'll see in the CPU schematic - there is a TX and RX programming feature - including RECORD /Playback function.

So the CPU isn't pin for pin compatible, but the functional blocks around it are still the same.

So the CPU has a few extra bells and whistles - and some features may not even have a trace for them - they're not part of that support level for the board it's being used with. But then too, they may have upgraded the board to fit this new CPU if only to provide a more durable - consistent part for their newer LX-based platform they seem to be heading in the direction of.

Just letting you know this is the best we can give you to help you - I only hope there's something in all of this that can help you fix that problem.

 

[longhaireddwb]

Thanks for the replys guys, but work has me running hard right now. Havent had a real chance to get back to it.
This radio doesnt say MAX on it anywhere so i dont think its that one. I did poke around and look at a few caps on that smaller board between the face plate and main board. Nothing stood out. No testing just looking to see if a cap looked like it was splitting or bad solder joints. Nope. Also seen most of the componants on that board are SMT. Beyond my abilities. That stuff is too small for these old shakey hands.

I hope to get back to it Wednesday but time will tell. I think i have another radio i can swap boards out with. I think that is my best bet if the problem is on that board. I like the old stuff better. Stick it in the hole, turn it over and solder. This tiny stuff is better but not for me.

 

[Handy Andy]

Then if its' an older one - usually one of two things is going on with that CPU area...

The on-board memory (non-volatile stuff) is bad, or as said earlier - those (d*mn) SMD resistors have failed...

Then so you know, it only has a limited lifetime left - the "reset" can only happen so many times before it just quits.

However the thing would use a WX monitor function when it still had power for the user if they set it up to warn them with alerts which means the "Reset" tells me that feature may be the reason why it failed - keeping power applied all the time.

Don't get me wrong - they are a good radio but this kind of symptom it has me thinking that this radio is on it's way out. It's unfortunate that this happened - so plan ahead to get another one. Before the thing forgets after too many times of "Reset" and if that follows my own experiences with them - the thing won't even power up to save your neck.

One case across my desk was from close proximity in lighting while driving in the radio using the radio - so these things are not immune to the static pokes from natures' best. (dry air, Mods from removing the Static Bleed resistor things like that can reduce it's lifetime) Not saying yours has had this type of work done to it, but people shortened down the life of a great radio just trying to get a few more watts out of something that they spend a LOT of $$$ on only to have it spend a lot of time in the shop or gets used for parts when it finally quits.

 

[longhaireddwb]

I hear ya. Give me an old Unidan any day.

 

[longhaireddwb]

I gave it back to the owner and told him to think about buying a new radio. As this one isn't going to last much longer. Then he tells me that he already bought one. A stryker of some kind. So I guess I got another radio for the parts box. I might still try to swap those boards out just to see what happens.

Although I have another one that the lights won't power up on the display. The text is there just really hard to see. Maybe I will swap displays? Options.

 

[Handy Andy]

It uses an LED Module - pretty specific for that Radio.

Stryker and Anytone are the others ones that use similar technology, but because of the Bezel - not sure of those sources can provide you the parts you need.

 

[longhaireddwb]

Maybe you miss understood, or I didn't explain well.

He bought another radio so the cobra 29lx he gave me for parts. Just to have on hand. I do have two of these 29lx radios in my parts box now and thought I could put the two together and make one that works properly. But, I've changed my mind.

He bought a Styker to replace his cobra. Wanted big power. I just don't get this big power mentality. But to each there own I guess.

Thanks to all who posted up and gave me a direction to go to. In the end it just came down to my abilities not being good enough to play with SMT and the fact that these cobras are a dime a dozzen.