Cobra 25 LTD Classic, no voltage change tuning

[Handy Andy]

Hmm, not sure of the "messing around" with it part, but stick with it.

Take a break from it every now and then - meditate about what were doing here in this thread trying to help you figure out the problems and get it straightened out.

Thanks for getting a Dummy load, only hope it can handle the wattage, didn't see any "ratings" on the front of the package, can only hope it can handle 5 watts.

Seen similar units wind up bad from just too much power or a long key up testing the radio - so always check to make sure it's 50 ohms BEFORE you use it!

 

[Mark A Bradley]

I have some older works from the CB Tricks days that may be able to help you decipher the wiring problems people seem to have with these radios.

Don't know why or how, you got into this predicament,
But, in your situation this info can possibly help...

View attachment 45558​

Pay close attention to the Mic plug to board wiring.

Schematically- they do apply several "RF bypass" caps...

View attachment 45560

So be aware of the Case to Board grounding issues and
know too, that they route wires to a spot on the Main PCB, that
is ISOLATED from both types of grounds
- Foil and Case (Chassis)
and they branch off from there...
​

Thank you I will look into it. Love the photo,,,

 

[baconater4749]

Hey,Andy I have to apologize and thank you at the same time, when you see what I am talking about. Our lovely 29 we have been trying to fix, someone pulled a ( they think this will be funny ) prank that I am the ending of it. Look at the pictures.And with the Caps,I just started,they or I changed every Cap and put in a S--- ones,and there are Diodes missing,and I put a new Diode in D11,nothing exploded,but there is Voltage running through it now.
And the best for all,the bunghole changed all the Pots also,as I wondered why every Pot was stuck at 12 mV,and non of them moved when adjusting the pot. I looked at all of the 29's I have in the locker,and all were the same but different than what is in this radio. You can see the Pot on the table by itself is what goes in the radios, the radio with the Pots are the ones that do not change?And the TX won't adjust either,is it the Pots,or something else?
Andy,is there something they could do to keep the pots from changing Voltage,I need to look. I have all the Caps done,and every one was the worst damaged Caps I have ever seen. One 470 26V had a 20% loss on my meter. That is terrible ,have you ever seen Caps that were all bad,I means severally damaged,and 2 -47 or 470 that were 6.3V, and the radio worked for about 5 mi Utes,then silence. Boy.I guess I am the Butt of his joke. I am feeling very sick right now,and being out in this heat don't help much. Talk soon,boy this person's did all that work to play a joke, I am assuming.
Now,do you think also I will have to change Pots,I do have a few junkers complete with all the Pots. Let me know,going into the Air-conditioning now,this heat is starting to suck. Talk soon, thank you again for all of your knowledge. I have 1 more question,but I will wait till later,oh and the fake Wax around everything
OH, THIS US IMPORTANT.
IC2 has been changed,you can see the dried up flux,and the IC Chip is on crooked, as one side is on further than the other,so obvious. I do have about ,5 of the TA7310AP in my stock,along with 40+ TA7222AP Power Amp's in stock. Cheaper in China,work just fine.

 

[Handy Andy]

There's a lot in the message, but thought this thread was about Cobra 25's ...

The AGE of those caps - and their ESR you're seeing now, is about time for them to give up - will need a recap.

I read you did change a lot of them so good job, I'm also confirming with you why they should be changed.

They will fail and if not taken care of, those caps can burn out the parts they work with - take out the stage if you try to keep them in there. Best to get rid of the older "grey can" caps - replace with new - it would not hurt to locate some 105C Heat/Temp rated types. Their a better quality - you might like the results better when it comes to audio.​

Now you showed me some pics of the Old 29 you're working on, or the other working on, either way it seems you're getting a better handle on things and the issues around the abuse it got - good to see you're sticking with it.

The Coil Can without the slug, couldn't find any reference in your post, then I see you said POTS - I think you mean the IF can - they removed the slug - that seems to be one of the many issues you have - no slugs to tune/change the tuning within the coil.

Is there a Fix for that? I don't know - might have to scavenge one off of a donor radio and see if it will work - no guarantees.

Now, D11 in a Cobra 29 is the LIMITER Diode, hope you installed it correctly. If it's not, it Won't let you talk, but you make a carrier - just no audio to go with it.

The TDA7310's those Mixer Amps - Cobra still kept a lot of the older design, but made a lot of resistor changes to match their newer supplier that replaced these. So if you have an older schematic you might want to look thru the PLL' VCO of both radios to find the resistors that they changed.

Because to make the newer radio work with the older Mixer chips, the Bias may need to be adjusted to handle the older - better - Toshiba parts.

A side to side comparison might seem a waste of time, but in reality it's not. In comparing the two, Old versus New, you can see how they revised the VCO to fit a newer tighter design specification.

You do have some work ahead of you.

BTW - this thread is for the Cobra 25's - I think you should go back to the old 29 thread you started so we don't confuse the rest of the group looking for answers about their 25's.

 

[work in progress]

OK,I will clarify both posts,as I got lost in the mix. The 25 had a bad Cap, and I had to replace R 22 core,the pot was not reading anything,so I decided to throw a Core at it before I changed the Pot, and it fixed that.I shall straighten the mess when I get a few minutes. Trimming Bushes now,big heavy, thick privacy Bushes,and it is 93 degrees out. I am not used to this stuff,the work and heat.
I have 1 more question, I have been trying to get 10.695,TX frequency I assume. But the Frequency meter, the radio won't,t come off 40,000+ MHz,I tried turning everything with no luck. I turned the ones I should,and nothing did nothing. I messed with, L19,,L20 ,,andL23, two of them had the wax removed,just enough to put a screw drive in it. ( The pots,19 and23 ). Any ideas,or is it me, but this is on 3 radios, and the one with the extras, I have that going, needs the same thing. Any suggestions sir. Back to the Bushes. Thanks man.

 

[Handy Andy]

The 10.69 signal? That is the IF RESULT - is is what you get.

So that means L19 / L22 / L23 and may even L24 are not working. But again, I asking about D13 and or D16 If you have Delta tune..D13 is the varactor that makes L19 produce 16~17MHz so you can EXTRACT 10.695 for IF then the main Xtal clock at L24/L23 is used that 10.240 is SUBTRACTED from the 10.695MHz to make 455kHz where L6 / L7 VR2 IF Gain control handles the last stage of that 455kHz to L8 and L9.

So in light of your situation again - for 29's - here's a reminder...

​

This will be the last time I hijack a thread about 25's for you and your issues with 29's - you should start another thread...

 

[baconater4749]

The 10.69 signal? That is the IF RESULT - is is what you get.

So that means L19 / L22 / L23 and may even L24 are not working. But again, I asking about D13 and or D16 If you have Delta tune..D13 is the varactor that makes L19 produce 16~17MHz so you can EXTRACT 10.695 for IF then the main Xtal clock at L24/L23 is used that 10.240 is SUBTRACTED from the 10.695MHz to make 455kHz where L6 / L7 VR2 IF Gain control handles the last stage of that 455kHz to L8 and L9.

So in light of your situation again - for 29's - here's a reminder...
View attachment 46010
​

This will be the last time I hijack a thread about 25's for you and your issues with 29's - you should start another thread...

Sorry for making a mess,I am getting lost on the transaction of a 25 or a 29. I purchased the 29 thinking the 25 and 29 were similar,but now I know they are way different. I can start a new 29 thread,and do the 25 clarification, so no one gets the radios mixed up with the adjustments.
You give me the go ahead, and I shall open a new thread on the 29. What did you have in mind?

 

[Mark A Bradley]

This is what I found on the 29 LTD Classic,2 days of messing around.OK,now with the 29,I found that the L24 can was bad, the can would not let me get the half of the first frequency, 27,185 MHz.I was able to get the receive frequency,18MHZ OK, I also found that the 10.240MHz crystal was acting up,would not let me lock in on the first stage also,I was able to move the frequency counter on the second stage of the transmit frequency also,,,not the first half.at this time, the frequency counter was going all over, the Crystal straightened up the searching of the first stage, that may be a indicator or a bad crystal,10,240 is bad, replaced, all settled down.
Keep an eye on the first stage of setting transmit, the frequency will run all over, look at cans L19,L23,these were bad. And the 10.240MHz crystal also needed replacement.
After doing this, I was able to get the first frequency to lock in, 10.240 locked, 27,185 locked, had the first and second half of the first sequence of setting the transmit, receive basically fell into place after setting the first frequency, the second half will come with the first half. This is what I did find messing with the radio for 2 days. 1 ,10.240MHz crystal, and a pair of cans L24,L23, for the transmit side of the radio.
This is all I can see with all of this questioning and hair pulling at this time. I would like to thank Handy Andy for all the help, he is a CB brain, would love to have a friend like that,a brain. This is confusing at first, but you have to put your mind to getting it going, I was scared to mess with anything on the radio, but after reading everything over, I dug into it, and that is all I found at this time.
Make sure you do some reading of ask questions before digging into a radio, it can be frustrating if you are a newbie,but hang in there. Again,thanks to Handy Andy for all his help and brain work, he is the life line of the Forum. I appreciate all the help from the other guys who were having fun with my posts, it is all in fun. I am a Kid at heart, but a go get em attitude.
I am going to open the radio back up and double check all the settings,also double check the can numbers.any issues, I will post them for you. Thanks to the Forum, and all associated with the help to all you radio heads out there.