Cobra 25ltd pll part # needed

[Smokinone]

Looking at the handy tester I assume the 2078 is toast.
I have good receive now, listening to the nonsense on channel 6...but I am grateful to be able to listen.

I did have a moment I though I lost the receive audio, but it was a broken wire on the external speaker...phew!

Looking at a 1969 final, I am sure the 2078 is no good as readings should look close to the below pic.

Getting closer to getting this working thanks to the help here.

The next step is to find a substitute for the 2078. Is there a drop in replacement, or what could I possibly use that would work with the 1957 driver? I have looked at the search engine offerings, but have found nothing other than an ELEFLOW 2078 from a guy in England, and some other ebay offerings that are questionable as I have gone that route before and got fakes.

I'm wondering if a 2sc2166 would work? They look close? Nope, answered my own question, it's a PNP

If I'm hitting the right pins on the T1, I am getting close to 400 ohms on both sides, so I think the T1 may still be functional.

Thanks.

 

[sunbulls]

The 2078 is an extremely common transistor. It’s also used in most 40 channel 5 watt walkies, cheap “help” radios, and many compacts that you can get for a song. To avoid fakes I usually reuse driver and final transistors I find in parts radios. Since you’re a DIY guy, it pays to have a few radios in your scrap box. They’re invaluable for broken slug and can replacement too.

 

[Smokinone]

The 2078 is an extremely common transistor. It’s also used in most 40 channel 5 watt walkies, cheap “help” radios, and many compacts that you can get for a song. To avoid fakes I usually reuse driver and final transistors I find in parts radios. Since you’re a DIY guy, it pays to have a few radios in your scrap box. They’re invaluable for broken slug and can replacement too.

Thanks. I ended up ordering a couple from epay from the multi star guy. He seems reputable. I checked a few of my parts radios but they all had the 1969 and 2166, 1306 and 1307 in them. Oh well, we'll see if this 25 will work after the new ones get here.

I just wish I had more knowledge of this radio stuff. I try, but thanks to the help of a few good guys I can muddle my way through. My late father in law was a tv/radio repair guy, and got me interested in this stuff. I just have a hard time grasping the circuits.

 

[Handy Andy]

That little tester you got is also sold in kit form - so if you put it together - nice job!

You're actually in pretty good shape.

Sorry about those caps, if you review them, by what I saw in your pictures were - well they looked like they had gotten hot or even burst (gassing)- you don't see too many caps cases shrink wrap "shrink" like that on the board unless there was a major failure.

One thing to consider in this, since you've replaced so many caps , good to know the radio still listens - you did great!

Now? We just have to get it working. By the sheer number of caps you're replacing - I'd be broke after the first two radios like what you've did in just one - trying to replace them all. Sadly this is why I said You may need to replace the caps" because of their age and once dried out - don't recover their values very well if ever.

Unfortunately, caps have a maximum voltage they can work with. So that means the radio becomes more vulnerable to the whims of the owners and installers. How? because of cost considerations - the manufacturer cheapens the radios' value by using working voltage ratings barely above the typical 12 charging system in a car. With the typical regulators used on todays cars, this is ok - but when installed in a semi with Dual-Battery system - older relay systems can shove over 24 volts into a radio like that - blowing the caps from the surges - the radio ends up like what you're encountering now.

I do not envy your position - been there and done that...still do...

 

[Smokinone]

That little tester you got is also sold in kit form - so if you put it together - nice job!

You're actually in pretty good shape.

Sorry about those caps, if you review them, by what I saw in your pictures were - well they looked like they had gotten hot or even burst (gassing)- you don't see too many caps cases shrink wrap "shrink" like that on the board unless there was a major failure.

One thing to consider in this, since you've replaced so many caps , good to know the radio still listens - you did great!

Now? We just have to get it working. By the sheer number of caps you're replacing - I'd be broke after the first two radios like what you've did in just one - trying to replace them all. Sadly this is why I said You may need to replace the caps" because of their age and once dried out - don't recover their values very well if ever.

Unfortunately, caps have a maximum voltage they can work with. So that means the radio becomes more vulnerable to the whims of the owners and installers. How? because of cost considerations - the manufacturer cheapens the radios' value by using working voltage ratings barely above the typical 12 charging system in a car. With the typical regulators used on todays cars, this is ok - but when installed in a semi with Dual-Battery system - older relay systems can shove over 24 volts into a radio like that - blowing the caps from the surges - the radio ends up like what you're encountering now.

I do not envy your position - been there and done that...still do...

Thanks Andy, I appreciate the flowers. I always try and go up in voltage and temp. Most of the ones I put back in were 25 volts and up, and 105 C, not all, some of what I had were 85 C, but they were all Nichicon, Rubycon, or Panasonic. Some were audio grade and power supply grade, I had a mixed bunch. But I still had to order some, and the 2078, so another week or so before I get to the transmit end of it.
Thanks for the helping hand. I hope I can still call on you when I get to that stage.

By the way, the radio was sitting on the dash of the car, and he moved here from SOCAL. I'm just outside Vegas so it do get warm, especially on the dash.

Thanks

 

[Handy Andy]

Hmmm, that heating effect can explain a lot...

From the Shake and Bake state too...amazing those parts just didn't fall out...

 

[Smokinone]

Well, back to this 25LTD. I replaced all the caps and now have audio on receive, and I have PA audio.
Received the 2078 from the multi star outlet on the bay:




Much better than the one that looked like a resistor. I installed the 2078, checked the alignment, step 1 and 2 are where they were, checks perfect.
However, still no transmit. I checked TR16 and TR9,and have the voltages ,at least close, to the ones in the voltage chart from CBTricks.

TR16 RX TX
E 9.7 8.3
B 9.13 9.0
C 0 8.95

TR9 RX TX
E 9.81 1.1
B 2.43 1.6
C 13.56 13.56

I don't have the 13V at the Collector of TR8... which is the 1957 driver, I have 5V. Well starts at 5 and slowly degrades to a little over 2V.

One other thing is that I do have the correct frequency at the antenna for each channel ie: 27.185 for channel 19 so there is some RF evidently getting through, just no power out.

Just checked R43 and have nothing there so it could be an issue with L14?

 

[Smokinone]

Well, back to this 25LTD. I replaced all the caps and now have audio on receive, and I have PA audio.
Received the 2078 from the multi star outlet on the bay:

View attachment 47554


Much better than the one that looked like a resistor. I installed the 2078, checked the alignment, step 1 and 2 are where they were, checks perfect.
However, still no transmit. I checked TR16 and TR9,and have the voltages ,at least close, to the ones in the voltage chart from CBTricks.

TR16 RX TX
E 9.7 8.3
B 9.13 9.0
C 0 8.95

TR9 RX TX
E 9.81 1.1
B 2.43 1.6
C 13.56 13.56

I don't have the 13V at the Collector of TR8... which is the 1957 driver, I have 5V. Well starts at 5 and slowly degrades to a little over 2V.

One other thing is that I do have the correct frequency at the antenna for each channel ie: 27.185 for channel 19 so there is some RF evidently getting through, just no power out.

Just checked R43 and have nothing there so it could be an issue with L14?

Found D10 and D17 to be bad, replaced that and 13V now at the Collector of the TR8 driver.

 

[Smokinone]

Well evidently that's the trick. I now have 5 watts deadkey and swings about 6-7. Into the dummie of course.

Thanks Andy for getting me here.
I have a few others I may try next including a Cobra 2000 I got off the bench of my late father-in-laws. I have no idea the condition other than he bought it new and was at the point of adding channels, but that was years and years ago, and I never knew him to put it on the air.

 

[Handy Andy]

For D10 and D17 to be open - tells me the board got wired, hooked up - backwards -

D17 Power protection Diode
D10 - Clipping/Spike diode to Driver and Final

Good catch!

 

[dss56]

The channels would not change with no output because it was not on a channel.

 

[danmcclain]

I have a cobra 25ltd mfg date 2013. The pll is bad I had a good one from a good 25ltd mfg date 2006 and worked.
I did get some info that a TC1906 would replace the cobra 25ltd pll well I tried 4 TC9106s and will not work it is not the replacement for the cobra 25ltd which i thought it was.

So I had a friend with a 25ltd mfg 2006 and we pulled his chip and tried in my 25ltd and worked great.
So I hope someone can tell me the correct part # for the pll replacement. Here is a picture of the chip with just cobra 25ltd on it no part #. I have tried all the schematics out there and many sites and all refer to the chip as IC3. I'm determined with your help to get this radio back on the air. Thanks for the help in advance
View attachment 27050

According to Sams PhotoFacts CB-207 the 25 LTD is listed along with 25 GTL and 25 LTD Classis. See attachment pic..Hope this works for you.

 

[danmcclain]

I have a cobra 25ltd mfg date 2013. The pll is bad I had a good one from a good 25ltd mfg date 2006 and worked.
I did get some info that a TC1906 would replace the cobra 25ltd pll well I tried 4 TC9106s and will not work it is not the replacement for the cobra 25ltd which i thought it was.

So I had a friend with a 25ltd mfg 2006 and we pulled his chip and tried in my 25ltd and worked great.
So I hope someone can tell me the correct part # for the pll replacement. Here is a picture of the chip with just cobra 25ltd on it no part #. I have tried all the schematics out there and many sites and all refer to the chip as IC3. I'm determined with your help to get this radio back on the air. Thanks for the help in advance
View attachment 27050

This is what I found in the Sams PhotoFact CB-207: