So I have a couple month old new cobra 25 ltd. I was testing amps and blew the radio meaning it would pop a fuse everytime radio was turned on. So I replaced the final, driver and pre driver transitor. The radio now turns on and the tx light changes when mic is keyed but shows no carrier/modulation and meter on the radio does not move. The receiver works fine and meter moves on receive. The pa works so I assume the audio chip is good. Any suggestions would be much appreciated! I replaced a couple 1000 uf caps but still no luck. Thanks!
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Cobra 25ltd no transmit
- Thread starter rflegacy
- Start date
If the receiver and PA still work, it does sound like the audio path is good.
If you have replaced the final and still have no transmit at all...
I would look in the area of D10 right below the audio transformer.
RCV/PA work so I would think that the transformer itself is good.
In transmit... that transformer takes the 13.8 vdc, modulates it by way of the audio chip, and sends it down through D10 to serve as the "modulated Vcc" for the final and driver. If D10 is smoked you won't get anything out of the final or driver.
Good luck!
Bob
I read in a book somewhere that a bad final has the potential to kill the transformer. See if the winding shows continuity.
Edit: if that turns out to be the case, and you find a replacement, it would be worth adding a fuse to the hot side of the transformer to prevent it from recurring. I seen a radio or two that actually had a fuse at the transformer for just that reason.
Edit: if that turns out to be the case, and you find a replacement, it would be worth adding a fuse to the hot side of the transformer to prevent it from recurring. I seen a radio or two that actually had a fuse at the transformer for just that reason.
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I would think that "no carrier" takes focus off audio trans. if tx light works, I'd check voltages at the collectors of final and driver, if okay then trans is likely good, or shorted between windings but not failed open. It's still offering volts. If the transistors are getting volts, look hard at the base. Check those volts. Just my 2 cents.
Check R40. When the final breaks down inside, it can smoke this resistor. Won't transmit without that part in the circuit.
73
73
That WOULD be a good check at this point.
As I look at that circuit.... with the +13 going into the transformer on the bottom, the audio IC tying in at the center tap... then the top of the upper winding goes off to D10 and the final/driver... I catch myself thinking that RCV and PA could work ... EVEN IF THAT UPPER half winding is smoked clean through... maybe even shorted.
I say that because the audio IC is going to be sinking current through the lower winding with the audio signal so THAT would probably pass through to the secondary EVEN if that upper primary half was toast.
With that... yes... check to make SURE that DC is making it out of that upper winding .... on to D10... and then ON to the final/driver collectors. Check that whole path.
Good call CG !!!
I would start with the replacement parts. What did you use as a pre-driver, driver and final and where did you get them? Most of the time when they blow they do not take out other parts (although it is VERY possible they could). My initial guess would be you picked up bad/fake parts unless you could somehow verify that they were good. It took me a few times to to get my head around the fake parts problem and to only by parts (usually for more money) from a known source like RF Parts. Five or six years ago I had a President Washington that had a bad final. I purchased replacement finals and drivers off of the bay. They of course were fakes. I then bought a second set from a web site. They were fakes. I ended up with another Washington that worked so I tried the driver and final from it. They of course worked. That at least told me the radio was ok. I then purchased a driver and final from RF Parts for tripple of what I had paid for the fakes. They of course worked. I've learned my lesson. Now there are 2 working Washingtons on the air somewhere.
Thank you all for all the pointers. Audio transformer was fine (rare for those to go out). I got the 2078 final on Amazon because rf parts doesn't carry 2sc2078 and got the driver 2314 at rf parts and the pre driver off a parts radio. Still no transmit. Well I use to do alot of mosfet conversion so I have alot of 2078s laying around. I put in a salvaged 2078 and magic.... radio works like a dream now. Thanks for everybody's input, much appreciated!
Before I replaced the final with the salvaged 2078 final, a ceramic capacitor c40 was smoked and fell apart when I replaced it. Now the question is why did my new sweet 16 straight 8 blow up this radio and a galaxy in the same fashion?
Sounds as if it tried to oscillate. How close it its construction to the factory original?
A failing relay can do this. The big one is not cheap. Don't see how the small (cheap) relay could be responsible. All it does is activate the big one, and provide bias current.
Mileage is worth knowing.
73
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