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Cobra 89xlr
- Thread starter Ben Hebert
- Start date
It would make sound when I would have it upside down and then I would put it right side up and wouldn't work now it doesn't it work at all so I don't know
Did you try poking around the board with the eraser-end of a pencil - and see if sound comes back - yet???
have not looked at the schematic for a relay in this radio. if it does wiggle it around and see if the RX or TX comes back. I have a mobile that the relay on it the leg was too short and the solder had came loose and when you pushed on the relay it would work. had to repair the relay leg to keep it working.
from reading this thread it sounds like you have a voltage supply issue, as in there are some branch circuits that are not getting the voltage they need.
in general, when troubleshooting a voltage supply issue, you test for correct voltages starting with the IC's and transistors, and when you find one that isn't what it should be, you unsolder that lead, lift it from the board (or push it up through the other side of the board) and then you test that component lead again.
if the voltage is now correct, there is something on the trace it was connected to that is open or shorted.
there is much more to this concept, but it's too involved to type it all up here in one post.
you are going to use this concept to determine if your driver and/or final transistors are shorted.
you should see your supply voltage on the middle leg of the driver and final transistors.
one leg goes to ground so obviously there should be no voltage on that one.
the other leg is the Base, and it should definitely NOT have the supply voltage on it.
so as it stands now, you have a short between the base and collectors of the driver and final transistors.
first thing to do is get out your multimeter and set it to test continuity. put one lead on the middle leg of the driver, and the other on the Base leg (the one not connected to ground).
by what you said, you should see a dead short here. so, unsolder the base and collector legs of the driver transistor, pull them up a bit so they are not touching the board, and again touch your meter leads to the traces on the board. (not the transistor leads)
is the short still there?
if yes, then with the driver still disconnected, repeat the test on the final transistor the same way you just did it.
if no, then you have found your shorted part. the driver transistor should be removed from the radio and tested.
if when you remove the legs of the final transistor, the short goes away, then you need to replace the final.
if the short is still there on the traces with both the driver and final transistor legs lifted, then you have a short somewhere else.
when you said that the modulation meter worked, that is a telltale sign.
try to determine where that circuit is getting its supply voltage from, and why it's voltage is there, but other branch circuits voltage is not.
using the SAMS will be very helpful here. look at the very first page of the schematic, and you will see the power supply sections of the radio.
see the numbers in black on the right with voltages listed next to them?
those are all the different supply voltages the radio needs.
so find the component they connect to and do voltage tests until you find the one that is not there.
then you have narrowed down your search to the areas of the main schematic that get their supply voltages from this "number".
so go to each one of these circuits, and find the active device at the heart of the circuit, and test it.
thats about as much troubleshooting advice i can give without going step by step through the radio, and it's tough to explain it all without writing a repair book LOL.
also look for unintentional solder bridges between traces, and backwards electrolytic capacitors.
best of luck.
LC
I do not have a pic of the inside of the radio so can not say. there should be 2 trans mounted to the case towards the back. one is the driver and one the final. normally the one closest to the so239 connector is the final. . you need to check the voltages on the legs and see what they are. this radio should also have another 3 legged trans for the out put also. so check all 3 and see what the voltages are. also look at the part numbers on them will help to let us know which is what one.if the AM regulator is out you will not have a dead key either.
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