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Cobra 139 LXR Capacitor question?

long night

Active Member
Jan 8, 2020
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Hi. I have a Cobra 139 LXR that receives well, has good carrier but very low modulation. I opened it up and see inside that capacitor C-304 is missing. Can someone tell me what this capacitor is used for? C-304 is a 3300 uF 25 volt capacitor located in the left side of the radio on a small PCB board. Would it be the cause of poor modulation?

Thanks Long Night
 

That is the main filter cap for the power supply. If you are running off AC power it needs to be there otherwise you will have quite a bit of hum in the radio. It is possible that someone installed one under the board. I have seen this done before when someone wanted to add more capacitance for better filtering but the cap was too big to fit on the top of the board.
 
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That is the main filter cap for the power supply. If you are running off AC power it needs to be there otherwise you will have quite a bit of hum in the radio. It is possible that someone installed one under the board. I have seen this done before when someone wanted to add more capacitance for better filtering but the cap was too big to fit on the top of the board.
Thank you tramd.
That is the main filter cap for the power supply. If you are running off AC power it needs to be there otherwise you will have quite a bit of hum in the radio. It is possible that someone installed one under the board. I have seen this done before when someone wanted to add more capacitance for better filtering but the cap was too big to fit on the top of the board.
 
Thank you Tramd Can you tell me which way the C-304 capacitor faces since mine is missing. I do not want to get the polarity backwards.
Long Night
 
I don't have a radio to look at but if you use an ohm meter you can find which solder pad is grounded. You will have to get a ground point from the main board as the chassis is not grounded directly to the boards in that radio. The ground pad will be on the larger circuit area of the power board. It also will have a black wire in that circuit that goes to the main board for ground which is where you could get ground from also. That pad will be where the negative lead of the cap (which will be the shorter lead and usually has arrows pointing down to it) will go. In a Washington and I believe also in the TRC 458 and 457 the negative side of the cap faces towards the front of the radio and most likely does it that radio also but make sure or you will damage the cap. Usually there will be a + sign on the top of the board to indicate the positive side which is the easiest way to determine orientation of the cap.
 
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...(I think you meant 139XLR)...

Well, you can do a simple ohmmeter check (continuity) with the power off and just locate foil ground...test the ohmmeter at 1X setting and find the lowest STEADY ohmic reading and that is your Caps "banded Cathode" side.

To help with most of the Electrolytic caps you're not sure how to install or orient correctly - find a shielded can case - RF cans work, and put one lead to it, and the other lead of the DVM to the caps "holes" - whichever one shows nearly dead short less than 2 ohms, that should be negative - the other hole would exhibit a ohmic reading that wildly changes.

And if you're in a spot where you need to know polarity of a cap you found bad, you can remove the cap and apply power to the board, and measure the voltage DIFFERENCE in the holes the cap goes. The side with the highest Positive value compared to negative - is where the UNBANDED or "+" mark (if tantalum) goes - this keeps the cap's plates properly set so it doesn't blow or cause excessive gassing and eventual failure.

If you need more help, CBRadio Magazine did up a short You Tube vid introducing you to the 139XLR, MikesRadioRepair did another - you can review these to get clues on how to proceed with the work you want to do or repair.

In light of your situation - the C304 cap the Banded end (Cathode) is towards the front of the Radio - right by the speaker - so when you install the new one it's banded end is right behind the speaker.

upload_2021-7-9_11-1-7.png

Sorry @tramd - posted this just after your's

But @long night ...

upload_2021-7-9_11-6-39.png

You commented about "great carrier but low modulation" Locate VR 8 - by the rear modulator speaker coil core - if you can adjust the carrier, see if turning down the carrier restores the modulation level - any carrier above 5 watts this radio tends to pinch audio up and nearly off.

Else more work around the Final and Driver to help with the RF tune up may just be all it needs.
 
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