Friend of mine doesn't have the proper power cord for his 138xlr and uses 2 clamps to hook up to power, says he got mixed up and crossed the wires and smoked the 2sc1419 transistor before he could get it disconnected. He replaced the 1419 but says there is still a short somewhere as he cant hook power to it without the connections sparking? I have not seen the radio, nor do I have the time to research it, long ride to get to it, Hoping someone here has come across the same issue and can offer some info of what to look for. Is there a blown protection diode in this radio that could be creating the short? He says everything else appears to look ok. Any help would be much appreciated....................Thank you
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Cobra 138XLR Info On Reversed Power Wire Repairs
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Did you look at D54 yet? If it is bad, the just replace it the same way it came out - paying strict attention to which side of the diode has the white stripe on it - and put the new one in the same way. Should be a 1N4003. He better beg or borrow a proper power cable unless he wants to fry the whole radio this time around . . .
Shadetree Mechanic
Delaware Base Station 808
The cords made by workman have a fuse in the positive and negative, maybe this is why?
over the past 30 years I bet I have changed out 1,000 or more of these diodes due to hooked up backwards.
I remember one guy about 15 years ago he came in and the diode was blown. I told him it had been hooked up backwards and he got mouthy and said he was not stupid. he knew how to hook up a radio and that was not what happened. I changed the diode and asked him if he wanted me to install it and he smarted off I can do it myself. no need for you to do it. I showed him it worked now. he took it out to install in his truck and with in a few minutes in the door he came cursing and having a fit said I had not fixed it. i changed the diode again then insisted I was going to put it in his truck. he followed me out to his truck and the 1st thing I did was look at where he had the power wires hooked at. which was simple since it was the 2 points trucks have the hookups at for a radio. low and behold he had them reversed at the hookup points. I showed him what was wrong, changed the wires around and his radio worked just fine. he then started saying his son had hooked the wires up for him and did not look there . he just knew the plug to the radio was right. he started thinking me and was a loyal customer after that.
so any thing is possible with a backwards hookup.
I remember one guy about 15 years ago he came in and the diode was blown. I told him it had been hooked up backwards and he got mouthy and said he was not stupid. he knew how to hook up a radio and that was not what happened. I changed the diode and asked him if he wanted me to install it and he smarted off I can do it myself. no need for you to do it. I showed him it worked now. he took it out to install in his truck and with in a few minutes in the door he came cursing and having a fit said I had not fixed it. i changed the diode again then insisted I was going to put it in his truck. he followed me out to his truck and the 1st thing I did was look at where he had the power wires hooked at. which was simple since it was the 2 points trucks have the hookups at for a radio. low and behold he had them reversed at the hookup points. I showed him what was wrong, changed the wires around and his radio worked just fine. he then started saying his son had hooked the wires up for him and did not look there . he just knew the plug to the radio was right. he started thinking me and was a loyal customer after that.
so any thing is possible with a backwards hookup.
Most amateur radios have fuses in both pos and neg wires. Never an issue since both fuses are effectively in series thru the radio. The reason for dual fuses has to do with the extremely low possibility of having starter current flow thru the radio to ground if you lose the main battery ground wire.
I got in the habit of asking the customer how big the fuse was when he got the wires reversed.
I would get a puzzled look, and explain that the higher the fuse rating the higher the price of the repair.
Biggest risk is that the protection diode will get a big enough surge of current to blow it apart.
This diode is connected to the DC power input. When the polarity is correct, no current flows through it, as if it were not there at all. But when the polarity is reversed, it becomes a near dead short. The common name for this sort of protection is "crowbar".
The diode will be rated for 1 Amp, typically. This means that the 2 or 3 or 4 Amps it takes to trip the correct-size fuse will decisively overload the diode, overheat it and cause it to fail, becoming a dead short.
For either the right OR the wrong polarity. If it gets enough current to rupture the body of the diode, the thing disintegrates and leaves an empty space between the two lead wires.
When this happens, you no longer have any protection.
If the reverse polarity voltage is now permitted to reach the circuits inside the radio, a series of dominoes begins to fall.
Electrolytic capacitors are polarity sensitive. Reversed polarity will make the water content of the fluid inside turn to steam and rupture the aluminum housing of the part. Might push the rubber plug out the bottom, or split the can. Might just explode and hurl fragments of aluminum foil in all directions.
Next casualties will be the large semiconductor devices, the audio-power chip, voltage regulators, final and driver transistors. Next will come the PLL chip and smaller transistors in the receiver circuits.
General rule is that the more current a device draws in normal use, the more likely it is to fail from reverse polarity.
We adopted the same precaution that Galaxy did around 20 years ago. We solder a 6-Amp rated diode directly across the pins on the inside of the power socket. The body of this diode has enough metal in it to prevent the "pop apart" risk, even with a large fuse. Besides, if you hook up the radio backwards with a 3 Amp fuse, you probably won't damage anything more than just the fuse.
Won't send a repaired mobile radio home without that particular upgrade.
Cheap insurance.
73
I would get a puzzled look, and explain that the higher the fuse rating the higher the price of the repair.
Biggest risk is that the protection diode will get a big enough surge of current to blow it apart.
This diode is connected to the DC power input. When the polarity is correct, no current flows through it, as if it were not there at all. But when the polarity is reversed, it becomes a near dead short. The common name for this sort of protection is "crowbar".
The diode will be rated for 1 Amp, typically. This means that the 2 or 3 or 4 Amps it takes to trip the correct-size fuse will decisively overload the diode, overheat it and cause it to fail, becoming a dead short.
For either the right OR the wrong polarity. If it gets enough current to rupture the body of the diode, the thing disintegrates and leaves an empty space between the two lead wires.
When this happens, you no longer have any protection.
If the reverse polarity voltage is now permitted to reach the circuits inside the radio, a series of dominoes begins to fall.
Electrolytic capacitors are polarity sensitive. Reversed polarity will make the water content of the fluid inside turn to steam and rupture the aluminum housing of the part. Might push the rubber plug out the bottom, or split the can. Might just explode and hurl fragments of aluminum foil in all directions.
Next casualties will be the large semiconductor devices, the audio-power chip, voltage regulators, final and driver transistors. Next will come the PLL chip and smaller transistors in the receiver circuits.
General rule is that the more current a device draws in normal use, the more likely it is to fail from reverse polarity.
We adopted the same precaution that Galaxy did around 20 years ago. We solder a 6-Amp rated diode directly across the pins on the inside of the power socket. The body of this diode has enough metal in it to prevent the "pop apart" risk, even with a large fuse. Besides, if you hook up the radio backwards with a 3 Amp fuse, you probably won't damage anything more than just the fuse.
Won't send a repaired mobile radio home without that particular upgrade.
Cheap insurance.
73
here is one seller I bought 100 from.
https://www.ebay.com/itm/100pcs-6A1...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649
https://www.ebay.com/itm/100pcs-6A1...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649
If I had to guess I would say there was no inline fuse, but that doesn't surprise me. Bought a power cord off that auction site and having it shipped to him
hopefully it isn't fried beyond fixing.
Any time we power up a radio for the first time after fixing this kind of fault we use a current-limited power supply turned down to one-half Amp. This is usually enough to run the receiver.
If we missed something, and there's still a dead short on the power input this limits further damage. Shorted capacitors may swell up when they short, or might not. If there are multiple failed parts putting a short circuit onto the power input, you'll have to find them one at a time.
Can't key the mike on that much current. But if it passes the receive-only test it should be safe to plug in the real power supply.
It's the best way to minimize the smoke you get from a smoke test.
73
If we missed something, and there's still a dead short on the power input this limits further damage. Shorted capacitors may swell up when they short, or might not. If there are multiple failed parts putting a short circuit onto the power input, you'll have to find them one at a time.
Can't key the mike on that much current. But if it passes the receive-only test it should be safe to plug in the real power supply.
It's the best way to minimize the smoke you get from a smoke test.
73
Standard protection diode found in most radios is rated for one Amp.
The fuse rating is always more than that. The current that has to pass through the protection diode before the fuse will trip always overheats the diode. Melts the silicon inside it and it becomes a dead short. For both the wrong and the right polarity.
Makes it a "one-time" protector. Has to be removed before the radio will run again. But it's a lot cheaper than all the other stuff in the radio that would be destroyed by reverse DC-power polarity.
73
The fuse rating is always more than that. The current that has to pass through the protection diode before the fuse will trip always overheats the diode. Melts the silicon inside it and it becomes a dead short. For both the wrong and the right polarity.
Makes it a "one-time" protector. Has to be removed before the radio will run again. But it's a lot cheaper than all the other stuff in the radio that would be destroyed by reverse DC-power polarity.
73
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