I have a CB radio. The plug is polarized so you can't plug it in backwards. Until they met me! This blew the fuse immediately. Now there's a shorted 1N4004 diode, which was installed into the circuit as a "Reverse Current Protection" device. In other words, plug in power backwards and you blow the fuse.

I have a be-gillion 1N4007's and wonder if I could install one of those in place of the 4004. What'cha think? I think it would just give more protection.

 

99.9% application will take the 1n4007's.
They are just that higher in reverse voltage rating.
That is all I keep on hand to cover all eventualities, (or most)!.
Max.

 

OK, thanks. Already replaced it. Checked the resistance with the radio powered off and am finding ~210Ω. The radio is supposed to have a 2 amp fuse but when I put a 2A, without even powering it on, just supply power, the fuse is blowing. I haven't even gotten as far as attempting to switch it on. Any advice?

 

Neither of those diodes, 1N4004 OR 1N 4007, is rated for more than 1 amp. That is probably why the one is short circuited. So while it is OK to substitute a 1N4007 that diode is still not rated at an adequate current for that application. Most CB radios use a 2 AMP FUSE and so the next time the diode might short and save the radio, or it might open and let the radio fry. I would choose a 5 amp diode at least. You don't need to heat sink it, but you do need the higher current rating to adequately protect the radio.

 

Any circuits available , or make model of radio?

 

New info: Battery in my meter was dying. Replaced it. With the radio switched off I'm seeing 218KΩ. Switched on I'm seeing 94Ω. That would look like (13.8 ÷ 94) 147 mA.

 

Any circuits available

Cobra 29LTD Classic. I don't have a schematic. I know that would help a lot.

 

There is a small chiclets capacitor in parallel with the diode. Could that have been affected? I don't see how.

 

Be back in an hour.

 

There is only C123 and D23 across the supply ...

Manual

https://elektrotanya.com/cobra_29ltd.pdf/download.html

 

There is only C123 and D23 across the supply ...

And C124, the chiclets cap.

C123 is 1000µF but I can't read the voltage on it without pulling it out of the circuit.

 

@Dodgydave Tried opening the link. Did a download but can't get more than the preview of the document. By the way, thanks for posting that. I can't see anything of value from the free preview.

 

@Dodgydave Tried opening the link. Did a download but can't get more than the preview of the document. By the way, thanks for posting that. I can't see anything of value from the free preview.

Ic4, Tr10 could be blown..
Here is the circuit diagram.

 

C124, a 1000µF, 25v Electrolytic cap doesn't show any signs of problems. Obviously it's old and maybe it's faulty. I'm looking to see if I have one on hand I can throw in there. I have a 2200µF cap. Given that it's across the power supply I don't think the extra capacitance would make much difference. May have a higher startup draw of current. But I'm sure somewhere in my hoards of stock parts I have the right cap.

 

OK, after turning the diode AROUND THE RIGHT WAY, I'm not blowing 2 amp fuses anymore. The system is now lighting up. All I need to do is connect it to an antenna and see if it receives.

Thanks for the help y'all.

 

OK, after turning the diode AROUND THE RIGHT WAY, I'm not blowing 2 amp fuses anymore.

Heck, beat me to it. I was going to suggest you confirm the diode polarity

 

Classic noob mistake...no wait you are not a noob...

Old age perhaps.

 

Back to the original question:
Usually with diodes like the 1N4000 family you can use the higher voltage parts without penalty. With Schottky barrier diodes the forward voltage is frequently somewhat higher for higher voltage parts within a series. With ultrafast recovery silicon P-N junction diodes the higher voltage parts are often slower recovery than others in the series.

 

Old age perhaps

Yeah… You nailed it. And I KNEW to make sure the bloody thing was oriented properly. And the resistances were throwing me off. I thought I had a bad cap, so I replaced it with like for like. I eventually thought to myself; 'is that diode in correctly?' Sure enough t-wasn't. But it's working now. Now that I fixed it right.