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Re: blown fuse and reverse polarity diode
Check around the pcboard where the dc goes to the board, sometimes the foil will burn. Also chec the transformer, Chec that one of the wires did not come off the plug in the back. When you replaced the protection diode did it blow again? If not keep checking for opens on the pc board. When you said it was dead do you mean no lights at all or no tx and rx?
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If the radio is blowing fuses then you are correct in that the protection diode may be burnt. the diode should be black and it should be located near the DC power connector. You should be able to replace it with a 1N4007 Diode.
press around board where power comes in to see if solder connections are weak. where the red and black wires come in, it's 12 volt power to the board its usually a reverse polarity diode, its black with a silver stripe around it, its job is to make sure it wasn't hooked up wrong, you could unsolder it MAKE SURE EVERTHING IS HOOKED UP CORRECTLY, turn it on for a test, if it works find a replacement diode.
I'm not familiar with the radio itself, but it seems from the symptoms that the audio processor/stage is the only common problem i.e: audio does not work. Do you have a schematic for the unit? If so, I would follow the DC path to the audio stage and troubleshoot in that area. The other stage that can be suspected is the regulator stage. Hope it helps a bit.
The black fuse means it blew from a fast, heavy short circuit instead of an overload of some sort. The high current vaporizes the wire inside and coats the glass, giving it the black look. In an overload, the fuse link melts.
A common cause is failure of the reverse-polarity diode in the radio. It's there to cause the fuse to blow if you get the battery connections backwards when hooking up the radio. The diode can short, and this will cause the blown fuse. If you check the fuse and find it's black again, you should check the diode with an ohmmeter. Look for the diode near where the battery lead connects to the circuit board in the radio.
Sounds like a "short" if it affects modulation + power. Try to inspect board with magnifier for solder bridges, and bad resistors. Change a few resistors near back with new ones. take out slugs near back finals like L10 or L20's not sure if its same as Uniden Pc68xl. but try that. I found turning L19 up to top gave great Modulation. clip D9, Sread coils 1/16 inch in back. Try to get lower carrier. Carrier does not mean anything much. Modulation is what you need. With less Carrier it may cool down that chip. It may be too wound out.
Make sure you're connecting the radio with the proper polarity. There is a protection diode inside that will cause the power supply fuse to blow or go into overcurrent shutdown if you have + and - reversed. It's also possible this diode has been shorted. You can't tell by looking at electronic components (unless they are physically damaged) whether they are good. An ohmmeter can be used to test for shorted diodes. Look at the board where the power leads connect for the diode.
Most devices that can be operated from external supplies have a diode across the the power jack that will short the source if the polarity is reversed.
Since this is a pretty small part without much chance of surviving the current available from a cigarette lighter socket, the only hope you have is that the fuse almost always found in these 'adaptors' will blow before the diode fails.
If the radios will not operate from batteries, I'm afraid they are badly damaged.
It son't hurt much now if you open the case and check near the power jack for burned parts. The diode(s) probably aren't there at all any more since they are commonly capable of no more than an amp or two and the typical fuse in those car adaptors is 5-10 amps, enough current to cause the diode to explode before the fuse blows.
I don't know where you reside, but in the US there are many sources for low-cost digital multimeters (<$20 US) that would have prevented your problem simply by checking the polarity of the jack before plugging it in.
I don't think the lighter was at fault; I believe the adaptor was wired for a particular item, just not for your radios.
It's always chancy to use a power source only because the plugs match.
Most devices have a small symbol molded into the plastic near the power jack showing which polarity input is correct.
D23 is the reverse protection diode and it shorting is the primary
reason your fuse blew. As you posted, power was applied in reverse,
hence other components would also fail. Pls check:
1. D22 - 9.1V Zener Diode
2. TR23 (2SC1096) 9 Volts regulator
3. D19 - 5.1 Zener Diode
4. IC4 (TA7222) - Audio Amp
5. TR14 (2SC2029B/10) - Finals
6. TR15 (2SC2028B/20) - Driver 7. Most capacitors over 220uF (like C123, C114 & C112)
Hope that this be of help/idea. Pls post back how things turned up or should you need additional information.
All LEDs are polarity sensitive since they are unidirectional (anode & cathode). If the LED was connected the other way around, then even with a known good new LED, it won't light up. At other times, it is possible that the LED has a limiting resistor and/or another diode in series with its supply line. You need to check if this is also open.