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How to diagnose and repair a radio?

A friend gave me a Marine VHF Radio Receiver which is working badly, and asked me if I can look into it and try to repair it (I'm not going to charge him).

The first time the radio is turned on, the reception works OK, but after a few minutes the reception dies, and turning it off and on again immediately won't fix the issue. I have to wait a few more minutes with the radio turned off to again start receiving OK. The squeltch is always at minimun, receiving noise.

When the radio dies there is a high pitch noise coming out of the speaker. The external speaker connection is wired directly to the same internal speaker circuitry.

Since the problem goes away after a few minutes I guess the issue is a broken component, like a capacitor or resistor.

I have no electronics background, just a few soldering skills. I want to learn how to repair stuff the DIY way. A few weeks ago I repaired an audio amplifier. The problem was simple, a burned track on the PCB.

What are the steps required to pinpoint the problem? My toolset is limited, I have no oscilloscope, and no capacitance and inductance meter, just a couple of cheap multimeters.

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Be safety aware!

Be alert to the fact that there may be dangerous or even lethal voltages inside the unit if you are working on it with the cover removed and you are testing with your DMM. (you do not state the make or model number of the radio so am assuming the most dangerous option!)

Also try not to inadvertently bridge between close connection points in the circuit with a test lead of your DMM when trying to measure voltages etc thereby shorting them out. You may cause more problems than before you started. We've all done it and it's a PITA.

First you should check online to see if there is a free schematic diagram or service manual available for download, or even if there is one that can be purchased cheaply. This will save a lot of time and angst in trying to locate the source of the problem. Search for (your make and model radio) service manual and see if there are any results.

Check for the obvious. Domed, bulging or leaking electrolytic capacitors, burnt or heat stressed other components. Dry solder joints and loose connections etc. Use a strong light and a magnifying glass if you have to.

Check if the circuit board has any voltage reference information printed on it, which may help in determining if the correct voltage levels are present etc.

As it works after it 'cools down' check for components that are feeling 'very hot' as soon as it has died. Do this with the power totally disconnected Be aware that some components might run hot anyway. Don't burn yourself. If you do find and suspect one, (it may look heat stressed already) the heat will have been caused by excess current flowing through it. The trick is to find out why. Is it the component itself that is the problem or something in the circuit path of the component that is the cause? If you want to use an Ohmmeter to test the resistance value of a particular resistor that you suspect for example, against what value is shown on it (resistor colour code here), ensure that the power is disconnected from the radio. Also it is better to unsolder and remove 1 leg of the component from the board to test it.

Hopefully this is good enough a start for you to be going on with. As you didn't state the make and model number it is hard to give any more detailed advice.

Welcome to the wonderful world of fault finding at component level without the aid of a circuit.

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