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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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    Sounds like some component is overheating. That would explain that you have to wait some time before it works again. Try to find out if a certain part gets really hot. If you can't pinpoint a component that way, use cold spray to cool components selectively. When you find the culprit it may be defective itself, or one of the parts surrounding it. Look for discolored resistors and leaking capacitors. – stevenvh Sep 03 '12 at 17:17
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    You have stated that the problem (the reception or the 'noise'?) appears after a few minutes, and then that it goes away after a few minutes!? First, getting a schematic can be tremendously helpful. One good trouble-shooting technique is to work backwards from the output. Build a simple audio oscillator and inject that signal at the speakers, then at the amplifier stage immediately before the speakers, etc. If it fails at some point you have narrowed down the problem area. If you get all the way back to the RF section you can then apply the same general technique with an appropriate RF signal. – MickeyfAgain_BeforeExitOfSO Sep 04 '12 at 02:06
  • @stevenvh, that should be a posted answer. And vzo, a good path to DIY fixing, is to get yourself some of those tools you lack. You can only go so far with guessing. (Though you can do pretty well if you're clever) – gbarry Sep 04 '12 at 17:25

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gbarry suggests I post this as an answer. Thanks for the support, gbarry!

You can do quite some troubleshooting without complicated tools. For a start we may not even need the multimeter.

If it works for a while and then stops working that sounds like some component is overheating. That would explain that you have to wait some time before it works again, so that it can cool down again. Try to find out if a certain part gets really hot. If you can't pinpoint a component that way, use cold spray to cool components selectively. Just spray on components until it works again. (You'll probably have to spray only briefly; the spray we use allegedly can cool down to -50 °C.) When you find the culprit it may be defective itself, or one of the parts surrounding it. Look for discolored resistors and leaking capacitors.

stevenvh
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If you have an external audio amplifier, try to unconnect the central wire from the volume potentiometer, and connect it to the external audio amplifier with the common GND. If you can ear continuosly that way you insulate the problem being in the audio or MF/RF section of your receiver.

Felice Pollano
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First, find out if the problem persists after disconnecting that added speaker.

Then, take @stevenvh's advice about overheating and the use of cold spray.

gbarry
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