Radio turns on when I turn knob, but very quickly turns off..

If you feel comfortable llooking under the hood on the radio, you can pretty easily diagnoose a bad power/volume switch. First, replace the external fuse with one around 3/4 or 1 amp. Find where the power switch terminals are on the back (or innermost) of the part. There will be just 2 terminals for the power, as opposed to 3 for the volume part. If you have a voltmeter, put the negative lead on a good ciuit ground, like a coil shield. With the power switch off, see which of the 2 power switch terminals has 12-or-so volts on it with the positive lead. The other one should be zero. While holding the meter lead on the terminal that has zero volts on it, turn on the power switch while watching the voltmeter. If it ticks up and stays around 12 volts, you probably have a problem further downstream. If the voltmeter ticks up briefly and drops to something around 5 or 6 volts, or lower, compare that reading with what is then on the incoming voltage side of the power switch. If the unswitched side is still at full voltage, you probably have a bad power switch; they do get dirty from arcing. You can test that theory by jumping the two power switch terminals with a small test lead, a miniature screwdriver, nearly anything conductive. If the radio comes on and stays on, you've diagnosed your problem. The small value fuse will keep something else that may be expensive from letting out all the smoke that the factory manufactures into expensive parts from escaping. If the radio seems OK, don't expect it to transmit with the small value fuse in place.

If this radio were on a commercial test bench, niceties like current-limiting power supplies and oscilloscopes would let a tech test more quickly and accurately, but this procedure will let you confirm or possibly rule out a high-rate failure item.
Hope that helps,
73

 

without having the radio its hard to say, just remember small fuse .5 imho, I would check the switch with a ohm meter before power, current limiting supplies are a wonderful thing

 

Why would you need a scope?

all you need is a can of freezeit and a voltmeter with a good power supply with a current limiter.

I'm thinking that this is a cold solder joint, nothing more.

 

so good old fashion tap, tap, tapping will probably find it, maybe a little shot of freeze mist or a blast from the desoldering hot air wand. If all else fails crank up the amps the smoke will lead the way

 

The scope is to enable quick checks on components like the ubiquitous 10.240 MHz rock (if that's used in this chassis), see if there's audio on the high side of the pot after the radio seems to stay powered up (if it does). I agree, it's more than what's needed for a go/no-go test on a power switch.
73

 

I get what both of you are saying but let's start with the cold solder joints at the power strip of the radio before the switch. I have never seen a bad power switch on a post 74 radio, cold solder joints yes and the same problem as the op described. I even seen a fuse with a cold solder joint.