Stereo fuses blow – wrong rating or symptom of failing regulator?

[Oldmogman]

My stereo has recently taken to blowing its one-amp fuses. I think it might be happening when I start the engine rather than when in use, but I've not yet established a pattern.

Of course, it could be because the fuse is the wrong rating (although I'm pretty sure it's always had a one-amp fuse), but I was wondering if it might be a symptom of impending problems with the voltage regulator?

The stereo is relatively low power and has two small speakers. The regulator was last replaced about 25 years ago.

All the other circuits/fuses (which are protected with higher-amp fuses) seem OK.

What are the symptoms of a failing voltage regulator? Is there any way of testing? Adjustment?

Suggestions please! (I’m not any sort of ‘sparky’ when it comes to car electrics but do have a basic circuit tester/voltage measuring meter.)

Thanks all

[Biggles1957]

I'd suspect some sort of short circuit if it just pops the fuse like that. Maybe a wire has chafed through the insulation and is shorting to earth?

[StillGotMy1stCar]

The fuse does appear to be too small, are you replacing it with a Fast blow type marked as F1A or Anti surge marked as T1A.
If you’re sure it has always had a 1 Amp fuse then it should be an Anti surge type.
Check the voltage across battery terminal after the car has been running for a while with some revs with all electrical items turned off, ideally you should have 13.5 to 14.5 volts if you get a voltage of over 15 volts you may have a regulator problem.
Regards John.

[Oldmogman]

Thanks John - that's great!

cheers

PJ

[Oldmogman]

Hi guys

Just checked the voltage output at the battery with the engine running - I'm getting 12 volts. Is this OK?

cheers

PJ

[TDV102]

Watts = Amps x Volts
A 50 W output would need a 5A fuse
A 25 W outbut would need a 2.5A fuse

Divide the wattage by 12 and go for the next integer up

[StillGotMy1stCar]

TDV102 Your statement is correct for the Wattage input of a device, bulbs etc, but not for the output of a device.
For it to work it would have to be a low power radio with output rated in RMS (Root Mean Square) then you’d need to add on power for the RF and IF stages, preamp and power amp stages and dial bulb, then if it’s got a cassette deck the power for the motor. It gets more complicated with higher power radios, some have both output rails moving, the separate power amps use inverters to raise the voltage to the output stage.
Sorry to ramble on, difficult to explain.
The proper way to select the correct fuse for an electronic device is too measure the peak current in the supply lead and times it by 1.5 for a glass type fuse.

Oldmogman, Dynamo’s don’t charge at idle speeds in fact you get a slight discharge, you need to give the engine some revs, the more you rev it the more the dynamo can deliver. No need to go silly though, for testing best have the revs equivalent to 25-30MPH in top gear. The voltage should rise above 13 volts 13.6 to 13.8 is ideal and if it reaches 15 V it’s overcharging. At 12 volts it is not charging unless the battery is very flat and hasn’t had enough charge put in to it.
Regards John