Bass Amp

[nigelbytes1]

I purchased this amp about 3 years ago, http://www.gear4music.com/Guitar-and-Bass/DISC-White-Horse-BP150-Bass-Guitar-Amplifier/8J5
It's never been gigged with in fact its had very little use at all apart from using it with headphones.
Recently I moved home and to my surprise the Amp doesn't work anymore
I switched it on only to hear a loud hum type noise through the speaker.
I tried playing my cd player into the aux in, but there's no sound at all apart from the hum. I removed the amp from the cabinet and tried it through my headphones and all i hear is the hum. no adjustment is possible.
Very strange because I'd used it maybe a month or so back and all I've done is move home.
Anyone any ideas ?

 

[Porque223]

Loud hum is likely a missing ground connection and you can check the cable continuity with a multimeter. Very low (power supply) hum is normal.

2nd & distant choice would be a power supply capacitor has failed to an open circuit and you can check this by shunting it with a known good cap of equal uF and working voltage.

 

[nigelbytes1]

Thanks for that reply, Having listened a little more it sounds like a loud hum with a purring kind of sound ? its quite loud.
I'm unsure how I can check the capacitors without replacing them but they look fine, no bubbling or leaks that I can see but others have mentioned they can go faulty so I'll see if I can get hold of some the same values. I tried with my multimeter but kind find any earth problems.
One thing I have noticed is after I power down the blue LED goes out straight away where as before it went out slowly.
Here's a couple of pictures,

 

[Porque223]

LED goes out straight away where as before it went out slowly.

A strong clue that says your filter caps (likely those big black 8200 uF units) have greatly diminished in value or maybe their solder joints have failed to a high resistance value. You need an oscilloscope to check power supply ripple but you may be able to check this with a true-RMS AC voltmeter. With no signal in you should have very low ripple, I'm guessing less than 5% of the voltage marked on those caps. The ripple is a sawtooth waveform so we'll need a formula to convert P-P values to RMS values.

Could also be a shorted power supply diode sucking the charge out of these caps. An in-circuit diode/capacitor checker would come in handy right about now, otherwise you need to start unsoldering components.

 

[Sam Gangee]

Luckily, it's a very basic transformer supply so fixing should be easy, provided that it hasn't damaged the amplifier. I recommend not powering it up again till you've replaced the faulty component.

If the hum is predominantly 50 Hz, one of the four rectifier diodes has failed. What is the number printed on them? Possibly 1N500x.

If the hum is predominantly 100 Hz, one of the large capacitors has failed. I would replace both with capacitors of the same physical size and rating. You need to know the value (8200uF) the voltage and the ripple-current rating. The latter will be impossible to determine other than from the manufacturer, so go for the highest you can find. It'll probably be Hobson's choice!

Let us know the voltage and dimensions if you need help in finding a replacement.

 

[Porque223]

...the value (8200uF) the voltage...

With 150W into 8 ohms they are probably >60 vdc.

 

[nigelbytes1]

I've managed to source a couple of Caps but will have to wait for post, I cant really test them myself so will replace also ordered IN5408 rectifier diodes, I just hope the amp hasn't blown, For the life of me I can't figure out whats happened.

 

[Sam Gangee]

I've explained what's probably happened. 50 Hz or 100 Hz?
As a bass guitarist you should know instantly.

 

[daxi]

Moved home, suddenly a hum...If it's not an earth issue. Get a filtered plug as a start..Things like florescent tubes, plasma TV's etc etc are all possible causes & the problem could be carried on the mains cable or radiated as a radio signal, so it may not be a simple cure.
Cheaper filtered adaptors are available, but Craplins, do them for £20 plus..
http://www.maplin.co.uk/6-way-flat-screen-and-hdtv-surge-protector-and-mains-filter-219077

 

[Sam Gangee]

You are ignoring all the other evidence that points to a PSU fault.

 

[Porque223]

You can check the caps yourself with a DVM, a 1 megohm resistor, a 12v battery and clock.

 

[Sam Gangee]

That method doesn't measure ESR. I've used a capacitor meter myself many times. One example was a PSU with a 1000uF/16v capacitor that measured 980uF. You'd think that should be OK, because it's well within tolerance, but its ESR measured a massive 10 Ohms and the Power Supply worked as soon as this culprit was replaced.

 

[Porque223]

That method doesn't measure ESR. I've used a capacitor meter myself many times. One example was a PSU with a 1000uF/16v capacitor that measured 980uF. You'd think that should be OK, because it's well within tolerance, but its ESR measured a massive 10 Ohms and the Power Supply worked as soon as this culprit was replaced.

What was your symptom?

For 1000 uF at 50 Hz you'd have less reactance than resistance. To measure this you'd need a multimeter, 230vac at 50 Hz, a load resistor, and a formula.

 

[nigelbytes1]

Caps and rectifier diodes now replaced but no difference, still loud hum/buzz so where to go from here ?

 

[Sam Gangee]

I think you should take it to a repair shop that has the necessary fault-finding equipment. It could be as simple as a broken connection but, as you can't see it, you are not going to be able to find it.