Bizzare amp hum

[hugbot]

My blackstars suddenly started humming loudly when plugged in, the hum is very strange though.

 

1)Its at a constant level, same volume regardless of where the amps master is.

 

2)Its only there when a cable is plugged into the input.

 

Any idea whats causing it?

[mike42]

I'd try a different cable first, just to be sure the cable itself isn't the problem. But, my first guess would actually be that you've got a broken solder joint, probably right where the jack mounts on the board. If that's the case it should be a simple fix, but the amp will have to come out of the cab to get to it.

[WRGKMC]

Bad cable or you lost a ground in the guitar some place.

[hugbot]

It does it with every cable I've tried.

[WRGKMC]

Then it may be a broken ground on the guitar jack inside the guitar (very commone) or a cracked solder joint to the jack

inside the guitar amp (also very common)

 

If you can plug the guitar into another amp to rule it out, it will narrow things down, otherwise you're going to have to dissasemble both and get an eyeball on it to see where the problem is.

 

What kind of guitar and what kind of amp?

[hugbot]

It does it with every guitar, its definitely the amp, not the guitar or cable.

 

also, if it was that kind of grounding hum coming from the guitar it would raise in volume as I raised the volume of the amp surely? The hum is at a constant level regardless of the amps volume.

 

And its a HT40 combo.

[WRGKMC]

OK then that explains it better. Still need to know what kind of amp.

 

Since the volume doesnt vary the hum the problem is likely in the power section of the amp.

 

If its a transistor amp, the power transistors may have blown and gone DC and is pumping 110vac straight into the speakers.

This is very bad because it will smoke the speakers voice coils in no time at all.

 

Do not power the amp on again with tha speakers connected or you can kiss the speakers goodbye.

 

The repair cost isnt that bad for blown outputs or a blown voltage regulator, maybe $100~150 for most amps plus parts.

 

You can test to see if this is what occured by disconnecting the speakers and connecting an AC meter

to the speaker output and kicking the amp on for a second. "Be careful" if the the power transistors are blown you'll be getting straight

110v out of a wall outlet from the speaker jack and it will kick you on your ass.

 

If you dont get AC out of the speaker jack then you may be looking at a bad power capacitor or cracked solder

joint. Thses require careful inspection to check for a cracked solder joint. Wiggleing components a little and looking

at the back of the board to see if the legs are cracked may find the problem. "Again, Be careful" caps retain

several hundred volts of static electricity and they would love nothing more than arch out through you to ground.

 

If you dont feel confortable making these checks, take it in and have it done.

My thought is if you have a clue to whats going on you then know what the tech is billing you for.

 

The cause of a power section blowing is usually load related. Connecting the wrong speaker impediance, too many speakers

for the amp design and it overheats the output transistors. It can also be the caps wearing out or vibration causes the solder

joints to crack, stuff like that.

[philbo]

Another possibility: One or more of the power supply filter caps have gone open. This would feed pulsating DC into the amp circuits instead of filtered DC. If the hum is 120 Hz rather than 60 Hz (for USA), its a fair bet that this is the problem.