Yamaha GA-15 ground noise even with 0 volume

[Pastrocchio]

Hi guys hope you can help me with this strange problem.
This little guitar amplifier started making this loud stable ground noise from nowhere.
The noise is always there from when i switch on the amp, does not change if I use the overdrive or the clean channel or if I mess with volume or freq knobs and with guitar plugged or unplugged.
The sound comes out pretty decent actually if i play something and if i turn up the volume enough, the music cover the noise but it's always there.
It's not a litlle rustling noise, it's a fairly loud BZZZZZZZ

The electrical system is grounded and I even tried changing house but the noise persist. I tought maybe a bad insulation or ground problem and i opened up the amp to take a look inside but eveything seems fine. The ground connectors are in place.

I can provide photos or audio sample, just ask me.
I am somewhat confident with circuits and electrical stuff and i own a digital multimeter so if you want me to test something, again ask.

Thanks for your time

 

[Mooly]

Without knowing anything of the internals, and taking into account the fact it still basically works, then I would probably start looking at basics such as the power supply. Perhaps a reservoir cap has deteriorated and is causing excess ripple on the supply. Do a visual check to see if any show signs of leakage or bulging.

Beyond that and you are going to need a circuit diagram and an oscilloscope to diagnose this properly.

 

[Pastrocchio]

Thanks for the answer. Capacitors look good to me, i attached some pictures.

Unfortunately i can't find the schematics nowhere. And for the oscillator maybe i can find one. What should i do with it if i obtain one? It's for testing the conversion from ac to dc?

 

[Mooly]

Oscillator ? you mean oscilloscope

https://en.wikipedia.org/wiki/Oscilloscope

Are you sure those caps are OK ? Its not always easy to make out in a photograph but has the vented top lifted on them ? The top should be flat.

 

[Pastrocchio]

Yeah i meant oscilloscope, sorry for the slip.
By the way now that you make me notice, the top is not so flat. The top looks like bent upward.
We found the problem?

 

[CapnDenny1]

It hard to imagine both caps just failing? Did it get plugged in to 220v by mistake or something?

They look bad, but there may be other damage as well, and there is also the root cause.

But I guess start with what you know, and replace those and take it from there.

You can probably get a schematic from Yamaha? You may have to buy it, but it's worth the $5 or whatever to be able to troubleshoot it and fix it.

 

[Mooly]

We found the problem?

Possibly ! You will only know by replacing them.

This is where its guesswork... using test equipment such as an oscilloscope would confirm (or otherwise) that there was a problem with them.

70 to 80% chance of success at this stage though

 

[Tarzan]

If the OP is writing from home, then the amp is connected to the 220V as is NORMAL here.
So that can't be the problem. Just bad caps.
There is a big chance that the solder joints are cracked to.
After replacement of the caps that is fixed also.

tip: mark all connectors that wil be disconnected during the repair with a permanent marker. A fine tip pen is just fine. Write the connector code on it, like P2 or J4 etc.
It makes the assembly a lot easier and without (costly) errors.

 

[geminni]

Those caps really look bad and I think you should replace them no matter what

I had buzzing sound caused by bad cap. Replacing it with the new one the buzzing has gone.

 

[Pastrocchio]

If the OP is writing from home, then the amp is connected to the 220V as is NORMAL here.

Yep 220-240 V is the standard here in Italy.

By the way the owner of the amp, a friend of mine, told me something that he should have told me before. The noise actually started after a power outage of a portable generator. The amp was plugged there when the generator ran out of gasoline.
So i guess that those caps really are the problem.

Thank you all for the answers