Buzzing Amp?

[chris20ire]

Hi everyone,I bought a second hand Vox advt digital amp about a year and a half ago,it's about 10 years old.It was working fine until recently when I noticed one speaker wasn't working.I switched speaker wires to check if it would work and it did,so that is a wiring issue,though I'm afraid to try fix it myself because the wires to the speaker(which have no visible damage anyway) are connected to a circuit board,and my soldering skills are not great.So I was forced to use it with one speaker when I noticed a buzzing(more noticable on distortion than clean) whenever I wasn't playing.This buzzing goes away somewhat when the guitar is turned down.Also if I plug the lead out of the input when the amp is on is produces some crazy buzzing noises that come and go and stop eventually until I plug in and start playing again.I also notice the noises from my guitar such as if I grab the whammy bar or turn a knob are coming through the amp more noticable than before.So I'm starting to think it's an earthing issue.I looked in the back and seen a screw in the amp frame on the bottom where I presume some kind of earthing wire was connected.As far as I know since I've owned the amp that wire was never there even when the amp made no buzzing.Should there be a wire there and where should I connect it to?Also Iv'e tried several guitar and the amp is the same so it's not the guitars.Any ideas would be greatly appreciated because there is no amp repair service in my town.Thanks.

 

[94Tremoverb]

It's unlikely to be the wiring - if it's a twin-speaker digital Vox it's stereo, and there are two separate power sections... so swapping the speaker wiring runs the other power section through the speaker. It's much more likely to be either: a dead power section; or: a faulty extension speaker jack - these put the extension speaker in series with the internal one, so a bad contact in the jack cuts out the speaker.

You can test the speaker jack like this: put any spare cable in the extension speaker jack. Now short the tip to the sleeve at the other end of the cable with something metal - pliers are good - while listening for background noise from the speaker. If it comes on, it's the jack and not the power amp. (NB: don't do this with an amp where the extension speaker is in parallel with the main!)

Both problems are common on these amps. If it's a blown power stage, it may be getting to the point of uneconomical to fix unfortunately, depending on how badly it's fried. If it's the jack, it's easy to fix but a right PITA to get to, you have to have the whole amp apart.

The noise from the input jack is dirty or corroded contacts in the jack switch in that one, which makes me think that it could very well be the same case for the extension speaker jack and not the power section. Contact cleaner and maybe a very small flat file are what you need.

Try this stuff first, and get it basically working. Worry about the buzz later .

 

[chris20ire]

Thanks for replying,I tried what you said with the external speaker jack.There were two jacks,on for left on one for right,so I tried the right because it was the right speaker is gone.Both speakers then produced a loud buzzing,is that normal?I'm a bit confused because the four speaker wires run to the circuit board so I thought they were powered from there somehow(I don't know much about electronics in amps) but sure enough what you said worked!Should I start on cleaning the jacks to see if it fixes the problem?Thanks again for replying.

 

[chris20ire]

I just examined all the jacks and there was no evidence of corrosion but I cleaned them anyway and the speaker still won't work.But like you said when both speakers came on when I plugged into the external jack,so I'm not sure what to do from here.

 

[94Tremoverb]

Do you ever use extension speakers? If not you could bypass the jack switches - solder wires across them.

The buzzing is more worrying though - that sounds like there might be some sort of power amp fault, maybe producing a DC output voltage. If so, it's possible that this has sent enough current through the jack to burn the contacts. If that is the case, and you re-connect it thoroughly, it will fry the speaker and/or the power stage more completely... so it's really important to find out why it's buzzing first.

This is going to be very difficult for me to troubleshoot for you from here! You really do need to find a tech if it's anything more serious than a jack or speaker problem - these amps are really not that reliable or well-built (sorry!) and you could easily end up with a lot of expensive damage.

 

[chris20ire]

I don't use extension speakers so I could try that.I forgot to mention that there are two inputs 'High' and 'Low' and the Low is considerably worse when I plug in there.And like I said before it's like the amp is detecting delicate sounds from my guitar more than is used to,for example if I'm screwing in the tremolo bar or especially if I use the pick up switch.This has become really noticable while playing.And if I don't touch my guitar the buzzing worsens and as soon as I touch something metal it goes quieter but is still there.All my guitars are correctly earthed.I think it's on it's last legs at this stage unfortunately.

 

[94Tremoverb]

Does it buzz with the extension speaker jack bypassed and no guitar plugged in? If not, there's no power stage fault. (Good news!)

If not, does it buzz with the guitar plugged in but the guitar volume turned down to zero? If not, this may point back to the possible lack of earthing on the amp. You really need to make sure this is correctly attached - the earth wire from the power cable socket should be screwed to the chassis via a large bolt/nut.

Odd that the problem is worse when using the low input than the high (it's normally the other way round), but this does make me think it's something to do with the earthing rather than a power stage fault. Don't give up on it yet .