1989 Mark III Green Stripe Noise

[RossettaStoned]

Just received my green stripe today from a seller on ebay and it's got a terrible background hum and odd popping crackling. It kinda sounds like wind going through an open door with some deep pops and cracks. It just gets louder and louder as the amp is on. The noise does not go away when the volume is off and gets worse as the GEQ controls are moved (the noise doesn't stop after moving them).The GEQ sliders are also REALLY scratchy and noisy. It's got a quite distorted low end when palm muting other than the typical high gain sounds and doesn't sound as tight as I'd expect. After a bit of googling it seems old amps have problems with filter caps going? Also I'm aware the tubes might need a change but as the seller said it was in full working order before it was sent I don't want to spend money on tubes to test out that theory and find out its something worse because it got damaged in transit (it was just wrapped in a cardboard box), end up sending it back and having a load of useless tubes. Or of course it could be the seller got rid of it because of this problem and lied (looking at his feedback it wouldn't surprise me).

Is there any way of doing any further trouble shooting without changing any tubes? Like cleaning the pots and sliders (I don't know exactly what to use for that either). There was also a point where there was considerably less gain than there should be and it came back after an hour or so of being off.

Sorry about the essay, just trying to give as much information as I can. Thanks

 

[CoG]

Changing tubes is kind of cost of doing business when you buy a used amp. You really shouldn't look for other problems until you have swapped all the power tubes, and then gotten at least one known good preamp tube to swap through all the positions.

 

[RossettaStoned]

Yea I realize that now and have swapped each preamp tube out so far to no avail. Also it was running with 6l6's in the outer pair and EL34s in the inner so I tried in half power and full power and swapped it over the right way and did the same. That didn't help at all so either a 6l6 and a EL34 has gone or all of them. I've found the noise almost goes completely when the GEQ is turned off (the sliders still make some noise though when moved). I'm taking it to an amp tech to give it a proper service which has kindly been provided by the seller so it should be fine.

I expect the tubes will need changing anyway so I'm wondering what tubes to go for exactly. I quite like the tone as it is but if anything I think it could be a bit tighter on the low end, it kind of blooms out when palm muting and I'm after that tight, percussive Petrucci attack.

 

[Boogie-Nights]

Just received my green stripe today from a seller on ebay and it's got a terrible background hum and odd popping crackling. It kinda sounds like wind going through an open door with some deep pops and cracks. It just gets louder and louder as the amp is on. The noise does not go away when the volume is off and gets worse as the GEQ controls are moved (the noise doesn't stop after moving them).The GEQ sliders are also REALLY scratchy and noisy. It's got a quite distorted low end when palm muting other than the typical high gain sounds and doesn't sound as tight as I'd expect. After a bit of googling it seems old amps have problems with filter caps going? Also I'm aware the tubes might need a change but as the seller said it was in full working order before it was sent I don't want to spend money on tubes to test out that theory and find out its something worse because it got damaged in transit (it was just wrapped in a cardboard box), end up sending it back and having a load of useless tubes. Or of course it could be the seller got rid of it because of this problem and lied (looking at his feedback it wouldn't surprise me).

Is there any way of doing any further trouble shooting without changing any tubes? Like cleaning the pots and sliders (I don't know exactly what to use for that either). There was also a point where there was considerably less gain than there should be and it came back after an hour or so of being off.

Sorry about the essay, just trying to give as much information as I can. Thanks

I had a simlar problem. Noise across all channels, increased with master volume (gain had a bit of an affect too). Mine was so bad you would never be able to use it at gig volume. Anyway!

I replaced all tubes and that didnt help. Its now in with a tech for a full re-cap (with R2 mod and adding a bias control), mine was a bit more serious though. Somewhere in its life the was a some kind of short in the bias circuit or power tube faluire and the PCB is burnt a bit. Noise was down to a bad repaire job!

Hoping to have it back anyday now