Hum / Buzz only when cord attached...

[Valvestate]

I have recently picked up a second hand Marshall VS265 3 channel chorus amp that has 2 x 12" speakers in it. I managed to fix a fault within one of the power amp circuits buy resoldering some dry joints and now the amp is working great!

However, I have one issue. When the amp is powered up with NO guitar cord plugged into the jack it is very quiet. When I plug a guitar cord in a reasonably large hum or buzz is introduced. If I plug a guitar into the other end of the cord the buzz remains with no change in pitch or volume.

The buzz/humm is not affected by the volume or gain settings on the amp. If I run the amp on the overdrive channels it is very slightly louder but again the gain and volume knobs on the amp don't make the buzz/hum any louder or softer if adjusted.

So, first off, is this an amp issue? I only have one (cheap $7.99) guitar cord however if I plug it into my Roland Micro Cube practice amp the buzz is not present there.

Any pointers would be most helpfull!

Thanks!

[booj]

Sounds like the amp has a problem, but of course the only way to prove that is to try a different cord. Here's the scat if that helps
VS265_1.pdf

VS265_2.pdf

VS265_3.pdf

VS265_4.pdf

VS265_5.pdf

VS265_6.pdf

[ThermionicScott]

Might not be the case here, but when you just plug in a cable and no instrument, you're attaching an antenna to the grid of the first gain stage, so it's normal to pick up noise that way.

- Scott

[Enzo]

PLug the guitar into the amp input, now turn the volume control on the guitar to zero. Does the hum remain?

Is the hum less when on the clean channel?

Not only can your guitar pick up tons of noise, and certainly a cord with no guitar on the other end will, but there are also muting contacts in the inpit jack to silence the amp when nothing is plugged in.

[Valvestate]

The hum or buzz I mentioned is more of a hiss really. As I say, it is only present when a speaker cord is attached however if I plug a guitar into the other end of the cord this does not change the hiss.

The hiss is present when volume and gain are at zero and does not seem to be affected by an increase in volume or gain.

HOWEVER! Since my last post I have found that the hiss goes away almost completely if I turn the REVERB knob down...More reverb = more hiss it seems.

If I disconnect the reverb tank then the hiss goes away. I plugged the reverb cable into another Marshall combo amp's reverb tank and the hiss is present with that one also...

The hiss is present on the CLEAN and BOTH overdrive channels. I would say it is actually very slightly worse on the clean channel.

Just FYI - I have replaced the pre-amp tube and that has not changed anything.