Help for amp section noise

[otto]

I have this noise on both channels of my marshall jcm800. Listen to attached file. You hear it on clean then crunch, then clean. Guitar is set on humbucker pick up. Noise is the same with different guitars, different cables, different places. I've replaced all tubes and all electrolytic caps, except the 50+50uF 500V filter ones (I don't have them now but noise has come from one day to the following so I don't think the problem is this). I've check some possible critical welds. Noise is the same if I connect a preamp in the return of the fx loop so it should be a problem in the amp section. See schem http://www.drtube.com/schematics/marshall/4210.gif
The 3 diodes in the power section are ok. Voltages on filte caps are ok.
What could I check?

[Enzo]

That whirring buzz? Sounds to me like either the amp or the guitar is sitting too close to a computer. And keep your cell phones off the top of the amp as well.

[otto]

That whirring buzz? Sounds to me like either the amp or the guitar is sitting too close to a computer. And keep your cell phones off the top of the amp as well.

it's not this. There is buzz everywhere

[otto]

is it right to read a AC value on filter caps (about 1000 volts) or should AC component be 0?

http://www.drtube.com/schematics/marshall/4210.gif

[Alex R]

Yeah can be a good few AC volts of ripple on the first stage or two - this is cancelled by the push-pull action of the power amp. Should be nearly all gone when you get to the early stage preamp filters.

The noise does sound like some kind of pickup from other equipment as it is higher-pitched than mains hum. Otherwise can't really help - hey, which of the panel controls change its tone or its volume? And does it only happen with a guitar plugged in?

[Garydean]

One time I found the preamp grid wires were too close to the chassis. Positioning them as far away from the chassis helped a lot for me one time.

[Enzo]

Did someone mount a fan in the amp?

There is really nothing in a MArshall amp that would make a noise like that, so I am drawn to an external source. Try a powr conditioning power strip and see if that helps. But this doesn't sound like something coming in through the mains.

Does the amp do this at someone else's house too?

[otto]

Yeah can be a good few AC volts of ripple on the first stage or two - this is cancelled by the push-pull action of the power amp. Should be nearly all gone when you get to the early stage preamp filters.

Sorry, I've not understood if is it right to read about 1000 AC volts on filter caps, shouldn't AC be zero?

The noise does sound like some kind of pickup from other equipment as it is higher-pitched than mains hum.

No, it comes with a different guitar too

which of the panel controls change its tone or its volume?

hum's volume goes with master volume and channels volumes

Does the amp do this at someone else's house too?

Yes

[Dave Curtis, dB AudioTech]

[QUOTE=otto;43255]Sorry, I've not understood if is it right to read about 1000 AC volts on filter caps, shouldn't AC be zero?


No, you should only see a volt at the most.

-DC

[otto]

New situation.
Sound was gone almost completely, I had to increase master volume a lot to hear something from the guitar, on both channels.
Switched the amp off. After some minutes the amp was ok, with no hum.
Now hum is back again.
Every pot of both channels influences it. All volume pots increase it. Both tone stacks have influence on it.
Gain on crunch channel has a strange effect when full up. In the first mp3 you hear it at 3/4 then when turned to 4/4 there is a whistle, this is a new effect, it wasn't there yesterday.
If I take off the input jack no sound and no hum. If I let the input jack in and I disconnect the cable from the guitar there is a whistle, listen to 2nd mp3.
On filter caps I read about 1000 VAC (the tester has a scale up to 750 VAC).
VDC is ok, 460 V.
Power transformer, output transformer and choke seem to be ok on voltages and resistances.
Could the problem be in filter caps?
Another thing is that reverb doesn't work, I've checked all connections and I don't find the problem, what could I check whithout an oscilloscope?
This is the schem again http://www.drtube.com/schematics/marshall/4210.gif
Thanks everybody for your help

[otto]

No, you should only see a volt at the most.

Even with power disconnected and filter caps discharging I read VAC about double of VDC

[Alex R]

Try another guitar.

1KvAC is a whole lot, man.

Respect to Dave but I think I've seen 5 or more AC volts of ripple on the centre tap of amps that run ok. But not a thousand! Check your meter by trying it on the mains voltage on the power on switch?

[otto]

Try another guitar.

it's the same
could it be filter caps?

Check your meter by trying it on the mains voltage on the power on switch?

it's ok, 220VAC
as said with power disconnected and filter caps discharging I read something as 5 VDC and 10 VAC

[Alex R]

Well, possibly it's some kind of internal feedback. First check - get hold of a new preamp valve and swap it for each one in there in turn, see if it goes away.

[Dave Curtis, dB AudioTech]

Try another guitar.

1KvAC is a whole lot, man.

Respect to Dave but I think I've seen 5 or more AC volts of ripple on the centre tap of amps that run ok. But not a thousand! Check your meter by trying it on the mains voltage on the power on switch?

Yeah 5's not uncommon. I guess I'm used to checking it after a cap job and don't normally check it otherwise.

I agree that 1000vac is a pile of EMF, and first wondered if the meter was wacky or something.