marshall noise (SS)

[496dart]

Ive got a solid state Marshall G100R CD head here thats oscillating at a high freq. The freq will change when the gain / volume/ contour knobs are turned, and will go away when i shut off the guitars volume.
What causes this in a ss amp ?

[Enzo]

Maybe it is just the guitar feeding back. Get as far from the amp as you can with the guitar. Turn sideways to the amp so the speaker see the least string length. Does that help the noise? If so, it is gain of the amp feeding back the pickups.

APply some other signal to the amp, like the output of a CD player or something. A tone generator is handy to have on the bench for exactly these situations.

Explore the input jack. is it on a little jack board? Make sure the ground connections are intact and getting to the mainboard. If the jack is on the main board, check it thoroughly.

[496dart]

i dont think its the pickups, i stood and turned my back about 6-8 feet away, also this is with the amp on 1 or2 (master). i will check the grounds tomorrow

[Enzo]

With high gain circuits, it can be very surprising how low the volume can be and it still feeds back.

[496dart]

grounds look good. This thing squeals like crazy. Its not the normal feedback, its a super loud oscillation that changes when you turn some of the knobs. This is in high gain mode

[Enzo]

But turning the guitar to zero makes it stop? Again, what happens if yuo apply a signal other than a guitar?

[496dart]

yes, turning the guitar off makes it stop. I ran the guitar into another amp-thats not the problem.
The only thing i could run thru it is a cd player and the amp did not oscillate .

turning the volume knob on the guitar changes the frequency of the oscillation/high pitched howling. Like i said, its not a normal feedback sound

[Rick Erickson]

Marshall G100RCD Oscillation

What causes this in a ss amp ?

Same things that cause it in a tube amp.
Sounds like a ground or power supply issue to me. Has this amp been serviced or modified before? Have input or output grounds been connected to chassis when they used to be isolated? Has/have the speaker(s) been altered or changed? Some combo amps require the speaker frames to be grounded or the amp will oscillate. I would look closely at all ground points for possible problems, then at filter & supply bypass caps.

RE

[Enzo]

I am not suggesting the guitar has a problem, but when you turn the guitar down and the noise stops, that means the GUITAR is the source of the noise, not the amp. It doesn't do it with a CD player, again becasue the CD player has no pickups to feed back. That is why I suggested it. A high gain circuit can feed back through guitar pickups even at quiet levels you can easily talk over. Feedback does not have to be loud. That the volume control affects the oscillation freq means that it is IN the feedback loop

[Rick Erickson]

when you turn the guitar down and the noise stops, that means the GUITAR is the source of the noise, not the amp.

What I gather from that is the noise or oscillation stops when the input is shorted or held at a low impedance - such as the output of a CD player. The problem still may originate inside the chassis, although it could be as simple as a bad connecting cable. I do hope you've tried another guitar and cable with this amp.

RE

[496dart]

Yes, ive tried a different guitar and cable. same thing. I will try to look at the amp this weekend if i get time

[Rick Erickson]

If in doubt about whether the amp is the problem: you can always try to find the same model amp at a local music store and see if your guitar has the same problem with it. btw - have you checked the connection between AC ground and the chassis? A no-ground condition can lead to oscillation problems, especially in high gain mode.

RE

[Enzo]

OK then, try this. You have the schemo? On the input jack, there is a contact that opens when a plug is inserted. The top set of contacts in the drawing. The line from that cutout contact snakes down to the middle of the second row, where it connects to the input to the tone stack driver. This is the gate of Q1, joined to C78, R65, and R67. This line is a mute you can do without.

Follow the trace back from the jack towards those other parts. That trace normally grounds off the signal path in the center there when nothing is plugged into the amp. But once we plug in, that trace is there with the relatively hot signal of that channel a millimeter or so away from the input jack curcuit. It can act like an antenna beaming the channel output back into the input.

If we cut that trace away from the jack, that antenna thing won't happen. Follow it back from the jack and cut a small gap in it as far from the jack as possible. Just don't sever the thing at a point where you interupt some other path too.

If that doesn't help, either leave it or it can be patched simply.

[496dart]

can you email me the scem ? I cant find it anywhere. 496dart@mchsi.com

all the grounds that i can see are good. This problem is only on OD2 mode.

Heres some weird things: i can hit the chassis and make the oscillation change and even stop. Also, i can stand in front of the amp(not touching anything) and clik on my solder gun and the amp will start oscillating ! its plugged into the same power strip.

[Enzo]

All that means is the amp is only borderline stable. The glitch from the gun or transient from the mechanical shock is enough to set it off.

Here is another try-it. When it is oscillating, try poking or grabbing the components on the board. You could have a microphonic cap or something.

It does it on the highest gain channel only? That is consistent with feedback in the amp.

Did you push on each and every part? Looking for solder joints failed. Did you flip- the board over and look at each solder joint closely? I use a magnifyer.