Marshall MB30 Motorboating sound with no input.

[DrTurbo]

Hello everyone,

Just joined on here and will be about when I can..

Hoping someone can shed some light onto a Marshall I have here at the moment.

Its an MB30 with the TDA2050 power amp IC, and for some reason it motorboats like crazy with no input. If I give it a sine wave of a level it will quieten and amplify well.

As soon as I remove the input signal it starts to motorboat again.

I've replaced the original TDA2050 as it had failled, probably because of this issue. I can only think its something on the input to stop this happening.

The power supply is all good.

If anyone has some advise it would be appreciated..

Thanks,

Mark

[Jon Snell]

I would check your ground points on the input jack.
Is it regardless of volume/tone settings?

[Chuck H]

I did a brief search for a schematic but was unable to find it. I did find a couple of Marshall-Bass 30 model schematics but it seems the "MB30" specifically is a different amp. A schematic (if you have one) would surely help.

[DrTurbo]

Thanks for that.

Yeh I'm struggling with finding a schematic too..

I will for sure check the grounding on the inout jack. If there is no input signal yes it will do it all the time for a few seconds and it seems the output IC goes into protection for a few seconds, then it starts again.
Regardless of input gain or tone it will continue to do this.

As sson as I feed it a signal it will amplify the signal quite happily with no motorboating.

There is a connector that links the main pcb to the rear line-out, send & return jacks, if this is unplugged it still does it.

Thanks..

Mark

[Jon Snell]

Sounds like a power supply decoupling fault if it is not affected by the gain control.
Take a look; https://elektrotanya.com/marshall_ba.../download.html

[DrTurbo]

No, none of the controls affect it with no input. As soon as I feed it a signal it quietens down and work fine, all controls as they should too, it just does it with no signal.

That schematic is about right for the output section, although this one looks to have a Zobel kind of network on the output, which also checks out ok..

Power supply seems good too, diodes good and caps reading correctly.

I guess it would need the power amp disconnecting somewhere to eliminate whare it is. I'll have a probe and see if I can find where the noise starts..

Thanks.

[g1]

Is the input jack shorting to ground when nothing is plugged in?

[DrTurbo]

Yes I'm pretty sure it is, if I have a guitar plugged in it willl still do it if you're not playing.
If I start to play then it seems to settle and amplify pretty well.

Its purely when the input signal is very low, really is strange..

Thanks again guys, if you've any more ideas I'll be glad to hear them....