Marshall MG 50 DFX bangs on switch on

Motor boating at low frequency can be caused by loss of capacitance, more likely in power line filters than the main reservoir, or grounding problems, and other issues too. Piggybacking the power smoothing caps especially after any droppers would be my first move. By that I mean touching a known good cap onto the existing one with it running. Be aware this could give you a hell of a wallop on the speaker, so a series R on that wouldnt go amiss.

NT

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I originally thought it was due to broken input socket so the input not shorting with no guitar lead connected, was open circuit, meant the input was unterminated. Replaced the socket with a more robust one with functioning switches but because powering up quite soon after initial powering, with me, this symptom did not appear on soak testing. Returned it to owner and of course a few days later and they powering up and bang,bang.....

I once had a similar thing happening, but can't recall now what it was on. Turned out to be a faulty opamp that was slamming over to one rail then back again, but only when it was cold. Does your problem happen irrespective of the volume / master setting ? This would at least give a bit of a clue as to whether a signal stage prob, or an output / power supply fault ...

Arfa

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It certainly sounds more like DC surges. Its annoying having to wait so long for electrolytes or whatever to reform, degrade or "plates" deform to be too close. With these amps its possible to disconnect all lines to the output decice and associated comps. After leaving overnight and powering up, this morning, monitoring the line out there was nothing wrong at power up, so preamp opamps probably OK. I Let the DC rails fall to

One of those very enigmatic faults. The 3 caps are on main +- rails and the bootstrap facility Changed all 3 caps and now a continuous high level buzz for always , no normal function however long amp is switched off. Put a higher ohmage speaker on output for a bit of leeway. Monitoring the stand-by line there was high amplitude oscillation during the buzzing. Cutting that connection allowing standby control i/p to go low then muted the o/p, as should be. Taking it high via a 3V battery and normal service resumed but whether the original fault is still there plus some feedback low-f oscillating loop involving the stand-by control ? Ther's no oscillation on the downstream side of the cut standby connection. Put 47uF on the standby line and it gradually ramps up to about 6V which is presumably as it should but no oscilllation. I'll have to trace the circuitry back down the standby line and hopefully find a duff cap there. Incidently any Americans reading this thread, what is thy're interpretation of the word duff? Every time I see reference to the word as Duff Beer on the Simpsons I wonder if it has the same meaning in the USA. In the UK One meaning as being unserviceable and the other meaning sexual in nature.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

connection.

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It seems as though there was a very small soot jet on the pcb tracing back to the collector of the little MPSA06 buffering the switch-on hold-off of the standby control. Otherwise cold "diode" checked out ok. Uprated that and associated 6.8V zener and 50V, 220uF cap. Now to see what happens tomorrow morning.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

no nasty noises this morning

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

I just spotted the tail end of this thread. These amps - in commom with many other SS Marshalls - sometimes make all sorts of crashes and bangs, and instability noises when the tone controls are moved or set to their extremes. Replacing preamp ic's usually cures the problem

Ron(UK)

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Hi!

interpretation

Well, it could certainly be taken to mean "defective", "faulty", or "not working properly"...it's not commonly used (that I know of) here in the States, but that doesn't mean a person can't interpret or discover the meaning of the term.

I've also heard the term "duff" used to refer to someone's hind end...as in "get off your duff and start working" or something like that.

William