Marshall MG 50 DFX bangs on switch on

[N Cook]

Only about 2 years old, solid state.
Series of loud bangs , about 2 a second, for a variable number of seconds
but about 10 or 20 seconds calms down to normal.
Only at switch on fropm cold, off and on , minutes or even an hour later ,
there is no bangs.
Other than failing power supply smoothing cap is this a known fault symptom
? as annoyingly difficult to induce such a fault. Other than changing the ps
caps any hints for zeroing in on such an intermittant fault, if further into
the amp ?

 

[[email protected]]

Only about 2 years old, solid state.
Series of loud bangs , about 2 a second, for a variable number of seconds
but about 10 or 20 seconds calms down to normal.
Only at switch on fropm cold, off and on , minutes or even an hour later ,
there is no bangs.
Other than failing power supply smoothing cap is this a known fault symptom
? as annoyingly difficult to induce such a fault. Other than changing the ps
caps any hints for zeroing in on such an intermittant fault, if further into
the amp ?

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

 

[N Cook]

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

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.....

 

[Arfa Daily]

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

 

[N Cook]

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

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 <02V after switching off, Reconnected the output
stage and switched on and bang/bang....
Suggests a problem at the output.
I will replace the small 47uF,63V secondary DC rail electrolytics on this
output board and leave a few hours before powering up again.
I cannot see what chemical ? problem there could be inside the TDA7293 that
would take hours to reform.

 

[N Cook]

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.

 

[N Cook]

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.

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.

 

[N Cook]

no nasty noises this morning

 

[Ron(UK)]

no nasty noises this morning

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)

 

[N Cook]

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)

2 mentions of op-amp problems in one thread, would that suggest excessive
dv/dt somewhere in these Marshalls ?

 

[William R. Walsh]

Hi!

Incidently any Americans reading this thread, what is thy're interpretation
of the word duff?

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