Marshall Lead 100 solid state amplifier 1978

[Madarchol]

Anyone help me with this? Its an old 4*12 combo which worked fine until
recently when it developed an intermittent click approx every 5
seconds. occasionally upon turning the power on it would just produce a
50Hz hum through the speakers.

Now that is all that it does! Grounds all seem to be fine. pre amp
filter caps, and all power supply components are OK also. There really
isnt that much inside this amp so I can replace it all (apart from the
output transistors which seem to be obsolete), but if anyone has some
time saving ideas I'd be grateful.

Thanks

 

[Arfa Daily]

Anyone help me with this? Its an old 4*12 combo which worked fine until
recently when it developed an intermittent click approx every 5
seconds. occasionally upon turning the power on it would just produce a
50Hz hum through the speakers.

Now that is all that it does! Grounds all seem to be fine. pre amp
filter caps, and all power supply components are OK also. There really
isnt that much inside this amp so I can replace it all (apart from the
output transistors which seem to be obsolete), but if anyone has some
time saving ideas I'd be grateful.

Thanks

Have you actually 'scoped the power supply rails to check for ripple, and
checked the voltages in case a leaky cap or leaky zener in the preamp supply
( a common problem on many makes of group amps ) is dragging down any of
them ?

Arfa

 

[Madarchol]

Power supply rails seem ok with power amp section isolated. When
connected the waveforms indicate a heavy load. The amp briefly worked
after a lot of hissing and then (after I connected an input signal)
returned to its usual state of hum so I'm off to recheck solder joints
& earthing.

Thanks

Duncan

 

[DaveM]

Power supply rails seem ok with power amp section isolated. When
connected the waveforms indicate a heavy load. The amp briefly worked
after a lot of hissing and then (after I connected an input signal)
returned to its usual state of hum so I'm off to recheck solder joints
& earthing.

Thanks

Duncan

Most likely you have one or more output transistors shorted. That's what's
loading the power supply. If you run it very long with speakers connected,
you run a good risk of burning the speakers up. Measure the DC voltage at
the speakers and see if there is a DC voltage present. If it's more than a
few millivolts, then check the output transistors, as well as the driver
transistors. You'll likely find one or more to be shorted.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!