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Marshall Lead 100 solid state amplifier 1978

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

Reply to
Madarchol
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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

Reply to
Arfa Daily

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

Arfa Daily wrote:

Reply to
Madarchol

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!!
Reply to
DaveM

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