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Marshall Micro Bass Amp - Electric shock???

tom1970

Aug 23, 2021
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Aug 23, 2021
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Hi all,
A friend asked me to take a look at his Marshall IBS 3505 30W Micro Bass amp. Schematic is attached. He had got a nasty shock while connecting an effects pedal to it (a Fender Metal pedal). Apparently he was holding up the pedal in both hands and turned it on and immediately jerked bolt upright and couldn't let go of the pedal. Lasted for a few seconds and then luckily an RCCB or MCB tripped and he was saved.

I've had a brief look at the amp. There was a transistor whose leads hadn't been trimmed properly but I don't think they were making contact with the chassis (i've clipped them now). But the mains lead was very loose in the back of the amp so it is possible that the Earth connection was dodgy. The earth wire connecting the mains socket to the chassis was sound. [I'm giving him a new mains lead that fits snug in the socket.]

There was no voltage between the tip and sleeve on the input to the amp - and I tested this a number of times and after leaving the amp on for about 30 mins.

Though it seems safe, I am a bit worried about giving it back to him without knowing exactly why he got this serious shock and without being able to reproduce the problem.

Any ideas as to a possible cause?
Could it be the AC/DC 9V adapter that he was using for the pedal? He said it was a cheap one that he had found somewhere in the house. Is it possible for it to fail so that 240v is going through the chassis of the pedal and hence the shielding on the guitar lead plus the chassis of the amp. If the mains lead on the amp was not grounding the chassis then it doesn't trip when it should and he ends up completlng the circuit??? DOes that make sense?
Unfortunately he had thrown away the adapter before bringing me the amp so I can't test it.

Any thoughts gratefully received,

Tom
 

Attachments

  • Marshall 3505 Schematic.pdf
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Bluejets

Oct 5, 2014
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Any incident like this should be reported to the relevant supply authority inspectors for investagation, not thrown out for discussion on an electronics forum.
Many countries it is mandatory requirement of workplace health and safety.
 

tom1970

Aug 23, 2021
36
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Any incident like this should be reported to the relevant supply authority inspectors for investagation, not thrown out for discussion on an electronics forum.
Many countries it is mandatory requirement of workplace health and safety.
Do you mean the electricity supplier?
I am going to advise that he has the house electrics tested as I feel something should have tripped immediately.
 

tom1970

Aug 23, 2021
36
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Aug 23, 2021
Messages
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Do you mean the electricity supplier?
I am going to advise that he has the house electrics tested as I feel something should have tripped immediately.
Or maybe not if the earth connection was compromised
 

kellys_eye

Jun 25, 2010
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Have the equipment PAT tested. If the stuff is being used 'professionally' then it should be anyway. Even if it's just private (amateur) use then a PAT test will show up any potential for faults that could be risky to the user.
 

Bluejets

Oct 5, 2014
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Do you mean the electricity supplier?
I am going to advise that he has the house electrics tested
Yes, the supply authority.
In many countries it is, as I say, mandatory (that is, it is the law) that incidents such as this are reported immediately and the proper inspections and outcomes applied.
Simply testing from a contractor is normally insufficient as there has already been a reportable incident.
 

tom1970

Aug 23, 2021
36
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Aug 23, 2021
Messages
36
Have the equipment PAT tested. If the stuff is being used 'professionally' then it should be anyway. Even if it's just private (amateur) use then a PAT test will show up any potential for faults that could be risky to the user.
Yes - definitely. I'll make sure he does that! Good call
 
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