Marshall Micro Bass Amp - Electric shock???

[taddy1970]

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 a pedal to it (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 intermittent. The earth wire connecting the mains socket to the chassis was sound. And 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

 

[Jon Snell]

From your description, it would seem to be DC voltage as mains or AC voltage will cause the muscles to contract and relax equally not contract for a period of time.
Rnsure the earth is bonded to the chassis of the amplifier and of course to the ground rail of the amplifier jack sockets.
I would then perform a PAT test to verify its safety.
The AC/DC adapter. If it has a drawing of a square within a square; ⧈ that means it cannot fail to either potential of the mains BUT if it is a cheap Chinesium switch mode type, there could be a number of issues with it including the class X, interference capacitor leaking to a midway point on the 240v mains. That could do it!
There again, PAT test it or at least use a Megger to test the integrity of the insulation.

 

[taddy1970]

Thanks so much for that Jon and I didn't know that about the adapter symbol - thanks again!

The earthing in the amp seems fine (chassis connection is solid plus there is continuity with the sleeves of the jack sockets. That said, the mains cable that he was using was a bit dodgy - felt very loose in the socket in the amp. This might have compromised the earth connection.
However, DC voltage rails on this things are only +15 and -15V and it is reading pretty much that (actually +14V and -14V). Even if that was somehow getting out surely it wouldn't be enough to shock him in that way (?).
The AC side of the power transformer seems absolutely fine in the amp - positive and negative just goes straight into the transformer via the internal fuse and switch and nowhere near the chassis.
Unfortunately, I haven't seen the adapter that he was using as he threw it away - frustrating as I cannot now say for certain what caused the problem.
The insulation on the new mains cable seems fine. I haven't got PAT testing equipment or a megger (not really sure what a megger is!). I only have a multimeter unfortunately. I should prob tell him to get it PAT tested before he uses it again.