Ground fault in old CD player?

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RhythMick
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#1 Ground fault in old CD player?

Post by RhythMick »

I was testing a passive balanced preamp I've built for a friend, but on playing music through it and attempting to scope the output my RCD trips.

I was taking a balanced output from an old TEAC VRDS player and after much elimination I think I've identified there's a ground fault somewhere in the TEAC. I wanted to check my logic with the more experienced folk here.

After various rounds of removing passive pre and then cables from the test rig, I've ended up with NOTHING else connected except the TEAC. Scope probes out of the way, not in circuit. No cables connected to the TEAC except the power (I'm remotely switching in case there's a dangerous fault). Variac is plugged in and a meter probe is in the earth socket on the Variac, but Variac is powered off and no power being taken from it. Just TEAC, standard wall socket and kettle lead.

TEAC turns on and starts playing the CD.

If I touch the grounded meter probe to the TEAC case the RCD trips. I think this proves the TEAC has a ground fault.

However If I use my multimeter to measure AC or DC voltage from earth to TEAC case the meter shows no AC and about 70mV DC, presumably that's the meter current through the small resistance.

Am I missing something? Is there any other explanation why grounding the TEAC case trips the RCD ?
RhythMick
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#2 Re: Ground fault in old CD player?

Post by RhythMick »

I thought of another explanation, a fault in the Variac earth post connection. Unlikely because the Variac wasn't in circuit when it first tripped.

Eliminated by using a different earth post.
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Nick
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#3 Re: Ground fault in old CD player?

Post by Nick »

Not sure what you mean about meter resistance.

70mv is more than enough to generate the 30ma that will trip the rcb. I would check any live to case caps in the ac input of the teac. Maybe worth looking at the 70mv with a scope to see just what sort of shape the DC is.
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RhythMick
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#4 Re: Ground fault in old CD player?

Post by RhythMick »

Nick wrote: Sun Mar 05, 2023 4:58 pm Not sure what you mean about meter resistance.

70mv is more than enough to generate the 30ma that will trip the rcb. I would check any live to case caps in the ac input of the teac. Maybe worth looking at the 70mv with a scope to see just what sort of shape the DC is.
Ok thanks for input Nick, I've not delved inside the TEAC before but sounds like now is the time.
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