Any idea why this PA on switch is tripping my fusebox?

[tom1970]

Hi everyone.
I'm trying to fix a Bell 5005 Stereo PA amplifier. It has a rotating ON switch at the front which seems to be causing the circuit breaker in my garage to trip every so often. I opened it up and the contacts looks a bit blackened so I cleaned them up (with 99% isopropanol + scraping with a screwdriver). It certainly help but after a few more tests at turning it off and on again it trip the breaker again. I've made a video. Here is the link...

Ideally I would like to be able to retain the existing switch but I'm not sure exactly why it is tripping the breaker. I have been looking into replacing it but there is no such part anymore and the cut out in the front panel is only 9mm diameter and I can't seem to find anything that will fit. Also as I show in the video the ON switch connects both the live and neutral to the primary windings of the transformer at the same time. Is this necessary or could I just permanent wire the neutral and then find a switch that just makes one connect on the live side. This is how my guitar amp is wired. Is there any reason why the Bell PA needs both sides connecting on the switch?

 

[Martaine2005]

Is that a fuse holder where the mains come in? Is it the correct size fuse?.
If not, put a fuse in series with the live cable before the transformer.
In theory, no reason why you can’t have the neutral straight through to the transformer. What is on the secondary side?. Bridge rectifier and large electrolytic capacitors?.
The switch contacts look very pitted and probably cause lots of arcing.
There are many ways and tools available to increase the switch hole size for a replacement.
It seems like inrush current is the main issue.

Martin

 

[Bluejets]

You need to be more specific when you say "tripping your fusebox".

It might mean blowing a wire fuse, tripping an overload circuit breaker or tripping an earth leakage device.
Doubtful the switch is at fault unless the rear is conducting to ground, no way we can know as you have it there with no more info.

 

[Martaine2005]

Ha, I should have watched the video properly!

Martin

 

[tom1970]

You need to be more specific when you say "tripping your fusebox".

It might mean blowing a wire fuse, tripping an overload circuit breaker or tripping an earth leakage device.
Doubtful the switch is at fault unless the rear is conducting to ground, no way we can know as you have it there with no more info.

Apologies. It's tripping the 30A MCB that supplies the garage. Reason I think it's the switch is because when I got it the switch was stuck on the ON position and it didn't trip anything. I got the switch working again and now it only trips when you go OFF to ON and not every time but every now and again. Also the contacts were blackened inside.

 

[tom1970]

Is that a fuse holder where the mains come in? Is it the correct size fuse?.
If not, put a fuse in series with the live cable before the transformer.
In theory, no reason why you can’t have the neutral straight through to the transformer. What is on the secondary side?. Bridge rectifier and large electrolytic capacitors?.
The switch contacts look very pitted and probably cause lots of arcing.
There are many ways and tools available to increase the switch hole size for a replacement.
It seems like inrush current is the main issue.

Martin

Thanks for that. So would a high inrush current be caused by the arcing and would that be tripping the MCB?
I think this is the schematic (attached - apologies as it is just a snippet as the full schematic file was too big for the forum) as I read elsewhere that the Bell 5005 PA became a Fostex 600 or the other way round. Photos of the interior look v similar so it probably is about right...

 

[Martaine2005]

According to the schematic, there is no reason for switching the neutral.
Yes, arcing can draw high currents. The voltage can drop across the arc causing even more current.
The switch looks very pitted and probably needs replacement.

Martin

 

[tom1970]

Just reading about inrush. Could it be that it is tripping because there is a thermister

You need to be more specific when you say "tripping your fusebox".

It might mean blowing a wire fuse, tripping an overload circuit breaker or tripping an earth leakage device.
Doubtful the switch is at fault unless the rear is conducting to ground, no way we can know as you have it there with no more info.

Just reading about inrush and ways of preventing it. I noticed that it trips the MCB not on the first switch on but on subsequent ones so could it be that there is a thermister in series with the primary and this is not cooling fast enough - or rather I need to leave it longer between tests. The capacitors are massive in this thing and they also discharge v quickly.

 

[tom1970]

According to the schematic, there is no reason for switching the neutral.
Yes, arcing can draw high currents. The voltage can drop across the arc causing even more current.
The switch looks very pitted and probably needs replacement.

Martin

Gonna be difficult to find anything that will fit the 9mm hole. Any ideas?

 

[Martaine2005]

Very likely. If the NTC is low resistance, it’s not limiting the current.
But that would also depend on how quickly the caps are discharging with their bleed resistors.
A 9mm hole can be increased with a stepped drill bit or a 9mm hole saw and a larger hole saw on the same arbor.
Test your set. Remove the switch and use the wall sockets switch.

Martin

 

[tom1970]

Very likely. If the NTC is low resistance, it’s not limiting the current.
But that would also depend on how quickly the caps are discharging with their bleed resistors.
A 9mm hole can be increased with a stepped drill bit or a 9mm hole saw and a larger hole saw on the same arbor.
Test your set. Remove the switch and use the wall sockets switch.

Martin

So there's no way to restore the contacts on the existing switch then?

 

[Martaine2005]

To be honest, not really. Like the old points on cars, you could use Emery paper or wet n dry etc. Same as relay contacts, once badly pitted its better to replace.

Martin

 

[tom1970]

To be honest, not really. Like the old points on cars, you could use Emery paper or wet n dry etc. Same as relay contacts, once badly pitted its better to replace.

Martin

Thanks for your help!

 

[Martaine2005]

No problem. Hope you sort it.

Martin

 

[kellys_eye]

Apologies. It's tripping the 30A MCB that supplies the garage.

Is there any other spec on the MCB? They usually have an A, B, C etc suffix that specifies the type of load that it can handle without nuisance tripping. Here in the UK our MCBs are usually 'C'-rated that allow for large inrush currents as would be caused by refrigerators or other large inductive loads.

A-types have no inrush capability, B are x5, C are x10 and D are x20.

 

[tom1970]

Is there any other spec on the MCB? They usually have an A, B, C etc suffix that specifies the type of load that it can handle without nuisance tripping. Here in the UK our MCBs are usually 'C'-rated that allow for large inrush currents as would be caused by refrigerators or other large inductive loads.

A-types have no inrush capability, B are x5, C are x10 and D are x20.

Ah - interesting - I didn't know that!. Just checked it. Actually it is a 20A MCB - and it's a B (B20). Might that explain it?

 

[kellys_eye]

Potentially - Our system is 230V-based so the situation you have would result in only HALF the current levels (in my country) that you experience (twice the voltage means half the current) and perhaps, as a result, wouldn't be an issue for us using a 'b' type MCB.

Changing to a 'c' type will, of course, improve the situation yet not stop the protection the 20A breaker gives to the wiring anyway so you have little to lose (other than the cost of a new MCB - a few $ - by trying.

I do note that the schematic shows some MASSIVE capacitors which the NTC should mitigate but apparently doesn't. Depending on where in the mains cycle you switch it on the surge could indeed be 100's amps!

I would note that, here in the UK, ALL our MCB's (at least in my property) are 'c' rated - maybe this is the standard, dunno (Shame on me, I should know as I passed my electricians exams but that was well over a decade ago. Maybe I should read my handbook!)

There is also the small possibility that your MCB is a touch over-sensitive. Without the proper test equipment to prove this (far too expensive) it's simpler and cheaper to fit the 'c' rated breaker and avoid future disappointment.

 

[Bluejets]

It's tripping the 30A MCB that supplies the garage.

If it's the earth leakage section causing the trip you need a test for insulation resistance possibly on the transformer.
I doubt such a small transformer would create a current inrush problem on the actual circuit breaker section of the earth leakage.
I have had isolation transformers on medical isolation units do this but they are much, much larger.
A way to verify if you think that is what it might be would be to run a 10ohm 5 watt resistor in series with the primary just for a test.

 

[Harald Kapp]

run a 10ohm 5 watt resistor in series with the primary just for a test.

Lacking that, use a filament light bulb (no LED, no fluorescent), e.g. 60 W in series with the primary. Also for testing only. The lamp will reduce the inrush current.

 

[tom1970]

Potentially - Our system is 230V-based so the situation you have would result in only HALF the current levels (in my country) that you experience (twice the voltage means half the current) and perhaps, as a result, wouldn't be an issue for us using a 'b' type MCB.

Changing to a 'c' type will, of course, improve the situation yet not stop the protection the 20A breaker gives to the wiring anyway so you have little to lose (other than the cost of a new MCB - a few $ - by trying.

I do note that the schematic shows some MASSIVE capacitors which the NTC should mitigate but apparently doesn't. Depending on where in the mains cycle you switch it on the surge could indeed be 100's amps!

I would note that, here in the UK, ALL our MCB's (at least in my property) are 'c' rated - maybe this is the standard, dunno (Shame on me, I should know as I passed my electricians exams but that was well over a decade ago. Maybe I should read my handbook!)

There is also the small possibility that your MCB is a touch over-sensitive. Without the proper test equipment to prove this (far too expensive) it's simpler and cheaper to fit the 'c' rated breaker and avoid future disappointment.

I'm actually in the UK too so the B should be OK. It looks like it is an issue with the contacts - see my next post...
Thanks for the help and info