Switch mode power supply from slot machine

[nathj]

Any electronics techs with switch mode experiance?

iv got an aristocrat slot machine that powers up ok but trips the circuit breaker when warmed up (about 5min from switching on) iv removed it from the machine so it wasn't loaded and it still does the same thing. any one know this fault with switch mode power supplies? or where a good place to start is??

cheers

[Skepticist]

First thing to check is the ESR of the electrolytic capacitors in the supply paying particular attention to those close to heatsinks as they're likely to be the most stressed. Frequently you'll notice bulging of the caps but don't rely on that as a true indication as I've encountered very bad caps with no external signs of trouble. The ESR meter is the only way to be sure.

If you don't have access to an ESR meter it may be cheaper to simply replace all the electrolytics on the secondary side with new 105C rated low ESR types.

Also pay close attention to any discoloured resistors (PCB may also be scorched under these) and check their values haven't drifted beyond tolerance.

Safety note: ensure the supply is not connected to the mains and all capacitors are safely discharged prior to doing any probing (protects you and your equipment).

[BCNZ]

Check the polarity of the mains cord. I had a similar fault some years ago with a digital title keyer in a video production facility.
The unit would randomly pop the breaker, for no apparent reason - and there was no pattern to when it would do it.

After a lot of inspection and testing I was sitting looking at the power cord and noticed it had a tapon (piggyback) plug on it.
On a hunch, I took the plug apart and noticed that the cable was the American type with white, black and green wires.
The white was connected to the phase pin, the black to the neutral and green to earth.

Equipment that has a transformer isn't too fussy about phase polarity (although it's not good practice to be switching the neutral)
but given that this unit had a switch mode power supply I had an idea that it could be a problem so I swapped the black and the white
around to where they should be.

For those who don't know, black is phase and white is neutral in the American colour coding for appliance cables.
I re-installed the unit and from that day on it never once tripped the breaker.

For OP, it's worth just checking, although I sense that your problem is more regular and probably points to the circuit drawing way too much current
at some point. This can be caused by a faulty optocoupler (used for PWM control feedback).

[nathj]

cheers guys my first move was to change the electrolitics out but just wanted confirmation.
the power supply was built in 1991 so the caps would be getting on. there is no apparent bulging or any of them wearing there insides on the out (electrolyte leaking) and no discoloration. but when i was an electronics tech 12yrs ago now this was a very common fault regardless of what equipment it was. ill try that first then change the opto couplers out which are also cheap.

its just really hard to fix something without a circuit diagram especially since my skills are abit rusty. ill report back asap

thanks nathan

[nathj]

Well ill be dammed

before i started replacing the caps i was looking at the power in and out, and the blue and brown wires were the wrong way around, it has been like this since manufacture because the security seal was still in place. it has been powered on for 45min now without tripping which is longer than before 5min.

thanks skeptic and studio but my money was on skeptic to be honest, now to load it up and see how she goes thank god i didn't have to start replacing caps

EDIT hi guys well it works fine now its been running for hours and no trip

[Skepticist]

Thanks for the quick feedback and I must add that one to my bag of tricks. The usual arrangement has two HV caps (usually ceramics or MKT) in series across active and neutral with their centre junction connected to ground and, as the MEN system has the neutral earthed, only one of those caps will be stressed if the earth is connected. Sounds a little like the cap that was connected to active is leaky enough to cause problems but swapping the active and neutral over has essentially replaced it with an unused one while the crook one is now not seeing any potential as long as the earth is connected.

Just a thought anyway

[BCNZ]

From memory, the way a switch mode power supply references to ground is the reason why the mains input must be polarity-correct and why they pop breakers
when they're swapped over.
When I did my apprenticeship the boss taught me to question everything. "Never assume that anything has to be right". This little pearl of wisdom has helped me out many times in my career and is still useful today.

Things like the mains power cord would normally be the first to be dismissed, on the basis that "it must be ok because it powers up".

I'm really pleased that you were able to spot that problem Nathj.