Metcal mx-500 soldering station heatsup when turned off!?

[Amitoha23]

Hello.

My name is Amit and need your help in understanding what is wrong with my mx500 stations.

They seem to heatup (the stations not the tips) even when the switch is turned off.so hot that the thermostat disconnecting the solder.
I have replaced Q1 Q2 as i thought. One of them is faulty . In the first 12 hours it was ok but then it started heating up again.

It seems that current is being consumed when its in off state as i can hear the transformer working. And of course the teansformer heats up but also the fets.
Thank you for your help
 
 

[Bratster]

The power switch on the mx500 is on the secondary side of the Transformer. So even while it is off you will still hear the Transformer hum.

However everything else should be off and it should only get a tiny bit warm.

Sounds like you have some fairly serious issues with that if even with the switch off it is staying on in some fashion.

Are either of the lights on when you turn the switch off?

Sent from my Nexus 6 using Tapatalk

[Assafl]

Well, the 7818 that powers the circuitry gets disconnected when the power switch is off. The power regulator LM2576HVT does get power (but should be powered down by Q12. Also I don't think the oscillator gets power (it gets the 18V regulated down to 7V or so by a 6.8v Zener) so in theory a scope should show no activity on the FETs.

I assume you have the attached schematics (but if not .